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Full text of "The Kelvin and temperature measurements"

Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 

[J. Res. Natl. Inst. Stand. Technol. 106, 105-149 (2001)] 

The Kelvin and Temperature Measurements 



Volume 106 



Number 1 



January-February 2001 



B. W. Mangum, G. T. Furukawa, 
K. G. Kreider, C. W. Meyer, D. C. 
Ripple, G. F. Strouse, W. L. Tew, 
M. R. Moldover, B. Carol Johnson, 
H. W. Yoon, C. E. Gibson, and 
R. D. Saunders 

National Institute of Standards and 

Technology, 

Gaithersburg, MD 20899-0001 

billy.mangiim@nist.gov 

george.furukawa@nist.gov 

kenneth.kreider@nist.gov 

christopher.meyer@nist.gov 

dean.ripple@nist.gov 

gregory.strouse@nist.gov 

weston.tew@nist.gov 

michae 1 . moldover@nist. gov 

c.johnson@nist.gov 

howard.yoon@nist.gov 

charles.gibson@nist.gov 

robert.saunders@nist.gov 



The International Temperature Scale of 
1990 (ITS-90) is defined from 0.65 K 
upwards to the highest temperature measur- 
able by spectral radiation thermometry, 
the radiation thermometry being based on 
the Planck radiation law. When it was 
developed, the ITS-90 represented thermo- 
dynamic temperatures as closely as pos- 
sible. Part I of this paper describes the real- 
ization of contact thermometry up to 
1234.93 K, the temperature range in which 
the ITS-90 is defined in terms of calibra- 
tion of thermometers at 15 fixed points and 
vapor pressure/temperature relations 
which are phase equilibrium states of pure 
substances. The realization is accom- 
plished by using fixed-point devices, con- 
taining samples of the highest available 
purity, and suitable temperature-controlled 
environments. All components are con- 
structed to achieve the defining equilibrium 
states of the samples for the calibration 
of thermometers. The high quality of the 
temperature realization and measure- 
ments is well documented. Various research 
efforts are described, including research 
to improve the uncertainty in thermody- 
namic temperatures by measuring the ve- 
locity of sound in gas up to 800 K, re- 
search in applying noise thermometry 
techniques, and research on thermocouples. 
Thermometer calibration services and 
high-purity samples and devices suitable for 
"on-site" thermometer calibration that 



are available to the thermometry commu- 
nity are described. Part II of the paper 
describes the realization of temperature 
above 1234.93 K for which the ITS-90 is 
defined in terms of the calibration of spec- 
troradiometers using reference blackbody 
sources that are at the temperature of the 
equilibrium liquid-solid phase transition 
of pure silver, gold, or copper. The realiza- 
tion of temperature from absolute spec- 
tral or total radiometry over the tempera- 
ture range from about 60 K to 3000 K is 
also described. The dissemination of the 
temperature scale using radiation ther- 
mometry from NIST to the customer is 
achieved by calibration of blackbody 
sources, tungsten- strip lamps, and pyrome- 
ters. As an example of the research ef- 
forts in absolute radiometry, which impacts 
the NIST spectral irradiance and radiance 
scales, results with filter radiometers and a 
high-temperature blackbody are summa- 
rized. 



Key words: acoustic thermometry; 
blackbody sources; calibrations; gas ther- 
mometry; Johnson noise thermometry; 
Kelvin; pyrometers; radiation thermometry; 
SPRTs; thermocouples 



Available online: http://www.nist.gov/jres 



Contents 



Introduction 

Part I. Contact Thermometry 

1. Introduction 

2. Thermodynamic Temperature 

3. International Temperature Scales 

3.1 International Temperature Scale of 1990 (ITS-90). . 
3.1.1 Realization of the ITS-90 at NIST 



109 
109 
109 

110 
110 
112 
112 



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Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 

3.1.1.1 Realization Below 84 K 112 

3.1.1.2 Realization in the Range 83 K to 1235 K 116 

4. Thermodynamic Temperature Measurements at NBS/NIST 119 

4.1 Thermodynamic Temperature Measurements Utilizing Ideal Gases 119 

4.2 Thermodynamic Temperature Measurements Utilizing Johnson Noise 121 

5. Device-Based Research 123 

5.1 Gas-Based Cryogenic Fixed Points 123 

5.2 (Standard) Platinum Resistance Thermometer [(S)PRT] 124 

5.3 Thermocouple Thermometry 125 

6. Maintenance and Dissemination of Temperature Scales 127 

6.1 Maintenance and Dissemination of the ITS-90 and Other Scales Below 84 K. . . 127 

6.1.1 Prior Scales 127 

6.1.2 The ITS-90 127 

6.2 Maintenance and Dissemination of the ITS-90 and Other Scales above 83 K, 
Evaluations of Fixed-Point Cells, and Uncertainties of Calibrations over the 
Range of Contact Thermometry 128 

6.2.1 Prior Scales 128 

6.2.2 The ITS-90 129 

6.2.2.1 Calibrations 129 

6.2.2.1.1 Resistance Thermometers 129 

6.2.2.1.2 Thermocouples 130 

6.2.2.1.3 Liquid-in-Glass Thermometers 130 

6.2.2.1.4 Digital Thermometers 130 

6.2.2.2 Non-Uniqueness 131 

6.2.2.3 Evaluation of Customer Fixed-Point Cells 131 

6.2.2.4 Measurement Assurance Program (MAP) 131 

6.2.2.5 Standard Reference Materials (SRMs) 132 

6.2.2.5.1 SRM ITS-90 Fixed-Point Metals 132 

6.2.2.5.2 Large SRM ITS-90 Fixed-Point CeUs 132 

6.2.2.5.3 SmaU SRM Fixed-Point CeUs 132 

6.2.2.5.4 SRM Thermometers 133 

7. Future Work in Contact Thermometry 133 

Part II. Non-Contact (Radiation) Thermometry 134 

8. Introduction 134 

9. Historical Developments 135 

10. Current Work at NIST in Non-Contact Thermometry 137 

10.1 Calibration Capabilities 137 

10.2 Research in the Field of Radiance Temperature 139 

11. Future Directions in Non-Contact Thermometry 141 

12. References 144 

List of Tables 

1 . Assigned values of temperatures of fixed points on various International 
Temperature Scales Ill 

2. NIST fixed-point devices, operating conditions, and measurement uncertainties . 116 

3. Capsule-standard-platinum-resistance-thermometer ITS-90 calibrations 129 

4. Cryogenic capsule-resistance-thermometer calibrations 129 

5. Long-stem-standard-platinum-resistance-thermometer ITS-90 calibrations 130 

6. Industrial-platinum-resistance-thermometer calibrations 130 

7. Thermocouple thermometer calibrations 131 

8. Liquid-in-glass thermometer calibrations 131 

9. SRM fixed-point metals 132 

10. Large SRM fixed-point cells 133 

106 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 

11. Small SRM fixed-point cells 133 

12. SRM thermometers 133 

13. A brief summary of radiometric quantities as they apply to non-contact 
thermometry 134 

14. Values for the constants encountered in radiometry, the standard uncertainties, 

and the relationship to fundamental constants 135 

15. The expanded uncertainties (k= 2), in kelvin, for radiance temperature 
determinations of the blackbodies in the LBIR facility 138 

16. The types of variable-temperature blackbodies available in the LLT facility .... 138 

17. Expanded uncertainty (k= 2) in radiance temperature for the Cs or Na 
pressure-controlled-heatpipe blackbody source at 800 °C 138 

18. Expanded uncertainty (k= 2) in radiance temperature for an argon-filled 

ribbon filament lamp in the RTCL 139 

19. Expanded uncertainty (k= 2) in radiance temperature for a typical radiation 
thermometer 140 

20. The component of uncertainty in radiance temperature due to the uncertainty 

in spectral radiance as a function of v^avelength and temperature 144 

List of Figures 

1. The differences between ITS-90 and the earlier EPT-76, IPTS-68, ITS-48, 

and ITS-27 112 

2. A schematic of the ITS-90 shov^ing the temperatures of the defining fixed 
points (or phase equilibrium states) on the scale and the temperature ranges 
defined by interpolating instruments and equations 113 

3. A schematic of the ITS-90 temperatures in the range specified for the 
platinum resistance thermometer, showing the various defined subranges 
and the temperatures of the defining fixed points required for calibration 

in the subrange 113 

4. Schematic diagram of the copper block containing ITS-90 realization cells 

for low-temperature fixed points 114 

5. Pressure measurement system for the Low Temperature ITS-90 Realization 
Facility 115 

6. Water triple-point cell in an ice bath contained in a silvered Dewar 117 

7. A schematic drawing of the argon triple-point apparatus for calibrating 

seven long-stem SPRTs and six capsule SPRTs 118 

8. Idealized liquid/solid equilibrium conditions inside fixed-point cells used 

in freezing and melting experiments 119 

9. A standard platinum resistance thermometer in an indium, tin or zinc 
freezing-point cell 119 

10. Aluminum or silver freezing-point cell 120 

1 1 . The difference between recent determinations of thermodynamic temperature 

and 7^90 in the range 200 K to 320 K 121 

12. Simplified cross section of the NIST acoustic thermometer, showing the 3 L 
resonator, the pressure vessel, and associated plumbing and electrical connections. 
The furnace surrounding the pressure vessel is not shown 122 

13. Large model of Meyers' thermometer coil 125 

14. Deviation of emf values at fixed points of the SRM 1749 Au/Pt thermocouples 
from the NIST reference ftmction 126 

15. Residuals of data from a spline polynomial that forms the basis for the 
NIST/IMGC reference function for Pt/Pd thermocouples 126 

16. The difference in mK between various historical temperature scales in the 
cryogenic range and the ITS-90 as realized by NIST 127 

17. A schematic of the freezing-point blackbody crucibles 137 

107 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 

18. Schematic of the NIST Radiation Temperature Calibration Laboratory, with 

the various sources mounted on a translation table 138 

19. A schematic of the filter radiometers for measurements of spectral irradiance . . 141 

20. The differences in the temperature of a high-temperature blackbody determined 
using ITS-90 and three irradiance-filter radiometers 142 

2 1 . The percent difference, as a function of wavelength, in spectral radiance of a 
high-temperature blackbody determined using a tungsten-strip lamp and the 
irradiance filter radiometers 142 

22. A schematic of SIRCUS, illustrating the flux-stabilized laser sources that are 
input to an integrating sphere to create a uniform, monochromatic source of 
spectral radiance 143 

List of Acronyms 



ACR 

BS 

CCT 

CIPM 

CSPRT 

CVGT 

DCDG 

EPT-76 

PASCAL 

FP 

GRT 

HACR 

HTBB 

HTSPRT 

ICVGT 

IMGC 

IPRT 

ITS-27 

ITS-48 

ITS-90 

IPTS-48(60) 

IPTS-68 

IPTS-68(75) 

JNT 

JQVS 

KTTS 

LBIR 

LLT 

LTRF 

MAP 

MP 

NBS 

NBS-39 Scale 

NBS-55 Scale 

NBS P2-20 Scale 
NHS 

NMi 
NPL 



absolute cryogenic radiometer 

Bureau of Standards 

Consultative Committee on Thermometry 

International Committee of Weights and Measures 

capsule standard platinum resistance thermometer 

constant- volume gas thermometer 

differential capacitance diaphragm gauge 

The 1976 Provisional 0.5 K to 30 K Temperature Scale 

Facility for Automated Spectroradiometric Calibrations 

freezing point 

germanium resistance thermometer 

High-Accuracy Cryogenic Radiometer 

high-temperature blackbody 

high-temperature standard platinum resistance thermometer 

interpolating constant-volume gas thermometer 

Istituto di Metrologia "G. Colonnetti," Torino, Italy 

Industrial platinum resistance thermometer 

International Temperature Scale of 1927 

International Temperature Scale of 1948 

International Temperature Scale of 1990 

International Practical Temperature Scale of 1948; text revision of 1 960 

International Practical Temperature Scale of 1968 

International Practical Temperature Scale of 1968; Amended Edition 

of 1975 

Johnson noise thermometry 

Josephson pulse-quantized voltage source 

Kelvin Thermodynamic Temperature Scale 

Low-Background InfraRed 

Low-Level Temperature 

Low Temperature ITS-90 Realization Facility 

Measurement Assurance Program 

melting point 

National Bureau of Standards 

The NBS 1939 Constant- Volume Gas Thermometer Scale 

The temperature scale resulting from a simple numerical modification 

of the NBS-39 Scale 

National Bureau of Standards Provisional Temperature Scale 2-20 

Normal Hydrogen Scale (or echelle normale) 

Netherlands Measurement Institute, Delft, The Netherlands 

National Physical Laboratory, Teddington, UK 



108 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



OFHC 
PEP 
PMT 
PTB 

RIRT 
RTCL 
RTD 
SIRCUS 

SMOW 

SPRT 

SQUID 

SRM 

TFTC 

TP 

VTBB 



oxygen-free high-conductivity 

photoelectric pyrometer 

photomultiplier tube 

Physikalisch-Technische Bundesanstalt, Braunschweig and Berlin, 

Germany 

Rhodium-iron resistance thermometer 

Radiation Temperature Calibration Laboratory 

resistance temperature detector 

Spectral irradiance and radiance responsivity calibrations with 

uniform sources 

standard mean ocean water 

standard platinum resistance thermometer 

superconducting quantum interference device 

Standard Reference Material 

thin-film thermocouple 

triple point 

variable-temperature blackbody 



Introduction 

This paper gives a brief review of the realization of 
the kelvin at the National Institute of Standards and 
Technology (NIST) and of current research and other 
activities in thermometry. (From 1934 to 1988, NIST 
was known as the National Bureau of Standards (NBS), 
and from 1903 to 1934 it was known as the Bureau of 
Standards (BS); from 1901 to 1903, it was known as the 
National Bureau of Standards.) The paper is in two 
parts. Part I concerns contact thermometry and the real- 
ization of the International Temperature Scale of 1990 
(ITS-90) [1] at temperatures below 1235 K. Part II 
concerns non-contact (radiation) thermometry and the 
realization of the ITS-90 at temperatures above 1234 K. 

NIST has been involved in the field of thermometry 
since shortly after the creation of NBS, and laboratory 
notebooks detailing calibrations of liquid-in-glass ther- 
mometers date back to 1904. Similarly, notebooks con- 
cerning calibrations of thermocouples date to 1909 and 
work on platinum resistance thermometers dates back to 
1907. Thus, temperature, one of the SI quantities for 
which NIST has the responsibility for disseminating its 
measurement unit — the kelvin — to U.S. industry, has 
been a feature of the NIST work throughout most of the 
existence of the organization. 



Part I. Contact Thermometry 



1. Introduction 

The quantity that is designated thermodynamic tem- 
perature is defined by the laws of thermodynamics; it is 



indicated by the symbol T, and has the unit kelvin, 
symbol K. The unit of thermodynamic temperature is 
defined to be the fraction 1/273.16 of the thermody- 
namic temperature of the triple point of water. It is 
common practice to express temperatures in terms of 
their differences from 273.15 K, the value for the ice 
point. A thermodynamic temperature T expressed in 
this manner is known as a Celsius temperature t, which 
is defined by the equation 



r/°C = r/K- 273.15. 



(1) 



The unit of Celsius temperature is the degree Celsius, 
symbol °C. The magnitude of the degree Celsius is 
defined to be the same as that of the kelvin. Measures 
of temperature that are defined to be consistent with the 
laws of thermodynamics are said to be thermodynamic 
temperatures. Thermodynamic temperatures, however, 
are very difficult to measure precisely and accurately. 
Consequently, internationally-agreed scales of tempera- 
ture, with temperatures on the scale as close to thermo- 
dynamic temperatures as possible at the time the scales 
are approved, are used to approximate the thermody- 
namic temperature. These international temperature 
scales are defined in terms of fixed points, vapor pres- 
sures of some liquefied gases, thermometers that can be 
measured very precisely and fairly easily, and equations 
that relate measurements of these thermometers to tem- 
peratures of the scale. 

The Thermometry Group of NIST has the responsi- 
bility to develop, establish, and maintain the standards 
for temperature measurements in the region of contact 
thermometry (below 1235 K) that are necessary for the 
Nation's industrial and scientific progress and, in coop- 
eration with other national laboratories, help establish 



109 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



international uniformity in temperature measurements 
and promulgate the adopted International Temperature 
Scale (ITS). To meet this responsibility, members of the 
staff conduct research on the improvement of ther- 
mometry and provide thermometry information to vari- 
ous national and international thermometry standards 
committees, e.g., the Consultative Committee on Ther- 
mometry (CCT) of the International Committee of 
Weights and Measures (CIPM), the International Elec- 
trotechnical Commission (lEC), the American Society 
for Testing and Materials (ASTM), and the American 
Society of Mechanical Engineers (ASME). The national 
thermometry community is informed of the interna- 
tional standards and methodology of measurements by 
publications, consultations, calibration services, work- 
shops, and thermometry seminars. 

This portion of this centennial article gives an 
overview of some of the efforts in contact thermometry 
at NIST. Developments at NBS/NIST that are important 
to thermometry, but not covered, include, e.g., the 
purification of platinum, the Mueller Bridge (widely 
used before the modern bridges were developed), purifi- 
cation by slow crystallization and zone refining, 
cryoscopic determination of purity of substances, ac 
bridge measurement of resistance, electronics and com- 
puters, and many other areas. For those who are inter- 
ested, hsts of NIST publications are available from the 
NIST Office of Information Services, and those who are 
interested in publications on thermometry may contact 
the authors. 



2. Thermodynamic Temperature 

Ultimately all physical properties should be referable 
to thermodynamic temperature. Thermodynamic tem- 
peratures can be accurately determined by: 

1) Pressure volume (PV) gas thermometry 

2) Velocity of sound (acoustic) gas thermometry 

3) Noise thermometry 

4) Total radiation thermometry 

and related methods, such as Boltzmann distribution of 
population of energy levels and spectroscopic tech- 
niques. Research projects involving all four methods 
have been conducted at NIST. Thermodynamic temper- 
ature measurements are difficult and time consuming 
and require dedicated effort. Some of these are dis- 
cussed in sections below. 



3. International Temperature Scales 

A conveniently and accurately reproducible interna- 
tional temperature scale is indispensable for interna- 



tional commerce and exchange of scientific and techni- 
cal information. Since the late nineteenth century, there 
has been a series of internationally recognized tempera- 
ture scales. Those scales are 

Chappuis' constant volume hydrogen gas thermome- 
ter scale made available in 1887 through mercury ther- 
mometers and referred to as echelle normale (NHS) [2]; 

International Temperature Scale of 1927 (ITS-27) [3]; 

International Temperature Scale of 1948 (ITS-48) [4]; 

1958 "^He Vapor Pressure Scale of Temperature [5]; 

International Practical Temperature Scale of 1948. 
Text Revision of 1960 [IPTS-48(60)] [6]; 

1962 ^He Vapor Pressure Scale of Temperature [7]; 

The International Practical Temperature Scale of 
1968 (IPTS-68) [8]; 

The International Practical Temperature Scale of 
1968. Amended Edition of 1975 [IPTS-68(75)] [9]; 

The 1976 Provisional 0.5 K to 30 K Temperature 
Scale (EPT-76) [10]; 

The International Temperature Scale of 1990(ITS-90) 
[1]; and 

The 2000 Provisional 1 mK to 1.7 K Temperature 
Scale [11]. 

Table 1 lists the fixed points and their assigned tem- 
peratures of all the International Temperature Scales 
that have been adopted. Except for the superconductive 
transition points, the fixed points are phase equilibrium 
states of pure substances. 

The NHS was based on verre dur (hard glass) mer- 
cury thermometers that had been compared to the nor- 
mal -hydrogen gas thermometer between °C and 
100 °C. The symbol °C that was used for this and the 
ITS-27 scale indicated degrees centigrade; the ITS-48 
changed the name of the symbol to degrees Celsius 
(after the Swedish astronomer who was one of the two 
persons who independently proposed the centigrade 
scale, the scale based on the definition that the differ- 
ence between the ice point and the boiling point of water 
was exactly 100 degrees). The International Tempera- 
ture Scales that followed the NHS were based on fixed 
points with assigned temperature values based on mea- 
surements of the thermodynamic temperature, standard 



110 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 

Table 1. Assigned values of temperatures of fixed points on various International Temperature Scales. The values of temperatures of NHS and 
ITS-27 are in degrees Centigrade; those of ITS-48 and IPTS-48 are in degrees Celsius; and those of IPTS-68, IPTS-68(75), EPT-76, and ITS-90 
are in kelvins 



Point 


NHS' 


IT^-lf 


ITS-48'^ 


IPTS-48'^ 


IPTS-68 


IPTS-68(75) 


EPT-76 


ITS-90 




trc 


trc 


trc 


trc 


r/K 


r/K 


r/K 


r/K 


CuFP'^ 
















1357.77 


AuFP 




1063 


1063.0 


1063 


1337.58 


1337.58 




1337.33 


AgFP 




960.5 


960.8 


960.8 


1235.08 


1235.08 




1234.93 


AlFP 
















933.473 


SBP'^ 




444.60 


444.600 


444.6 










ZnFP 








419.505" 


692.73 


692.73 




692.677 


SnFP 










505.1181P 


505.1181P 




505.078 


InFP 
















429.7485 


H2O BP^ 


100 


100.000 


100 


100 


373.15 


373.15 






GaMP' 
















302.9146 


H2O TP^ 








0.01 


273.16 


273.16 




273.16 


H2O MP'' 





0.000 















HgTP 
















234.3156 


O2BP' 




-182.97 


-182.970 


-182.97 


90.188 


90.188 






ArTP 












83.798^^ 




83.8058 


O2TP 










54.361 


54.361 




54.3584 


NeBP^ 










27.102 


27.102 


27.102 




NeTP 














24.5591 


24.5561 


e-H2 BP'' 










20.28 


20.28 


20.2734 


20.3 


e-H2 BP' 










17.042 


17.042 


17.0373 


17.0 


e-H2 TP 










13.81 


13.81 


13.8044 


13.8033 


Pb SP™ 














7.1999 




^HeBP 














4 9991 


4.2 


InSP 














3.4145 




'HeBP 
















3.2 


AISP 














1.1796 




ZnSP 














0.851 




CdSP 














0.519 





'NHS: Normal hydrogen scale. 

^ For these temperatures, the ice point was 273.16 °K. 

^ FP: Freezing point. 

'^BP: Boiling point at 101 325 Pa. 

' H2O BP: Steam point. 

^MP: Melting point at 101 325 Pa. 

^ TP: Triple point. 

"^ H2O MP: Ice point, saturated with air at 101 325 Pa. 

' Redefined in 1975 to condensation point (CP). 

^ Ne BP: Natural isotopic composition. 

^ e-H2: Equilibrium composition of the ortho/para species. 

' Boiling point at reduced pressure, at /? = 33 330.6 Pa. 

™ SP: Superconductive transition point. 

" Alternative to S BP. 

P Alternative to H2O BP. 

'^ Alternative to the O2 BP. 



thermometers and interpolation equations. Until the 
ITS-27 was adopted in 1927, BS maintained NHS, 
adopted by the CIPM in 1887, using 16 verre dur mer- 
cury thermometers. 

The purpose of the ITS-27 and of the subsequent 
International Temperature Scales has been well ex- 
pressed in the introduction to the ITS-48 [4]: 

"The experimental difficulties inherent in the mea- 
surement of temperature on the thermodynamic scale 
led to the adoption in 1927, by the Seventh General 



Conference of Weights and Measures, of a practical 
scale which was named the International Tempera- 
ture Scale. This scale was intended to be as nearly 
identical with the thermodynamic centigrade scale as 
was possible with the knowledge then available. It 
was designed to be conveniently and accurately re- 
producible and to provide means for specifying any 
temperature on the International Scale within much 
narrower limits than was possible on the thermody- 
namic scale." 



Ill 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



Figure 1 shows differences between the ITS-90 and 
the earlier EPT-76, IPTS-68, ITS-48, and ITS-27. The 
difference between ITS-90 and IPTS-68 reflects the 
more recent determination of the difference in the ther- 
mocouple range of the IPTS-68 (630.615 °C to 
1064.18 °C) [12]. 

3.1 International Temperature Scale of 1990 
(ITS-90) 

The ITS-90 extends upwards from 0.65 K to the 
highest temperature measurable by spectral radiation 
thermometry in terms of the Planck radiation law. The 
ITS-90 is defined in terms of 17 fixed points; vapor 
pressure/temperature relations of equilibrium-hydrogen 
(e-H2), "^He, and ^He; "^He or ^He constant-volume gas 
thermometers (CVGTs); standard platinum resistance 
thermometers (SPRTs); and radiation thermometers. 
The Pt-10 % Rh vs. Pt thermocouple that formerly de- 
fined the region from 630 °C to the Au freezing point 
(FP) has been replaced by high-temperature SPRTs 
(HTSPRTs). The spectral radiation thermometer can be 
referenced to either the Ag, Au, or Cu FP. Figure 2 is a 
schematic representation of the ITS-90 showing the 
defining fixed points and temperature ranges defined by 
interpolation thermometers and equations. The SPRT is 
the only contact-type interpolating instrument of the 
ITS-90 that directly disseminates the scale. In the previ- 
ous International Temperature Scales, the standard 
Pt-Rh/Pt thermocouple served also in that position. 



The ITS-90 is designed with a number of ranges and 
subranges that overlap, giving different definitions of 
Tgo that have equal status. The temperature differences 
that may arise are of negligible practical importance. 
Figure 3 shows the temperature range specified for 
SPRTs, with various defined subranges, and tempera- 
tures of the defining fixed points that are required for 
calibration. 

3.1.1 Realization of the ITS-90 at NIST 

3.1.1.1 Realization Below 84 K 

Below 84 K, the ITS-90 has four different definitions 

(1) ^He vapor-pressure thermometry (0.65 K to 3.2 K) 

(2) "^He vapor-pressure thermometry (1.25 K to 5.0 K) 

(3) interpolating constant-volume gas thermometry 
(3.0 K to 24.5561 K), with calibrations at a ^He or "^He 
vapor-pressure point between 3 K and 5 K, at the e-H2 
triple point (TP) (13.8033 K), and at the Ne TP 
(24.5561 K); and (4) TPs over the range 13.8033 K to 
83.8058 K, plus two additional temperatures close to 
17.035 K and 20.27 K (determined either by using a 
gas thermometer or the specified temperature -vapor 
pressure relationship of equilibrium-hydrogen — See 
Table 1), at which capsule standard platinum resistance 
thermometers (CSPRTs) are calibrated and used for in- 
terpolation between the points. In order for a CSPRT to 
be used below 84 K, however, it must be calibrated also 
at the TPs of Hg and H2O. 



0.5 T- 



U 



0.0 



, -0.5 



E^-1.0 "— 



.0 
-1.5 



-EPT-76-rr8-90 
-IPTS-48-rrS-90 

-rrs-27-rra-90 




Temperature / K 



Fig. 1. The differences between ITS-90 and EPT-76, IPTS-68, ITS-48, and ITS-27. 



112 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 




30 50 , 100 



5O00 1000D 



Fig. 2. A schematic of the ITS-90 showing the temperatures of the defining fixed points (or phase equilibrium states) on the scale and the 
temperature ranges defmed by interpolating instruments and equations. For assigned values of defming temperatures, see Table 1. 



"I — I I I r 1 1[| 



• Calibration Points 
Interpolation Range 



H-l-*- 



-♦ — *- 



a d> " ^- 



>— *- 



o 



S 



s s - 
' I 



100 



I I I I llll — 

273.16 
/ H2O 
* I ♦ » » » 

♦ !♦ » ♦ 

♦ !-♦-♦ 



« — M I 



I I 



I I 

I I 
I I 

I I 

I I 



-♦♦I 

-^\ ! ! I 
&: i 8:8: 8: 8:8: 

«>• life" ?i 

Si § i i i i 

I 1 I I r I I il 



1000 



Temperature, (K, ITS-90) 



-| i — I M M I 



1 I I I I II 

10000 



Fig. 3. A schematic of the ITS-90 temperatures in the range specified for the platinum 
resistance thermometer, showing the various defmed subranges and the temperatures of the 
defming fixed points required for calibration in the subrange. 



Over certain temperature ranges, there is overlap be- 
tween two or more definitions (see Fig. 2). All defini- 
tions are considered equally valid over their respective 
ranges, allowing the possibility of non-uniqueness in the 
ITS-90 in the overlap ranges [13]. 

In order to fully realize the ITS-90 below 84 K, NIST 
began construction of its Low Temperature ITS-90 Real- 



ization Facility (LTRF) in 1990. A brief description of 
the facility can be found in Ref [14]. The LTRF was 
designed to realize the ITS-90 below 84 K using the 
guidelines published in Guidelines for Realizing the In- 
ternational Temperature Scale of 1990 (ITS-90) [15] and 
in Supplementary Information for the International 
Temperature Scale of 1990 [16]. The centerpiece of the 



113 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



LTRF is a gold-plated cylindrical, oxygen-free high- 
conductivity (OFHC) copper block that contains seven 
sample cells for realizing the ITS -90 over this range (see 
Fig. 4). The largest is 1000 cm^ for the interpolating 
constant-volume gas thermometer (ICVGT). Four cells 
are used for realizing the triple points of Ar (20 cm^), O2 
(20 cm^), Ne (3 cm^) and e-H2 (3 cm^). There is also a 
^He vapor-pressure cell (3 cm^) and a "^He vapor-pres- 
sure cell (3 cm^). The e-H2 triple-point cell is used also 
for realizing the e-H2 vapor-pressure points at 
r- 17.035 K and at 7-20.27 K for calibrating 
CSPRTs. The e-H2 cell contains about 0.5 cm^ of ferric 
hydroxide powder, a catalyst for the conversion of ortho- 
hydrogen and para-hydrogen to their equilibrium distri- 
bution. The OFHC Cu block contains six thermometer 
wells, one at the top of the block, four at mid-height, and 
one at the bottom. The top and bottom wells can accom- 
modate rhodium-iron resistance thermometers (RIRTs) 
and the mid-height wells can accommodate either 
CSPRTs or RIRTs. The resistances of the thermometers 
are measured with a commercial ac bridge using a stan- 
dard resistor calibrated at NIST. The OFHC Cu block is 
surrounded by three copper shields in a vacuum space. 
The outer shield is immersed in an appropriate cryo- 
genic liquid. Cooling of the copper block is accom- 
plished by exchange gases for temperatures above 12 K 
and by a continuously recirculating ^He refrigerator for 
lower temperatures. Heating is performed with a resis- 
tive-wire heater wrapped around the Cu block. 



Togas handling 
system 




-—Two 20 cm^ cells 
(Ar, O2) 



Thermometer well 



Four 3 cm^ cells 
(Ne, H2, 3He. ^He) 

Four 
thermometer wells 



Gas 

thermometer 
cell (1000 cm3) 



Thermometer well 



Fig. 4. Schematic diagram of the copper block with ITS-90 realiza- 
tion cells. 



Thfifinal 
shieki 



The ICVGT and vapor-pressure realizations require a 
pressure-measurement system (see Fig. 5), which is a 
combination of a piston gauge and a differential capaci- 
tance diaphragm gauge (DCDG). The piston gauge gen- 
erates an accurately known pressure, P , and the DCDG 
measures the pressure difference between that of the 
cell and that generated by the piston gauge. The piston 
gauge pressure can be made to be very close to that of 
the cell, so that the pressure difference across the 
DCDG is small (< 20 Pa). With such a system, the 
relative standard uncertainty (k= 1) in the absolute pres- 
sure measurement is 12 X 10~^ and in the relative pres- 
sure measurement, it is 3 X 10 "^. All cells requiring 
pressure measurement have individual DCDGs but use 
the same piston gauge. 

A description of the triple -point realizations can be 
found in Ref. [17]. For these realizations, the Cu block 
is thermally isolated from the shields around it to make 
the heating of the block adiabatic. Before each melt, the 
Cu block is cooled to a temperature that is several 
kelvins below the triple -point temperature of the sam- 
ple. It is then heated to a temperature that is slightly 
below the triple -point temperature and kept there for 
several hours to permit equilibration. Then the tempera- 
ture is increased through the triple -point transition by 
successive constant increments of heat. After each incre- 
ment of heat, the cell is allowed to come to thermal 
equilibrium. During this time, the temperature is moni- 
tored with one of the resistance thermometers. The size 
of the heat increments is typically 1/12 the heat-of-fu- 
sion. The period of time allowed for reaching thermal 
equilibrium after each heat increment is determined ex- 
perimentally. At the end of the waiting period, the resis- 
tance R of the monitoring thermometer is measured. 
Data consisting of these final equilibrium resistance 
readings as a function of applied heat are used to deter- 
mine the beginning of the melt, the end of the melt, and 
the heat-of-fusion. Subsequently, plots of thermometer 
resistance as a function of 1/F, where F is the fraction 
of material melted, are made. The final resistance is 
extrapolated to 1/F = 1 to provide the triple-point resis- 
tance Rtp. At an appropriate point on the plateau of one 
of the melts, the resistances of all CSPRTs in the Cu 
block are measured. These resistances are corrected to 
correspond to 1/F = 1 by using the readings of the mon- 
itoring thermometer. Expanded uncertainties {k = 2) of 
realization for the triple points are 0.07 mK for Ar, 
0.06 mK for O2, 0.21 mK for Ne, and 0.15 mK for e-Hs. 

Procedures for realizing the ITS-90 using the ICVGT 
are described in Ref. [18]. The ICVGT is filled with 
approximately 0.16 mol of "^He. The measurements with 
the ICVGT are at intervals of about 1 K. At each point, 
the temperature of the OFHC Cu block is brought to the 



114 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



Valve 



Bulb 



To filling 
system 



i 



J^ 



Differential 

capacitance 

diaphragm 

gauge 



II 



Piston 
gauge 

Pp = Mg/A 



ws 



EX1= 



PriP.^Po) 



X 



To 
vacuum 



Px = Pp^Po^Pcdg 



To gas 
manifold 



Fig. 5. Pressure measurement system for the Low Temperature ITS -90 Realization 
Facility. 



selected temperature. The cryostat is allowed to equili- 
brate and then the resistances of the thermometers are 
measured. The pressure measurement is performed by 
first balancing the piston gauge and then measuring the 
pressure difference across the DCDG. Corrections to 
this pressure measurement are then made for the dead 
space between the ICVGT and the DCDG. Corrections 
are also made for the aerostatic pressure head and for the 
thermomolecular pressure difference. Measurements 
are made of gas pressures that correspond to the ITS-90 
fixed points (5.0 K, 13.8033 K, and 24.5561 K) to 
calibrate the gas thermometer. The 5.0 K point involves 
the measurement of "^He vapor pressure. The latter two 
fixed points are triple-points of e-H2 and Ne, respec- 
tively. In practice, the three fixed-points are realized in 
the copper block first, and then the readings of the 
resistance thermometers are used to set the block tem- 
perature to the fixed-point temperatures to calibrate the 
gas thermometer. Once the ICVGT has been calibrated, 
the ITS-90 is realized with it by using the measured 
pressures and Eq. (4) of Ref [1]. The RIRTs in the Cu 
block are then calibrated in terms of the ICVGT. The 
uncertainty (k = 2) of measurements with the ICVGT 
varies from 0.09 mK at 5 K to 0.26 mK at 24.5561 K. 
The procedure used in the vapor-pressure/tempera- 
ture measurements of ^He, "^He and e-H2 is described in 
Refs. [14,19]. For each vapor-pressure point, the Cu 
block is brought to the selected temperature and left to 
equilibrate. The resistances of the thermometers are 
then measured. The pressure is measured as described 
above for the ICVGT. Corrections to the measured pres- 



sure are made for the aerostatic pressure head and for 
the thermomolecular pressure difference. The ITS-90 
temperature is then obtained from the measured pres- 
sure and Eqs. (6) or (11) of Ref [1], and that value is 
assigned to the corresponding resistance of the ther- 
mometers. The uncertainties (k = 2) for the He vapor- 
pressure realizations are 0.1 mK or less over 97 % of the 
ranges of the ITS-90 definitions. In the lower 3 % of the 
ranges, the uncertainties increase to as high as 0.3 mK 
because of the increasing thermomolecular pressure 
correction. The uncertainties (k = 2) in the two e-H2 
vapor pressures near 17.0 K and 20.3 K are 0.15 mK. 

The LTRF was designed to calibrate in-house 
"reference-standard" resistance thermometers consist- 
ing of selected CSPRTs and RIRTs for NIST only. Cus- 
tomer thermometers are calibrated against these resis- 
tance thermometers in a comparator block located in a 
separate facility (see Sec. 6.1.2). Realization of the 
ITS-90 in the cryogenic range was completed in 1996, 
and since that year the scale below 84 K that is dissem- 
inated by NIST is traceable to the realization measure- 
ments made in the LTRF. NIST intends to realize the 
ITS-90 below 84 K in the LTRF to re-calibrate the 
reference-standard resistance thermometers at 5 year 
inter\^ls to minimize scale uncertainties due to possible 
drifts of the thermometers. 

The LTRF has been used also for studies of the scale, 
in particular the non-uniqueness of the ITS-90 [13] over 
the ranges of definition overlap. Results were published 
in 1996 [14] on the non-uniqueness over the range 
1.25 K to 3.2 K, in which the scale is defined by the 



115 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



vapor-pressure/temperature relations of both ^He and 
"^He. The non-uniqueness over this range was found to 
vary between 0.1 mK and 0.3 mK. Results also were 
published [19] on non-uniqueness over the range 
13.8033 K to 24.5561 K, in which the ITS-90 is defined 
by both ICVGTs and SPRTs. The maximum non- 
uniqueness found over this range was 1.55 mK, which 
occurs at 15 K. 

To date, NIST is the only national laboratory to real- 
ize the ITS-90, as it is defined, from 0.65 K to 84 K in 
its entirety and it also is the only laboratory that has 
published determinations of the ITS-90 non-uniqueness 
below 25 K. 



3.1.1.2 Realization in tlie Range 83 K to 1235 K 

The SPRT range from 83 K to 1235 K is defined by 
nine fixed points: Ag FP, Al FP, Zn FP, Sn FP, In FP, Ga 
melting point (MP), H2O TP, Hg TP and Ar TR Samples 
of the highest purity are selected. Container materials 
were selected that would not contaminate the sample at 
the operating temperatures and are strong enough to 
endure multiple freezing and melting. Purified graphite 
was selected for Ag, Al, Zn, Sn and In; Teflon for Ga; 
borosilicate glass for H2O and Hg; and copper for Ar. 
Stainless steel is also used with Hg. The graphite con- 
tainer and its sample are protected from oxidation with 
an atmosphere of argon or helium gas. Table 2 lists the 
purity of the fixed-point substances, the container and 
holder materials, the amount of sample used, the immer- 



sion depth of the SPRTs in the thermometer well of the 
sample container, the controlled operating environment, 
and the uncertainties associated with the measurements 
using the fixed-point devices. 

The H2O TP is the most important fixed point of the 
ITS-90. The Kelvin Thermodynamic Temperature Scale 
(KTTS) is defined by assigning 273 . 1 6 K to the H2O TP, 
making the kelvin equal to 1/273.16 of the H2O TP 
temperature. All thermodynamic thermometry is refer- 
enced either directly or indirectly to this temperature. In 
the SPRT range, temperatures are determined in terms 
of the ratio of the observed resistance /? (rgo) at T90 to the 
resistance R (273 .16 K) at the H2O TP, i.e., 
W{T9o) = R{T9o)/R{273A6 K), and the resistance -ratio 
reference function , which was designed to closely repre- 
sent thermodynamic temperatures (see Refs. [1,20,21] 
for details). Figure 6 is a schematic of how a H2O TP 
cell of NBS design is used for calibrating an SPRT. For 
measurements with SPRTs at NIST, four H2O TP cells 
are maintained in a water bath held at 0.007 °C. For 
details of application and measurements at the H2O TP, 
see Ref. [22]. 

The Ar TP is realized by a method different from the 
others. The apparatus is operated immersed in liquid 
nitrogen. The outer vacuum jacket surrounds three sets 
of thermal radiation shields around the 300 cm-^ copper 
sample cell, containing 1 5 mol of Ar, into which seven 
long, thin-wall stainless steel thermometer wells were 
inserted and soldered. During operation, the tempera- 
ture of the tubes that extend above the sample cell is 



Table 2. NIST fixed-point devices, operating conditions, and measurement uncertainties. The expanded uncertainty (k = 2) is denoted by U 



Fixed 


(mass 


Container 


Amount 


Immersion 


Holder 


Furnace or 


Type A 


TypeB 


U 


point 


fraction) % 


material 


of sample 


depth (cm) 


material 


bath 


(mK) 


(mK) 


(mK) 


AgFP 


99.9999+ 


graphite 


1.5 kg 


13.3 


Inconef'' 


sodium heat pipe 


0.50 


0.17 


1.06 


AlFP 


99.9999+ 


graphite 


0.4 kg 


16.7 


Inconef 


sodium heat pipe 


0.28 


0.16 


0.64 


ZnFP 


99.9999+ 


graphite 


1.0 kg 


18 


glass^ 


three zone 


0.18 


0.10 


0.41 


SnFF 


99.9999+ 


graphite 


1.0 kg 


18 


glass^ 


three zone 


0.12 


0.02 


0.24 


InFP 


99.9999+ 


Teflon 


1.5 kg 


19 


ss'^ 


three zone 


0.04 


0.03 


0.10 


GaTP 


99.99999 


Teflon 


0.9 kg 


13 


glass^ 


single zone 


0.02 


0.01 


0.04 


H2OTP 


99.99999 


glass"^ 


0.50 kg 


31.5 




maintenance bath 


0.003 


0.01 


0.02 


HgTP 


99.999999 


glass'^''^ 


2.3 kg 


17 


ss'^ 


alcohol bath 


0.07 


0.01 


0.14 


ArTP 


99.9999 


copper 


15 mol 


10.9 




Dewar 


0.03 


0.03 


0.08 



^ For protection, the graphite container of Ag and Al are placed inside silica- glass cells before placing in the Inconel holder. 

^ Borosilicate glass. 

^ Stainless steel is also used. 

^ ss: stainless steel. 



^ Certain commercial equipment, instruments, or materials are identi- 
fied in this paper to foster understanding. Such identification does not 
imply recommendation or endorsement by the National Institute of 
Standards and Technology, nor does it imply that the materials or 
equipment identified are necessarily the best available for the purpose. 



116 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 




Fig. 6. Water TP cell in an ice bath contained in a silvered Dewar. 
A — platinum resistance thermometer; B — heavy black felt shield 
against ambient radiation; C — ^polyethylene tube for guiding the SPRT 
into the thermometer well; D — water vapor; E — borosilicate glass 
cell; F — water from the ice bath; G — thermometer well (precision 
bore); H — ice mantle; I — air-free water; J — aluminum bushing with 
internal taper at upper end to guide the SPRT into the close-fitting 
inner bore; K — ^polyurethane sponge; L — finely divided ice and water. 

controlled close to the Ar TP temperature to temper the 
sheath of the SPRT. Figure 7 is a schematic of the 
apparatus for calibrating SPRTs at the Ar TP. The TP 
temperature realized with this apparatus agrees to 
within 0.1 mK with those Ar TPs obtained with sealed 
cells (see Refs. [23,24] and Sec. 5.1). 

In the development of fixed-point devices at 
NBS/NIST to achieve the best measurement accuracy in 
the calibration of SPRTs, attention has been given to 
having multiple phase-equilibrium surfaces to provide 
uniform surface temperatures for the SPRT. The H2O 
TP of Fig. 6 shows two equal-temperature equilibrium 



surfaces, one at the inner liquid-solid interface at the 
inner melt and the other at the outer surface of the 
mantle. Likewise, in the realization of the FP or the MP 
of metal fixed points, the operating procedure is de- 
signed to surround the SPRT in the sample container 
well by two equal-temperature equilibrium surfaces. 
Figure 8 is an idealized representation. 

The wells for the long-stem SPRT are made suffi- 
ciently deep to eliminate "stem conduction." The depth 
of the thermometer well of the container for the high- 
purity fixed-point substance is limited. To temper the 
SPRT sheath that extends above the sample container, 
the container is placed inside a long tubular "holder" 
that is inserted into a deep tube furnace or liquid bath 
operated at a temperature within 1 K of the FP or MP 
of the sample. A borosilicate glass holder is used with 
Zn FP, Sn FP and In FP graphite containers (see Fig. 9) 
and an Inconel metal holder is used with Al FP and Ag 
FP graphite containers (see Fig. 10). As an added pro- 
tection, the graphite containers of Ag and Al are com- 
pletely enclosed in silica glass before being placed in- 
side Inconel metal holders. The sheath of the SPRT is 
tempered in the thermometer guide tube, which is cen- 
trally mounted above the thermometer well of the sam- 
ple container. The guide tube is heated close to the 
furnace temperature by thermal bridges of graphite 
disks between the holder and the guide tube. In the cases 
of the holder for the Al FP and the Ag FP devices, twelve 
Inconel metal disk thermal radiation traps are mounted 
on the guide tube. Platinum disks, however, are pre- 
ferred in order to eliminate the possibility of contamina- 
tion of the thermometer. The stem conduction is consid- 
ered eliminated when readings of the SPRT at different 
depths of immersion at the bottom 3 cm to 8 cm of the 
well (depending upon the SPRT and fixed-point device) 
correspond to the effect of the hydrostatic head (see 
Refs. [1,25]). 

In the realization of the freezing point of Ag, Al, Zn, 
Sn or In for the calibration of (HT)SPRTs, the sample is 
melted overnight in the furnace held about 5 K above 
the freezing point. In the morning with a "check 
(HT)SPRT" in the thermometer well, the furnace tem- 
perature is reduced to initiate the freeze. When recales- 
cence is observed, the furnace temperature is set to 
within 1 K below the freezing-point temperature. The 
check (HT)SPRT is removed and two cold silica-glass 
rods are successively inserted into the thermometer well 
for about 5 min each to form a thin layer of solid metal 
around the thermometer well. The cold check 
(HT)SPRT is then reinserted into the cell and the equi- 
librium temperature measurements are made. The read- 
ing should agree with previous freezing-point tempera- 
tures of the fixed-point device or devices of the same 



117 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



TO SAMPLE 
RESERVOIR 



COVER PLATE 

LIQUID NITROGEN 
DEWAR PLATE 



TO HIGH VACUUM 
AND ELECTRICAL 
LEAD TERMINALS 

PORT FOR FILLING 
LIQUID NITROGEN 




SILICONE"0" RING 
.SEALS FOR SPRTS(7) 



67 cm 



LONG STEM SPRT 
WELLS (7) 

HE GAS MANIFOLD 
TO SPRT WELLS 



TEMPERATURE 
CONTROLLED SHIELD 

PASSIVE SHIELD 
SAMPLE CELL SHIELD 



SAMPLE CELL 



FFLES 

CAPSULE SPRT 
WELLS (6) 

VACUUM CAN 

SUPER INSULATED 
DEWAR 



Fig. 7. A schematic drawing of the argon triple-point apparatus for calibrating seven long-stem SPRTs and six capsule 
SPRTs. Six long-stem SPRTs surround a central SPRT well, which is sufficiently large to accommodate a holder for 
calibrating a capsule SPRT. At the bottom of the sample cell, six capsule SPRT wells are circularly arranged between the 
long-stem SPRT wells. 



metal to within 0.1 mK. As shown by Fig. 8, two equal- 
temperature equilibrium interfaces are formed by the 
procedure. Usually for a given freeze for all of the 
metals except Ag, about six test (HT)SPRTs, that are 
first preheated close to the fixed-point temperature, are 
successively inserted into the fixed-point device and 
calibrated. For Ag, usually only one HTSPRT is cali- 
brated per freeze. After measurements on the test 
(HT)SPRTs have been completed, the resistance of the 
check (HT)SPRT is read. The second reading of the 
check (HT)SPRT must agree with that of the first to 
within 0.1 mK; otherwise the calibrations are repeated. 
In the case of Sn, which supercools about 25 K, the 
Sn FP device is pulled out of the furnace to initiate the 
freeze. When recalescence is observed, the cell is rein- 



serted into the furnace that is operating within 1 K 
below the freezing point. 

In the case where temperatures are observed at melt- 
ing conditions, e.g., the TP of Ga or Hg, the metal 
sample is frozen first; then the fixed-point device is 
inserted into a deep bath that is controlled about 1 K 
above the melting point. Next, a long heater is inserted 
into the thermometer well to form an inner melt. The 
bath liquid tempers the SPRT sheath that extends above 
the fixed-point device. Figure 8 shows the two equal- 
temperature equilibrium surfaces for melting experi- 
ments. For details of freezing and melting experiments 
with metal fixed-point ceUs, see Refs. [26-28]. Table 2 
lists the total of Type A and Type B uncertainties, along 



118 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 




Melting 



Freezing 



Fig. 8. Idealized liquid/solid (L/S) equilibrium condition^ inside 
fixed-point cells used in freezing and melting experiments. In freezing 
experiments, as the solid layer on the crucible wall thickens, its L/S 
interface approaches the L/S interface of the thin solid layer on the 
thermometer well. Similarly, in melting experiments, as melting ad- 
vances, the outer L/S interface approaches the inner L/S interface. 

with the expanded uncertainties U (k = 2) in measure- 
ments with each of the fixed-point devices. Type A 
uncertainties represent many measurements of check 
(HT)SPRTs that are associated with each of the fixed- 
point cells. Type B uncertainties reflect principally the 
effect of impurities in the fixed-point samples and the 
effect of physical and thermal geometry on the 
(HT)SPRT in the fixed-point device during measure- 
ments. 



4. Thermodynamic Temperature 
Measurements at NBS/NIST 



4.1 Thermodynamic Temperature Measurements 
Utilizing Ideal Gases 

The quantity that is termed temperature is well-de- 
fined by the laws of thermodynamics; measures of tem- 
perature that are defined to be consistent with the laws 
of thermodynamics are said to be thermodynamic tem- 
peratures. The measurement of thermodynamic temper- 
ature is based on a physical system that can be created 
in the laboratory and whose temperature is related to a 
set of measurable properties. The difference between 




Fig. 9. An SPRT in an indium, tin or zinc freezing-point cell. A — 
SPRT; B — to helium gas supply and pressure gauge; C — thermome- 
ter/helium gas seal with silicone rubber; D — silicone rubber stopper; 
E — thermal insulation (Fiberfrax); F — thermometer guide tube [pre- 
cision bore tube, ground (matte finish) to uniform outside diameter]; 
G — heat shunt (graphite) in close contract with F and with H; H — 
borosilicate glass cell (holder) [precision bore tube, ground (matte 
finish) to uniform outside diameter]; I — graphite lid (cap) for the 
graphite crucible; J — graphite thermometer well; K — metal sample; 
L — graphite crucible; M — thermal insulation (Fiberfrax paper) be- 
tween the graphite crucible and the borosilicate glass holder. 

temperature on the ITS-90, denoted Tgo, and the thermo- 
dynamic temperature, denoted T, can be determined by 
placing laboratory thermometers calibrated on the 
ITS-90 in the same apparatus that is used to determine 
T. Once the difference (T — Tgo) is known for a range of 
temperatures, this information can be used to improve 
future versions of the international temperature scale. 

Early thermodynamic thermometers were based on 
the equation of state of an ideal gas, for which determi- 
nations of gas density and pressure enabled determina- 
tion of the gas temperature. Significant experimental 
contributions by NBS began with the work of Hoge and 
Brickwedde [29], who calibrated an ensemble of resis- 
tance thermometers against a gas thermometer to estab- 



119 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 




Fig. 10. Aluminum or silver freezing-point cell. A — matte -finished 
silica-glass pumping tube; B — thermal insulation (Fiberfrax); C — 
matte- finished, silica glass thermometer guide tube; D — twelve In- 
conel radiation shields; E — thirteen silica-glass spacers; F — silica- 
glass envelope with a matte -finished, silica-glass re-entrant well; G — 
graphite cap for the graphite crucible; H — graphite re-entrant well; 
I — metal sample; J — graphite crucible; K — silica-glass tape for cush- 
ioning; L — Inconel protecting tube. 

lish a scale (known as the NBS-39 Scale) for the calibra- 
tion of thermometers from 14 K to 83 K. At a later date, 
a program in gas thermometry at temperatures above 
273 K was begun at NBS, as described in a review by 
Schooley [30] of gas thermometry work at NBS/NIST 
up to 1990. This research program culminated in the 
results of Guildner and Edsinger [31] from 273 K to 
730 K and in the results of Edsinger and Schooley [32] 
from 503 K to 933 K using constant-volume gas ther- 



mometry. These results formed the basis of the ITS-90 
from 373 K to 730 K, and also served as a reference 
point near 730 K for the radiometry work that defined 
the ITS-90 at higher temperatures. Unfortunately, the 
two sets of CVGT data differ by an amount equal to 
12 mK at 500 K and rising to 30 mK at 730 K, which 
is much larger than the combined measurement uncer- 
tainty and which limits the thermodynamic accuracy of 
the ITS-90. The source of the discrepancy between the 
CVGT results has not been resolved. 

An alternative to CVGT is the acoustic thermometer, 
which again relies on a simple relationship between 
thermodynamic temperature and measurable properties 
of the gas. The property to be measured in this case is 
the speed of sound w of a monatomic gas. Early mea- 
surements at NBS relied on an acoustic interferometry 
technique to measure thermodynamic temperatures 
from 2 K to 20 K [33]. To achieve higher accuracy, the 
value of w may be determined from measurements of the 
frequencies of acoustic resonances in a gas-filled spher- 
ical shell of volume V , a technique developed by Mold- 
over and coworkers [34]. In the limit of zero gas density, 
kinetic theory and hydrodynamics give the dependence 
of M on T: 



mu 



ykT, 



(2) 



where m is the mass of one molecule, y is the specific 
heat ratio, and k is the Boltzmann constant. For 
monatomic gases 7=5/3. Measurements of the frequen- 
cies of microwave resonances within the same shell 
determine the thermal expansion of the resonator cavity. 
The equation linking the measured frequencies to T, 
neglecting small corrections, is 

T-w ["(T-w)] lV{T^)j l/a(7'w)J 









(3) 



where T^ is the triple point of water (273.16 K exactly) 
and /a and/m are the acoustic and microwave resonance 
frequencies. 

As shown in Fig. 11, recent acoustic thermometry 
results at NIST [34] have determined thermodynamic 
temperature with a standard uncertainty of 0.6 mK in 
the temperature range 217 K to 303 K. The discrepan- 
cies of the CVGT work and the recent success at mea- 
suring thermodynamic temperatures near 270 K with an 
acoustic thermometer have motivated the development 
of an acoustic thermometer for determining the thermo- 
dynamic temperature above 500 K [35]. 



120 



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ical shell is a significant possibility. Microwave 
measurements that are concurrent with the acoustic 
measurements are used to correct for creep at each 
datum point. For the acoustic measurements, novel 
capacitance transducers have been developed that 
utilize a monocrystalline silicon diaphragm and alu- 
mina insulators, enabling operation at temperatures 
up to 800 K. 



• NIST Acouslic {1 999) O NBS Gas {1 976) 

V UCL Acouslic (1994) D NML Gas (1987) 

O PRM! Gas (1989;1995) A NPL Total Rad (1988) 

NBS Acoustic (1988) 




Fig. 11. The difference between recent determinations of thermody- 
namic temperature and T90 in the range 200 K to 320 K. Citations can 
be found in Ref. [28]. 

Distinct advantages of acoustic thermometry over 
earlier CVGT work include higher precision, the ability 
to conduct experiments with continuously flowing gas 
to maintain purity, and the ability to use microwave 
resonances to characterize the volume of the resonator 
cavity in situ . The present NIST effort seeks to greatly 
expand the temperature range of precision acoustic ther- 
mometry and to benefit from the lessons learned while 
conducting the lower temperature measurements. The 
NIST acoustic thermometer, shown in Fig. 12, has the 
following features: 

A. Operation up to 800 K. Discrepancies between the 
NBS/NIST CVGT data become significant at tem- 
peratures above 500 K. Measurements at the zinc 
freezing point (692.677 K) are desirable, because 
the determined value of (T — Tgo) at the fixed-point 
temperature does not depend on the non-uniqueness 
of the SPRTs [13], which is a measure of the inter- 
polation error between fixed points on the 
ITS-90. 

B. Continuous purging of the resonator cavity. Contam- 
ination of the gas in the resonator is proportional to 
its residence time, or inversely proportional to flow 
rate. Continuous purging reduces gas residence time 
approximately two orders of magnitude relative to 
the residence time in CVGT experiments. Sensitive 
pressure control techniques are used to limit adia- 
batic temperature variations in the gas, caused by 
pressure fluctuations, to 0.5 mK or less. 

C. Direct measurement of impurities in the gas exiting 
the resonator. A gas chromatography system can 
detect impurities in the sample gas with a mole frac- 
tion sensitivity better than 0.5 X 10"^. 

D. Simultaneous microwave and acoustic measure- 
ments. At elevated temperatures, creep of the spher- 



E. Stable and inert materials. We use no elastomers, 
which have been a significant source of outgassing 
in previous acoustic thermometers. The materials 
exposed to high temperatures include stainless steel, 
copper, alumina, platinum, and gold. 

F. Well-characterized resonator temperature. Up to 
five long-stem SPRTs, calibrated on the ITS-90, 
may be used to measure the resonator shell temper- 
ature. To minimize temperature fluctuations and 
spatial variations, the pressure vessel is encased in 
three concentric aluminum shells that are actively 
temperature controlled, and the thermal couplings 
between the aluminum shells, the SPRTs, and the 
spherical resonator have been carefully modeled. 

G. A resonator cavity of approximately 3 L. Previous 
measurements with resonators of at least this volume 
agree well with theoretical predictions of the acous- 
tical losses. 

This acoustic thermometer has been fabricated and 
successfully tested up to 500 K. Work continues with a 
goal of measuring (T - Tgo) over the range 273 K to 
800 K with a standard uncertainty not exceeding 0.6 mK 
near 273 K and 3 mK at 800 K. These measurements, 
we hope, will contribute to significant improvements in 
the thermodynamic accuracy of the next international 
temperature scale. 

4.2 Thermodynamic Temperature Measurements 
Utilizing Jolinson Noise 

The random fluctuations in current and voltage in a 
normal conductor, generally known as "Johnson noise" 
[36], are a result of the thermally activated motion of the 
conduction band electrons. Consequently, as first shown 
by Nyquist [37], the mean square noise voltage <V^> 
across a resistance R ina frequency band A/ is directly 
proportional to its absolute temperature T, in the low 
frequency-high temperature limit (hf« kT), or 



{V^) = 4kRTAf, 



(4) 



where k is the Boltzmann constant. Numerous applica- 
tions of Johnson noise thermometry (JNT) utilizing the 



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Journal of Research of the National Institute of Standards and Technology 



Pressure vesse] 



SPRT tube 



SPRT tube 
(1 of 3 on 
equator) 



10 cm 



Microwave 
transducer — 
(1 of 3) 



Gas ]ine 




'CoaY 



SPRT iuW > 



Resonator 
shell 

Gas line 
(1 of 3) 



Acoustic 
transducer 
(1 of 2) 



Resonator 
support 



Fig. 12. Simplified cross section of the NIST acoustic thermometer, showing the 3 L resonator, 
the pressure vessel, and associated plumbing and electrical connections. The furnace surround- 
ing the pressure vessel is not shown. 



Nyquist relation have been developed in the last 50 years 
since the 1949 publication by Garrison and Lawson [38] 
describing the first practical instrument. The signifi- 
cance of this work was recognized early on by the NBS 
staff. In particular, Hogue [39] was the first to critically 
examine the limitations inherent in the measurement 
technique utilized by Garrison and Lawson. The sub- 
tleties of amplifier gain and noise level being dependent 
on source impedance, as described by Hogue, were sub- 
sequently taken into account in later JNT designs. 

Many of these early efforts are described in the re- 
view article by Kamper [40]. 

Kamper and Zimmermann [41], working at the NBS 
Boulder Laboratories, were also the first to apply the 
high sensitivity inherent in the Josephson effect to mea- 
suring temperatures in the range of 4 K and below. 
Soulen [42] later refined this technique into a special 



type of JNT instrument known as an "R-SQUID," 
which was used to establish thermodynamic tempera- 
ture between 520 mK and 6.5 mK. 

Despite the great technological advances during the 
last few decades, the general measurement problems of 
JNT have remained highly challenging due to the ex- 
traordinarily small signal level, which is only about 
1.26 nV/VHz for 100 II at 273 K. Until recently, the 
benchmark for accuracy in practically all JNT instru- 
ments was 0.1 %. This fact has relegated JNT as a ther- 
modynamic technique to the fringes of contact ther- 
mometry (i.e., T<1 K or r> 1000 K), where the 
generally more accurate gas-based techniques are not 
practical. At the same time, some specialized industrial 
applications of JNT have been developed [43] which 
take advantage of the primary thermometer status of 
JNTs in order to solve difficult calibration problems in 



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Journal of Research of the National Institute of Standards and Technology 



high-temperature and highly-ionizing-radiation environ- 
ments. For these applications, such as in nuclear and 
fossil fuel reactor environments, an uncertainty of 0. 1 % 
is very competitive with all other types of industrial 
contact thermometers available [e.g., platinum resis- 
tance temperature detectors (RTDs) and base metal 
thermocouples]. 

Recently, increasing amounts of technical sophistica- 
tion and digital processing techniques have been brought 
to bear on the JNT problem [44]. As a result, it is now 
possible for a JNT system to achieve relative uncertain- 
ties, using switched-input noise-correlation techniques, 
which are smaller than 0.01 % over a broad range of 
temperatures [45]. The significance of these advances, 
originating at the Forschungzentrum Julich in Germany, 
has been recognized by various national metrology labo- 
ratories in Europe as well as by the staff at NIST. A 
European collaboration between the researchers at the 
Netherlands Measurement Institute (NMi), the 
Physikalisch-Technische Bundesanstalt (PTB), and the 
Forschungzentrum Julich has recently demonstrated 
thermodynamic fixed-point determinations using a 
Julich designed JNT system with relative uncertainties 
of (5 to 7) X 10"' [45] at the Ga MP, Zn FP, Ag FP, and 
PdFR 

Starting in late 1999, NIST initiated a program in 
JNT designed to advance the state-of-the-art using the 
recent advances in digital synthesis and signal process- 
ing techniques, together with advances in the Josephson 
pulse-Quantized Voltage Source (JQVS) [46]. The goal 
of the project is to create a JNT measurement system 
capable of achieving relative uncertainties of 1 X 10"' 
in the range of temperatures between 83.8 K and 430 K. 
In addition, NIST will explore the potential for industrial 
level applications of this technology in those extreme 
and/or remote environments where the temperature must 
be accurately known over long periods of time without 
access to either fixed points or replacement of probes. 



5. Device-Based Research 



5.1 Gas-Based Cryogenic Fixed Points 

The triple points of certain chemically-pure elements 
and compounds, when realized via the sealed-cell tech- 
nique, produce compact, transportable fixed-point stan- 
dards in the range between 13.8 K and 216.6 K. These 
substances are gases at standard temperature and pres- 
sure (273.15 K, 101.325 kPa) and realizations of their 
triple points require cryogenic techniques. Sealed-cell 
techniques are well suited for the realization of four 
of the defining fixed points of the ITS-90 [1]: 
Ar (83.8058 K), O2 (54.3584 K), Ne (24.5561 K) and 
e-H2 (13.8033 K). In addition, the triple points of several 



other substances such as e-D2, N2, Kr, Xe, and CO2, 
while not defining fixed points on the ITS-90, are po- 
tentially useful for temperature scale research [47], e.g., 
the non-uniqueness of portions of the ITS-90 [13]. 
These fixed points are useful also for international scale 
comparisons [48], scale maintenance, and dissemina- 
tion. 

The inherent stability of the triple point results from 
all three phases of the sample being in thermal equi- 
librium. When a pure material of fixed amount attains 
the triple -point temperature, there are no remaining de- 
grees of freedom in which the three phases may coexist. 
Heat may be absorbed or emitted by the sample under- 
going melting or freezing under its own saturated vapor 
without a change in temperature. The latent heat of 
fusion that accompanies the first order phase transition 
provides a stable plateau in temperature, useful for cali- 
brating thermometers. 

Previous work at NBS/NIST has included realizations 
of the triple points of Ar [24], O2 [49], Xe [50], and Ne 
[51] using sealed ceUs of various designs. The funda- 
mental theory and conventional practice of sealed cells 
has recently been reviewed by Pavese [52]. The generic 
sealed cell consists of a permanently sealed pressure 
vessel with a ballast volume; a sample volume for the 
condensed portion of the sample; a thermometer well 
insert; a heat exchanger; and a heating element. In the 
NBS/NIST sealed-cell designs discussed here, the vol- 
ume of the pressure vessel is primarily ballast, ranging 
from 20 cm^ to 50 cm^, and the cells contain the pressure 
of the room temperature gas. Storage pressures need not 
exceed 12 MPa at 300 K for ceUs of this size, which hold 
samples of 0.2 mol or less. 

The thermometer well inserts are large enough to 
accommodate three capsule-type thermometers, either 
CSPRTs or RIRTs. The insert exchanges heat with the 
solid and liquid phases of the sample by confining the 
condensed sample to form an annular mantle surround- 
ing the thermometer well insert. The heat exchange 
surface is optimized between the competing require- 
ments of maximum surface area and minimum flow 
impedance in the annular sample space. In the latest 
NIST designs, this is accomplished through a double 
helical groove geometry. 

Current capabilities at NIST related to sealed cells 
include two all-metal gas handling manifold systems; a 
cryostat adapted for adiabatic measurements of melting 
plateaus using sealed cells; and a variety of cells made 
from type 316L stainless steel and oxygen-free copper. 
The gas manifold systems include one general purpose 
manifold, GM-1, suitable for any of the gases mentioned 
above except for H2 and D2. The GM-1 can fill cells in 
either gas phase or condensed phase and includes a 
high-temperature vacuum bake-out furnace for service 
up to 450 °C. The other gas manifold, GM-2, is a special 



123 



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Journal of Research of the National Institute of Standards and Technology 



system designed for H2 service using only condensed 
phase filling. The cryostat has an operating temperature 
range sufficient to realize all the triple points mentioned 
here and a sufficiently large sample space to accommo- 
date up to three sealed cells at once. The cells currently 
being used at NIST are suitable for any of the above 
gases, with the exception of D2, which requires special 
materials and considerations (see below). 

The current sealed-cell research and development ef- 
forts at NIST are focused on the production of chemi- 
cally-pure H2 samples of mass fraction 99.9999 % using 
conventional spin-exchange catalysts of alpha ferric hy- 
droxide. A related research topic at NIST concerns the 
analysis of the actual isotopic purity of prepared H2 
samples relative to the deuterium to hydrogen (D/H) 
ratio of 156 |uimol/mol, derived from Standard Mean 
Ocean Water (SMOW), as specified by the ITS-90. Rel- 
ative isotopic abundance in e-H2 is a source of uncer- 
tainty in the ITS-90 due to the high sample to sample 
variation in the D/H ratio (e.g., 40 |uimol/mol to 125 
|uimol/mol) of commercial gas bottles of high chemical 
purity H2. These variations are due to the different 
methods of synthesis employed commercially and the 
commensurate variations in the relative depletion of the 
heavier isotope with respect to an equivalent SMOW 
composition. 

NIST is a participant in an international comparison 
of sealed triple-point cells ongoing at the PTB. As of 
this writing, NIST sealed cells of Ar, O2, and Ne have 
been compared with other cells at PTB, and there are 
plans to include an e-H2 cell. This comparison was 
originally conceived as an EUROMET project, but later 
it was expanded to include some non-EU countries. 

Another active area of sealed-cell research is a collab- 
oration with the Istituto di Metrologia "G. Colonnetti" 
(IMGC) to disseminate 0.05 mol samples of D2 with 
mass fraction 99.998 %. This D2 gas was originally pre- 
pared in 1986 [53] through a special process developed 
at the U.S. Department of Energy's Mound Laboratory 
in Miamisburg, OH, which was designed to minimize 
contamination by the lighter isotope. The IMGC is 
transferring some of this gas from a storage cylinder 
into a number of sealed cells of different design for 
international dissemination, including cells to be used at 
NIST. Isotopically-pure deuterium is particularly chal- 
lenging due to the presence of HD impurities from H2 
contaminant gas in the nominal iron hydroxide catalysts 
as well as in the stainless steel cells themselves. Conse- 
quently, one is forced to use relatively weaker catalysts 
such as Gd203 which contain no water of hydration. In 
addition, special cell construction materials such as re- 
inforced oxygen-free copper or vacuum-arc re-melt 
stainless steel are necessary to avoid H2 contamination 
of the D2. The long-term viability of deuterium sealed 



cells for triple-point standards, as prepared and stored 
with these considerations in mind, has not yet been 
conclusively determined. 

5.2 (Standard) Platinum Resistance Thermometer 
[(S)PRT] 

In the investigations in 1881 by Callendar and in 1909 
at BS, PRTs wound on mica crosses were used to mea- 
sure the freezing-point temperatures of metals up to 
1100 °C [54,55]. The reproducibility was on the order 
of 0.1 °C to 0.3 °C. The stability was dependent on the 
purity of the platinum wire and how well the platinum 
wire was protected from contamination by its supports 
and surroundings. Since that time, developments in plat- 
inum resistance thermometry have resulted in many im- 
provements: higher purity of the Pt wire; smaller size of 
the Pt resistance element; supports for the Pt resistance 
coils that are nearly free of contamination and that main- 
tain the Pt resistance coil in a nearly strain-free state; 
and increased accuracy of resistance measurements and 
of representation of the thermodynamic temperatures. 

In 1932, Meyers of NBS described the design of an 
SPRT element consisting of a small helical Pt coil that 
was 5 mm in diameter and 32 mm in length and that was 
wound in a strain-free manner on a notched ruby mica 
cross [56]. Figure 13 is a photograph of the SPRT ele- 
ment. The size is comparable to that of most mercury 
thermometers. The element is mounted inside borosili- 
cate or silica-glass tubes for long-stem SPRTs or is 
inserted into Pt tubes for CSPRTs. Both SPRTs have 
been commercially available since that time. The work 
of McLaren on reducing light transmission (piping) in 
glass sheathed SPRTs [57] and on eliminating external 
illumination of SPRTs [58], and the work of Berry on 
the thermal strain [59] and oxidation effects [60] in 
SPRTs have contributed much to achieving greater ac- 
curacy with SPRTs. Platinum resistance elements of 
other coil forms have been introduced, but SPRTs of 
Meyers' design seem to give the best reproducibility 
below about 600 °C. 

Investigations have been conducted at NBS and in 
other national laboratories to extend the SPRT scale to 
the Au FP [61-63]. The electrical resistivity of insula- 
tion supports is less at high temperatures. High-temper- 
ature SPRTs of 25.5 n, 2.5 H, and 0.25 H of several 
designs have been made and tested for the effects of 
insulation leakage [64,65]. On prolonged exposure to 
high temperatures, the Pt wire became susceptible to 
mechanical and thermal shock [66]. In some cases, 
grain boundaries were visible [67]. The removal of 
strains that were introduced during the manufacture of 
the Pt wire and in winding the Pt coil requires prolonged 
heating at high temperatures. Slow cooling of the 



124 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 













1 ^mttttta 


■ 


E 


m^ 


-~fli^ 




EZ. 






CM i 

1 I I 1 1 1 1 1 1 1 1 1 


2 

1 1 I t 1 1 1 1 1 1 1 


Mil 


•3 
Mil 


Ib 


""'''■ '%- ^ 


*>- 










mi 


m 




.t_. ■ , ■'" '^=~—-^^^^^^^^^^^^^M 









Fig. 13. Large model of Meyers' thermometer coil. A — mounted on mandrel; B — ^re- 
moved from mandrel (from Ref. [50]). 



HTSPRT from high temperatures is required to avoid 
freezing-in high-temperature lattice vacancies. 

Oxygen is added to the heat exchange gas inside the 
SPRT sheath to maintain any metal impurities that 
might be present in the oxidized state. Free metals will 
alloy with Pt, especially at high temperatures. The oxy- 
gen also oxidizes the Pt, forming an oxide that has 
greater resistivity than Pt. Thus, when the SPRT coil is 
oxidized, its resistance is greater than when it is less 
oxidized. The error in the resistance ratio is small or 
negligible when the degree of oxidation is the same for 
the two resistance measurements that are required. The 
rate of oxidation seems to be the greatest in the range 
300 °C to 400 °C [68]. With SPRTs filled to about one 
third of an atmosphere of dry air as an exchange gas, the 
Pt oxide is decomposed at about 500 °C. Slow cooling of 
HTSPRTs to about 500 °C and quickly cooling to the 
ambient temperature and then to the H2O TP should 
yield an accurate resistance ratio for the high tempera- 
ture observation. See Table 2 for uncertainties (Type A) 
of measurements that can be achieved with SPRTs and 
HTSPRTs in different fixed-point cells up to the Ag FP. 

5.3 Thermocouple Thermometry 

Historically, much of the research at NBS and NIST 
in thermocouple thermometry has focused on the deter- 
mination of reference functions for a variety of thermo- 
couple types. A thermocouple reference function, giving 
thermoelectric emf as a function of temperature, serves 
two purposes: it is a standard that thermocouples are 
manufactured to match, to within a specified tolerance, 
and it is a tool for calibration of thermocouples. With an 
accurate reference function, a thermocouple may be cal- 
ibrated at only a small set of temperature values, and the 
thermoelectric emf at intermediate temperatures may be 
obtained between these values by first interpolating the 
deviation of the emf from the reference function, and 



then adding the deviation to the reference function 
value. 

Each of the reference functions for the letter-desig- 
nated thermocouple types are based in part on research 
performed at NBS. Major NBS contributions include 
(see citations in Ref [69]) 

1. establishment of the first reference functions for 
types E, K, and N thermocouples; 

2. improvement of the reference functions for types B, 
R, S, J, and T thermocouples; and 

3. determination of reference functions for all of the 
base metal thermocouple types (E, J, K, N, and T) 
from °C to temperatures as low as —270 °C. 

All of the internationally-standardized and letter-des- 
ignated thermocouple types have been adjusted to the 
ITS-90 temperature scale by NIST researchers and are 
now disseminated both in NIST publications [69] and in 
national [70] and international standards [71]. 

In addition to the work on reference functions, 
NBS/NIST researchers have made significant contribu- 
tions to the development of calibration and fabrication 
techniques for high-temperature tungsten-rhenium alloy 
thermocouples [72], and in characterization of the drift 
of thermocouple emf values at elevated temperatures 
[73]. 

Although thermocouple thermometers are exceed- 
ingly simple in construction and have been in use for 
over a century, recent work at NIST [74-76], stimulated 
by publications of McLaren and Murdock [77], has doc- 
umented the fabrication and use of thermocouples with 
uncertainties an order of magnitude better than previous 
reference standard thermocouples. Alloy thermocouples 
are limited in performance because oxidation or vapor- 
ization of one of the alloy components at high tempera- 
ture alters the thermoelectric properties. Thermocou- 
ples fabricated from pure elements, either gold vs 



125 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



platinum (Au/Pt) or platinum vs palladium (Pt/Pd), do 
not suffer from preferential oxidation or ^^porization. 
With careful annealing to place the thermoelements into 
a homogeneous and well-controlled physical state, and 
with careful measurements of the emf, expanded uncer- 
tainties (k = 2) as small as 10 mK at 960 °C are attain- 
able. 

A set of Au/Pt thermocouples, a^^ilable from NIST as 
Standard Reference Material 1749, were recently fabri- 
cated and calibrated at NIST. The calibration results, 
expressed as deviations from the NIST reference func- 
tion, are shown in Fig. 14. The variations of emf values 
between the different thermocouples do not exceed the 
equivalent of 8.5 mK, an indication both of the repro- 
ducibility of the annealed state of the thermocouples and 
of the uniformity of the commercially-available gold and 
platinum wire used in their construction. For each ther- 
mocouple, the deviation of emf from the reference func- 
tion can be accurately modeled by a quadratic function. 
The expanded uncertainty (k= 2) of this set of thermo- 
couples is the equivalent of 8 mK from °C to 962 °C, 
and then rising to 14 mK at 1000 °C. In comparison, a 
platinum-rhodium thermocouple can be calibrated to an 
expanded uncertainty not less than 0.1 K. 



> 0.0 



-0.1 



I -0.2 



-03 













o 


Ice 














r-- 


Sn 


Zn 

o 


Al 

o 










'"-■■-. o 

° ■■■* 

i ° 


-4 


{ 




o 




In 




o"'" 


-10m°c"" 





200 



400 600 

Temperature / '^C 



800 



1000 



Fig. 14. Deviation of emf values at fixed points of the SRM 1749 
Au/Pt thermocouples from the NIST reference function. Full circle: 
average of 18 thermocouples; open circle: maximum and minimum 
values. The uncertainty bars indicate ± 1 standard deviation. 

Au/Pt thermocouples are the most accurate thermo- 
couples a^^ilable, but the melting point of gold at 
1064 °C does not allow use at temperatures exceeding 
1000 °C. Pt/Pd thermocouples have uncertainties ap- 
proaching those of Au/Pt thermocouples, and have a 
maximum usage temperature of 1500 °C. A recent col- 
laboration between NIST and IMGC (Italy) led to the 
development of a reference function [76] for Pt/Pd ther- 
mocouples for the temperature range °C to 1500 °C, 
with expanded uncertainties not exceeding the equiva- 
lent of 11 mK up to 1050 °C, and rising smoothly to 
0.3 °C at 1500 °C. Figure 15 shows residuals of the data 



from a spline function that forms the basis of the refer- 
ence function. Up to 1064 °C, the data were obtained 
from measurements of the Pt/Pd thermocouples in 
fixed-point cells, and by comparison against SPRTs in 
stirred-liquid baths and against Au/Pt thermocouples in 
a copper isothermal block. From 800 °C to 1500 °C, the 
data were obtained by comparison measurements of the 
Pt/Pd thermocouples against a radiometer, which was 
calibrated on the ITS-90. 




1600 



Fig. 15. Residuals of data from a spline polynomial that forms the 
basis for the NIST/IMGC reference function for Pt/Pd thermocouples. 
Open triangle: SPRT comparison; open square: Au/Pt TC compari- 
son; open circle: IMGC radiometry; full circle: fixed points. 

Commercialization of pure element thermocouples 
has been successful, but recommended procedures still 
need to be developed and disseminated for optimal use 
of these thermocouples in standards laboratories or in 
such demanding environments as semiconductor pro- 
cessing. Future work at NIST will be in this direction. 

Another active area of thermocouple research is the 
development and application of thin-film thermocouples 
(TFTCs). As a consequence of the sub-micrometer 
thickness of TFTCs, these sensors have a very fast re- 
sponse time and do not thermally perturb the object 
being measured. Projects on TFTCs have included 
transparent TFTCs [78]; corrosion resistant TFTCs [79]; 
high-temperature metal silicide TFTCs [80]; and high- 
output intermetallic TFTCs [81]. NIST work has also 
pioneered improved methods for calibrating TFTCs 
[82], bonding of TFTCs to oxides, and calibrating ra- 
diometers [83]. The development of a thin-film/wire 
thermocouple wafer for calibrating light-pipe radiation 
thermometers is an ongoing project but it has already 
achieved smaller uncertainties (standard uncertainty of 
2.1 °C at 900 °C) than any currently-existing commer- 
cial technology [84]. 



126 



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Journal of Research of the National Institute of Standards and Technology 



6. Maintenance and Dissemination of 
Temperature Scales 

6.1 Maintenance and Dissemination of the ITS-90 
and Other Scales Below 84 K 



reference thermometers amongst themselves. The mea- 
surement system for comparison then also serves as the 
means of providing comparison calibrations for cus- 
tomers' thermometers, which has been the customary 
approach at NBS/NIST. 



6.1.1 Prior Scales 

The capability for performing calibrations of ther- 
mometers in the cryogenic range (i.e., T< 120 K) has 
been maintained at NBS/NIST since 1939 starting with 
the National Bureau of Standards Constant- Volume Gas 
Thermometer Scale of 1939 (NBS-39 Scale) [29]. 
Later, scientific and technical refinements, both within 
NBS/NIST and internationally, were carried over into 
the dissemination of the following scales in the cryo- 
genic range: the National Bureau of Standards 1955 
Scale (NBS-55) [85]; the National Bureau of Standards 
Provisional Temperature Scale 2-20 (NBS P2-20) [33]; 
IPTS-68 [86]; EPT-76 [87]; and finally the ITS-90 [88]. 
All of these scales were either laboratory thermody- 
namic scales or international practical scales that were 
generally too complicated to be realized outside of the 
national laboratory environment. These complications 
necessitated the use of reference thermometers with 
which to maintain these scales for calibration purposes. 
Such an approach then requires the use of a system of 
scale maintenance that periodically checks the refer- 
ences against known fixed points, and compares the 



6.1.2 The ITS-90 

In the case of the ITS-90, the inherent complications 
in its realization below 24.5561 K prevented any na- 
tional laboratory from completing a full realization, ac- 
cording to the definitions, before 1996. In fact, the only 
laboratory to do so even by the year 2000 is NIST [89]. 
As a consequence, between January 1990 and October 
1996 the ITS-90 below 83.8 K was disseminated from 
NIST by a "wire scale" approximation [90], usually 
referred to as 'TTS-90W" with temperatures denoted 
Tgow From October 1996 onward, temperatures T^q of 
the "as defined" ITS-90 below 83.8 K were dissemi- 
nated [91], as a result of the completed NIST realiza- 
tions of the ITS-90 from 0.65 K to 83.8058 K [14,18]. 
This scale change shifted the disseminated temperatures 
by less than 1 mK over this range. The ITS-90 sub- 
ranges at temperatures at or above 83.8058 K, as dis- 
seminated from NIST, continued according to definition 
during this time and were unchanged by this lower tem- 
perature scale shift. Figure 16 is a summary of all previ- 
ous temperature scales disseminated from NBS/NIST 
since approximately 1965 over the range 0.5 K to 90 K. 



K -2 



-10 





A 
























—IPTS-68 
♦ NBS-IPTS-68 

■ NBS-P2-20 
He VP 58/62 






ir 


V* ♦ 


f^ 


^^ 










■ / 


y 


f 


^^ 1 


'"\ 






.-■ 


■ V 


/ 






> 


^ 








■ ■ 
■ 


---''^■' 










\ 








1 ■ 


^ % 20 




30 


40 


50 


\* 70 


80 




90 


\ \' - 


\_ 












\ 


♦ 


♦ 


^ 




'~~~~- 



7-90/ K 



Fig. 16. The difference in mK between various historical temperature scales in the cryogenic range and the ITS-90 as 
realized by NIST. The IPTS-68 curve represents the version as disseminated from the National Physical Laboratory (UK) 
(NPL), a different version was disseminated from the NBS (NBS-IPTS-68). The 1 958 and 1 962 He vapor-pressure scales 
(VP 58/62) were based on a vapor pressure relation for "^He and ^He. The NBS P2-20 scale was a provisional scale based 
on acoustic gas thermometry from 2 K to 20 K. The EPT-76 was another provisional scale based on paramagnetic 
susceptibility. 

127 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



Between 1972 and 1992, an ultra-low temperature 
scale below 1 K was developed at NBS/NIST. This scale 
was derived from thermodynamic temperature determi- 
nations using a SQUID-based Johnson noise thermome- 
ter [92], ^^Co y-ray anisotropy, and paramagnetic salt 
susceptibility [93]. These thermodynamic, or nearly 
thermodynamic, measurements were used to derive a 
^He melting curve relation for the melting pressure pm 
and temperature T, over the range 0.006 K to 0.7 K [94]. 
The difference between a temperature T on this scale 
and the NIST realization of the ITS-90 at Tgo = 0.65 K is 
approximately 1 mK. Calibrations from NIST on this 
scale are no longer available. 

The current NIST calibration capabilities in the cryo- 
genic range cover most types of cryogenic resistance 
thermometers, including all types of capsule SPRTs for 
temperatures from 13.8 K and higher and RIRTs over 
temperatures between 0.65 K to 83.8 K. The calibra- 
tions in this range are performed in a recirculating ^He 
cryostat via an OFHC copper comparison block in vac- 
uum. All low temperature comparison calibrations are 
arranged according to batches with no more than two 
such batch calibrations being scheduled per year. 

Resistance thermometers made of rhodium with 
0.5 % iron, known as RIRTs, were first developed by 
Rusby [95] in 1975 and are now available commercially. 
Because of their high sensitivity and stability for 
r<24 K, NBS/NIST began using RIRTs in 1976 as 
reference thermometers for its EPT-76 over its range 
[10,87]. Stability tests on the RIRTs used at NIST were 
performed by Pfeiffer [96], who determined the differ- 
ences between the temperatures indicated by the NIST 
reference RIRTs in 1982 and in 1990. He determined 
that the two RIRTs had undergone a maximum relative 
drift of 0.15 mK over that 8 year period. After NIST 
realized the ITS-90 below 24 K in 1996, it has calibrated 
customer thermometers using reference RIRTs that have 
been calibrated in its LTRF. Measurements of RIRT 
resistances are made with a commercial ac bridge, typ- 
ically using currents of 0.2 mA and 0.283 mA for 
T> 1 K and 0.1414 mA and 0.2 mA for T< 1 K. Cali- 
brations are made at 1 K inter\^ls, and an 11th order 
polynomial series is fitted to the results. In 1999, NIST 
participated in the CCT Key Comparison 1, which is 
comparing ^^rious national laboratories' realizations of 
the ITS-90; for these comparisons the NIST realization 
was represented by RIRTs calibrated in the NIST LTRF. 

The ITS-90 in this range is maintained at NIST on a 
set of highly stable reference SPRTs and RIRTs. The 
reference RIRTs have been calibrated on the ITS-90 
using the following defined sub-ranges: the ^He vapor 
pressure scale from 0.65 K to 2.0 K; the "^He vapor 
pressure scale from 2.0 K to 5.0 K; and the ICVGT scale 
from 5.0 K to 24.5561 K. Reference SPRTs are cali- 



brated on the ITS-90 using all fixed points within the 
sub-range of 13.8033 K to 273.16 K. Since 13.8033 K 
(e-H2 TP) and 24.5561 K (Ne TP) are calibration points 
for both the SPRT sub-range as well as for the ICVGT, 
the two reference scales agree at these points to within 
the stated uncertainty for the calibration. For tempera- 
tures above 13.8033 K, NIST disseminates the SPRT 
definition of the ITS-90 using the hydrogen ^^por pres- 
sure definition for the points near 17.0 K and 20.3 K. 

This same definition is also available as an SRM in 
the form of a NIST-calibrated capsule SPRT over the 
range 13.8 K to 430 K. The SRM 1750 [97] incorpo- 
rates a calibrated capsule SPRT and an adapter probe for 
use in immersion-type fixed-point cells such as triple 
point of water cells. These SRMs are available to cus- 
tomers through the Standard Reference Materials Pro- 
gram for immediate use. This eliminates the need to 
wait for NIST cryogenic batch comparison calibrations 
to be scheduled. 

The NIST calibration uncertainties for RIRTs, as well 
as for the lowest three SPRT sub-ranges, have been re- 
vised recently according to the most recent NIST ITS-90 
realization results. These expanded uncertainties (k = 2) 
do not exceed 0.7 mK between 0.65 K and 273.16 K. A 
detailed assessment of the calibration uncertainties for 
capsule thermometers is presented in the NIST internal 
report NISTIR 6138 [98]. Table 3 gives the ranges of 
calibrations of CSPRTs, the expanded uncertainty U at 
the fixed points, and the maximum uncertainty over the 
various ranges from 13.8033 K to 505.078 K [99]. Infor- 
mation similar to that provided for CSPRTs is given for 
RIRTs and Germanium Resistance Thermometers 
(GRTs) for the ranges from 0.65 K to 84 K in Table 4 
[99]. 

6.2 Maintenance and Dissemination of the ITS-90 
and Other Scales Above 83 K, Evaluations of 
Fixed-Point Cells, and Uncertainties of 
Calibrations Over the Range of Contact 
Thermometry 

6.2.1 Prior Scales 

In the area of contact thermometry for this range of 
temperature, NBS/BS maintained the NHS by means of 
1 6 special Hg-in-glass thermometers calibrated at °C 
and 100 °C (centigrade) on the NHS. The ITS-27 and 
the ITS-48 were maintained by means of the oxygen 
boiling point, the ice point (0 °C), and the boiling points 
of H2O and S [100]. The IPTS-48(60) was maintained 
by the wire scale below °C, by the triple point of H2O, 
the boiling point of H2O, and the freezing point of Zn. 
The triple point of H2O (0.01 °C) was introduced into 
the IPTS-48(60), replacing the ice point as a means of 



128 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



Table 3. Capsule standard platinum resistance thermometer ITS-' 
denoted by U 



) calibrations. Vapor pressure is denoted by VP and the expanded uncertainty {k = 2) is 



ITS-90 Fixed Points 


e-H2 TP 


e-H2 VP 


e-H2 VP 


NeTP 


O2TP 


ArTP 


HgTP 


H2OTP 


GaMP 


InFP 


SnFP 




ITS-90 assigned 


13.8033 


17.0 


20.3 


24.5561 


54.3584 


83.8058 


234.3156 


0.01 


302.9146 


429.7485 


505.078 




temperature (K) 




























U 


U 


U 


U 


U 


U 


U 


U 


U 


U 


U 


MaxC/ 


ITS-90 subranges 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 



13.8033 K to 273.16 K 
24.5561 K to 273.16 K 
54.3584 K to 273.16 K 
83.8058 K to 273.16 K 
234.3156 K to 302.9146 K 
273.15 K to 302.9146 K 
273.15 K to 429.7485 K 
273.15 K to 505.078 K 



0.22 
0.22 



0.21 



0.22 



0.26 
0.26 



0.18 
0.18 
0.18 



0.08 
0.08 
0.08 
0.08 



0.20 
0.20 
0.20 
0.20 
0.20 



0.02 
0.02 
0.02 
0.02 
0.02 
0.02 
0.02 
0.02 



0.04 
0.04 



0.10 
0.10 



0.24 



0.62 
0.39 
0.29 
0.39 
0.20 
0.04 
0.10 
0.24 



Table 4. Cryogenic capsule resistance thermometer calibrations. The 
expanded uncertainty {k = 2), is denoted by U 



Thermometer 
type 



Temperature 
range (K) 



U 
(mK) 



RIRTs 
RIRTs 
GRTs 
CRTs 



0.65 to 24.6 


0.46 


0.65 to 84 


0.46 


0.65 to 24.6 


0.46 


0.65 to 84 


0.46 



determining °C. The IPTS-68 and the IPTS-68(75) 
were maintained by means of a wire scale below °C, 
the triple point of H2O, and the freezing points of Sn and 
Zn [101]. 

Over this range of temperature, BS/NBS/NIST of- 
fered precise and accurate calibrations of thermometers. 
Between the times of the adoption of the IPTS-68 and 
the ITS-90, NBS/NIST also provided evaluation and 
certification of materials as SRMs and provided a mea- 
surement assurance program on the IPTS-68. 

6.2.2 The ITS-90 

In this range of temperature, NIST maintains the 
ITS-90 through sets of fixed-point cells at each of the 
defining fixed points of the scale [25]. See Sees. 3.1.1.2 
and 5.1 concerning the Ar TP apparatus. 

NIST offers precise and accurate calibrations of ther- 
mometers, evaluation and certification of materials as 
SRMs, and provides a measurement assurance program. 
Also, customers' fixed-point cells are evaluated. 

6.2.2.1 Calibrations 

At NIST, long-stem and capsule SPRTs have been 
calibrated on the ITS-90 in the range 83.8058 K to 



1234.93 K since the adoption of the scale in 1990 [25]. 
Over this range of temperature, NIST has the capability 
for precise and accurate calibrations of essentially any 
type of thermometer used in contact measurements. 
These include resistance thermometers of the usual 
types (standard and industrial grade) over their custom- 
ary temperature ranges, both noble-metal and base- 
metal thermocouples, liquid-in-glass thermometers, and 
the various types of digital thermometers. The ther- 
mometers listed here are calibrated either directly 
against the ITS-90 defining fixed points or by compari- 
son with thermometers that have been calibrated against 
the ITS-90 fixed points, whichever is appropriate. 

The methods of calibration, temperature ranges of 
calibration, and the associated uncertainties for some of 
these thermometers are as follows. 

6.2.2.1.1 Resistance Thermometers 

Standard platinum resistance thermometers 

Since the scale came into effect in 1990, long-stem 
and capsule SPRTs have been calibrated on the ITS-90 
in the range 83.8058 K to 1234.93 K, as appropriate, 
taking into account the effects of hydrostatic head and 
self-heating and, for FPs and MPs, any deviation of 
the gas pressure from 101 325 Pa. Table 2, given in 
Sec. 3.1.1.2, shows that the uncertainty of calibration at 
the fixed points, using current equipment and measure- 
ment practices, is highly satisfactory. Table 5 gives the 
ranges of calibrations of (HT)SPRTs, the uncertainties 
at the fixed points, and the maximum uncertainty over 
the various ranges up to 1234.93 K (961.78 °C) [99]. 
Table 3 gives similar information over the various ranges 
from 83.8058 K to 505.078 K for CSPRTs [99]. 



129 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



Table 5. Long-stem standard platinum resistance thermometer ITS -90 calibrations. The 


expanded uncertainty {k 


= 2) is denoted by U 




ITS-90 Fixed Points 




ArTP 


HgTP 


H2OTP 


GaMP 


InFP 


SnFP 


ZnFP 


AlFP 


AgFP 




ITS-90 assigned 




-189.3442 


-38.8344 


0.01 


29.7646 


156.5985 


231.928 


419.527 


660.323 


961.78 




temperature (^C) 




























U 


U 


U 


U 


U 


U 


U 


U 


U 


Max U 


ITS-90 subranges 




(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


(mK) 


-189.3442^Cto0.01 


°C 


0.08 


0.14 


0.02 














0.27 


-38.8344 °C to 






0.14 


0.02 


0.04 












0.14 


29.7646 ^C 
























"C to 29.7646 "C 








0.02 


0.04 












0.04 


O^Cto 156.5985 T 








0.02 




0.10 










0.10 


0°C to 231.928 °C 








0.02 




0.10 


0.24 








0.24 


0°C to 419.527 °C 








0.02 






0.24 


0.41 






0.41 


"C to 660.323 "C 








0.02 






0.24 


0.41 


0.64 




0.64 


0°C to 961.78 ^C 








0.02 






0.24 


0.41 


0.64 


1.06 


1.06 



Industrial platinum resistance thermometers (IPRTs) 
and thermistors 

NIST calibrates IPRTs within the range from 77 K to 
835 K (- 196 °C to 552 °C), as desired by the customer, 
and thermistors over any part of the range from 77 K to 
435 K (- 196 °C to 162 °C). Since the resistance-tem- 
perature relationships of thermistors are essentially ex- 
ponential, the total range for any given thermistor is not 
large. The various ranges available and the uncertainties 
for those calibrations are given in Table 6 [102]. 

6.2.2.1.2 Thermocouples 

The types of thermocouples, the methods of calibra- 
tion, the ranges of calibration and the uncertainties of 
calibration that are offered at NIST are given in Table 7 
[103]. Note also that NIST has the capability to calibrate 
types of thermocouples other than those indicated in 
Table 7, e.g., Au/Pt and W/Re thermocouples. 



6.2.2.1.3 Liquid-in- Glass Thermometers 

NIST has the capability of calibrating both total im- 
mersion and partial immersion liquid-in-glass ther- 
mometers over their entire range. The information con- 
cerning these calibrations is given in Table 8 [104]. Note 
that NIST does not calibrate household-type thermome- 
ters, nor does it calibrate fever thermometers; it cali- 
brates only precision- type scientific thermometers. 



6.2.2.1.4 Digital Thermometers 

NIST has the capability of calibrating digital ther- 
mometers (quartz, resistance and thermocouple types) 
over their entire range. The information concerning 
those calibrations is the same as that given in Table 6 
[102], except the uncertainty may be limited by the 
digital display. 



Table 6. Industrial platinum resistance thermometer calibrations. The expanded uncertainty (k = 2) is denoted by U 

Comparison calibration 



Comparison bath 


LN2 


Cryostat 


Cryostat 


Cryostat 


Water 


Oil 


Salt 


Temperature range 


-196 T 


O^C 


-70 "C 


-80 X 


0.5 ^C 


95 ^C 


300 ^C 






to 


to 


to 


to 


to 


to 






-70 °C 


-80 "C 


-97 X 


95°C 


300°C 


550°C 



f/(mT) 



2.3 



2.3 



2.4 



4.8 



7.5 



Fixed-point calibration 



Fixed point 
Temperature 

f/(mT) 



ice point 
0"C 

1.8 



H2OTP 
0.01 "C 

1.4 



GaMP 
29.7646 ^C 



130 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



Table 7. 


Thermocouple thermometer calibrations. The expanded uncertainty {k 


= 2) is denoted 


by U 






Thermocouple 


Temperature 


Type of 


U 


Thermocouple 


Temperature 


Type of 


U 


type 




range (°C) 


calibration 


CO 




type 


range fC) 


calibration 


CO 


S 




to 1100 


Fixed point 


0.1 




E 


to 1000 


Comparison 


0.9 


S 




Oto 1100 


Comparison 


0.3 












S 




1100 to 1450 


Extrapolation 


1.6 




J 


to 760 


Comparison 


0.7 


R 




Oto 1100 


Fixed point 


0.1 




K 


Oto 1100 


Comparison 


1 


R 




Oto 1100 


Comparison 


0.3 












R 




1100 to 1450 


Extrapolation 


1.6 




N 


Oto 1100 


Comparison 


1 


B 




to 800 


Comparison 


0.3 




T 


to 400 


Comparison 


0.4 


B 




800 to 1100 


Comparison 


0.3 












B 




800 to 1550 


Comparison 


1.6 




All 


-196 


Comparison 


0.4 


B 




1550 to 1750 


Extrapolation 


2.4 




All 
All 


-110 to 315 
315 to 550 


Comparison 
Comparison 


0.4 
0.5 



Table 8. Liquid- in- glass thermometer calibrations. The expanded uncertainty (k = 2) is denoted by U 



Thermometer 


Thermomoter 


Temperature 


Thermometer 


u 


type 


liquid 


range (X) 


graduation 


CO 


CO 


Total immersion 


mercury 


Oto 100 


0.1 




0.02 


Total immersion 


mercury 


Oto 100 


0.2 




0.02 


Total immersion 


mercury 


100 to 200 


0.2 




0.06 


Total immersion 


mercury 


200 to 300 


0.5 




0.05 


Total immersion 


mercury 


300 to 500 


1.0 




0.16 


Total immersion 


mercury 


-35 to 550 


0.1 




0.02 


Partial immersion 


mercury 


-35 to 150 


0.1 




0.1 


Partial immersion 


mercury 


150 to 550 


0.1 




0.2 


Total immersion 


organic 


-196 too 


0.1 




0.2 


Partial immersion 


organic 


-100 too 


0.1 




0.3 



6.2.2.2 Non-Uniqueness 

The uncertainty of calibration is sufficiently small for 
the (HT)SPRTs, CSPRTs and RIRTs to determine the 
differences in indicated temperatures as given by the 
different thermometers at temperatures between the 
fixed points, i.e., the non-uniqueness of the ITS-90 [13]. 
The estimated uncertainty of the ITS-90 due to non- 
uniqueness is given to be within ± 0.5 mK between 
13.8 K and 273 K; within ± 1 mK from 273 K to 
693 K; within ± 3 mK from 693 K to 933 K; and within 
± 5 mK from 933 K to 1235 K [20]. Experiments are 
in progress at NIST to determine the non-uniqueness of 
HTSPRTs between 900 K and 1235 K. As a cautionary 
note, we point out that due to flaws in the capillary, 
there could be a small non-uniqueness between calibra- 
tion points in liquid-in-glass thermometers also. 

6.2.2.3 Evaluation of Customer Fixed-Point Cells 

In addition to calibration of thermometers, we 
evaluate fixed-point cells of Ar (83.8058 K), Hg 



(234.3156 K), water (273.16 K), Ga (302.9146 K), In 
(429.7485 K), Sn (505.078 K), Zn (692.677 K), Al 
(933.473 K) and Ag (1234.93 K). The expanded uncer- 
tainty (^ = 2) of these evaluations are at the 0.1 mK to 
1 mK level. 

6.2.2.4 Measurement Assurance Program (MAP) 

We also offer a service that evaluates the complete 
measurement system of the participant. This is done 
through a Measurement Assurance Program (MAP) in 
which we send to the participant a set of three calibrated 
thermometers, which the participant then calibrates ac- 
cording to his/her own procedures. The participant re- 
turns the thermometers to NIST along with his/her own 
results. After a re-calibration of the thermometers, the 
participant's results are analyzed in comparison with the 
NIST results. The results include details about system- 
atic errors in the participant's laboratory, as well as 
statements on the participant's other uncertainties. The 
participant is sent the results of the analysis. 



131 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



6.2.2.5 Standard Reference Materials (SRMs) 

As part of a program to disseminate the ITS-90, NIST 
provides not only a calibration service but also thermo- 
metric fixed-point cells, high-purity metals for con- 
structing fixed-point cells, and thermometers that are 
certified as SRMs for the temperature range from 
-259.3476 °C to 1768 °C. SRM high-purity metals of 
mercury, gallium, indium, tin, zinc, aluminum and sil- 
ver, with mass fractions ^ 99.9999 %, may be used to 
construct ITS-90 fixed-point cells that cover the temper- 
ature range from -38.8344 °C to 961.78 °C. SRMs of 
large freezing-point cells of tin (231.928 °C) and zinc 
(419.527 °C) are for use in calibrating SPRTs in accor- 
dance with the ITS-90. SRMs of small fixed-point cells 
are for use in calibrating small thermometers, e.g., ther- 
mistors and IPRTs, over the temperature range from 
29.7646 °C to 156.5985 °C. SRM thermometers (cap- 
sule SPRT, clinical laboratory, Pt thermoelement and 
Au/Pt thermocouple) have been calibrated in terms of 
the ITS-90 and cover the temperature range from 
-259.3476 °C to 1768 °C. 



6.2.2.5.1 SRMs of ITS-90 Fixed-Point Metals 

A single lot (20 kg to 89 kg) of high-purity metal 
(mass fraction ^ 99.9999 %) for each of the seven 
metals that define the ITS-90 from -38.8344 °C to 
961.78 °C has been certified as fixed-point-standard 
SRMs. Table 9 shows the SRM number assigned to each 
metal, the freezing-point temperature of each metal, the 
SRM unit sample size, the purity of the sample, and the 
expanded uncertainty (k = 2) associated with the freez- 
ing-point temperature of the SRM metal in fixed-point 
cells (as tested using randomly-selected samples from 
the lot). These SRMs were developed for the fabrication 
of freezing-point cells of the ITS-90 defining fixed 
points and for their use in the calibration of (HT)SPRTs 
and other thermometers requiring high-accuracy cali- 
brations. In the case of SRM 740 and 741, these SRMs 



were replaced with higher-purity, tear-drop, shot sam- 
ples designated SRM 740a and 741a, respectively. Fur- 
ther information can be found in Refs. [26,105-107]. 

6.2.2.5.2 SRMs of Large ITS-90 Fixed-Point Cells 

Large SRM fixed-point cells containing high-purity 
(mass fraction ^ 99.9999 %) Sn and Zn were developed 
for use as ITS-90 defining fixed-point cells to calibrate 
(HT)SPRTs. The cells were fabricated and certified by 
measurements in the manner described in Ref. [27]. 
They are designated as SRM 1747 (Sn freezing-point 
cell) and SRM 1748 (Zn freezing-point cell). They, to- 
gether with the triple point of water cell, are used to 
calibrate SPRTs from °C to 420 °C, used for part of 
the calibration of SPRTs from °C to 661 °C, or used 
for part of the calibration of HTSPRTs from °C to 
962 °C. 

Table 10 shows the serial number (s/n) assigned to 
each SRM freezing-point cell and the expanded uncer- 
tainties {k = 2) assigned to the freezing-point tempera- 
ture of the cells. Figure 9 shows a cutaway drawing of 
the fixed-point cells. The distance from the inside bot- 
tom of the graphite re-entrant well to the top of the 
liquid level of the metal of the cells is 20.5 cm. These 
fixed-point cells are designed to fit into most commer- 
cially-available fixed-point-cell furnaces. 

6.2.2.5.3 Small SRM Fixed-Point Cells 

A series of six small, sealed fixed-point cells were 
developed to cover the temperature range from 
29.7646 °C to 156.5985 °C for the purpose of calibrat- 
ing small thermometers and for use as check points in 
which to verify the calibration of small thermometers 
(e.g. thermistors, diode thermometers, industrial-grade 
resistance thermometers). Materials were chosen that 
have a freezing-point, melting-point, or a triple-point 
temperature, closely matching critical temperature 



Table 9. SRM fixed-point metals. The expanded uncertainty {k = 2) is denoted by U 







Fixed-point 








SRM 




temperature 


Unit 


(mass 


U 


number 


Metal 


CO 


size (g) 


fi-acdon) % 


(mK) 


743 


Hg 


-38.8344 


680 


99.999 999 


0.15 


1751 


Ga 


29.7646 


200 


99.999 995 


0.04 


1745 


In 


156.5985 


200 


99.999 99 


0.12 


741 


Sn 


231.928 


1300 


99.999 9 


1.0 


741a 


Sn 


231.928 


200 


99.999 97 


0.25 


740 


Zn 


419.527 


350 


99.999 9 


1.0 


740a 


Zn 


419.527 


200 


99.999 947 


0.7 


1744 


Al 


660.323 


200 


99.999 96 


0.7 


1746 


Ag 


961.78 


300 


99.999 974 


1.1 



132 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 

Table 10. Large SRM fixed-point cells. The expanded uncertainty (k = 2) is denoted by U 



SRM 174 






SRM 1748 






fixed-point 


Freezing-point 


U 


fixed-point 


Freezing-point 


U 


cell, s/n 


CO 


(mK) 


cell, s/n 


CO 


(mK) 


Sn95-1 


231.928 


0.36 


Zn95-1 


419.527 


1.01 


Sn 95-2 


231.928 


0.39 


Zn 95-2 


419.527 


1.12 


Sn 95-3 


231.928 


0.37 


Zn 95-3 


419.527 


0.98 


Sn 95-4 


231.928 


0.40 


Zn 95-4 


419.527 


0.94 


Sn 95-5 


231.928 


0.40 


Zn 95-5 


419.527 


1.14 



values used in clinical, biomedical, or pharmaceutical 
laboratories as reference-temperature check points. Ad- 
ditionally, these cells may be used for the calibration of 
thermometers that do not adhere to the definition of the 
ITS-90. Two of these SRM ceUs (SRM 1968 and SRM 
1971) contain high-purity metals that have defining 
fixed points on the ITS-90 and the other four are sec- 
ondary fixed points. Table 1 1 gives the SRM number, 
sample material, cell type, fixed-point temperature, and 
expanded uncertainty (k = 2) of the fixed-point temper- 
ature of the cell. 

6.2.2.5.4 SRM Thermometers 

Four different SRM thermometers were developed as 
a means to disseminate NIST-calibrated devices to cover 
the temperature range from -259.3476 °C to 1000 °C. 
Each of these four SRMs was chosen for specific areas 
of industrial interest. Table 12 gives the SRM device 
number, thermometer type, usable temperature range 



and uncertainty in the measured temperature associated 
with the SRM. 



7. Future Work in Contact Thermometry 

Work is in progress and/or planned in areas described 
below. 

There is a large discrepancy in the PV gas thermome- 
try measurements in the range 500 K to 800 K on which 
the ITS-90 is based. The acoustic thermometry work 
will be continued to determine the difference (T — Tgo) 
from 273 K to 800 K to resolve the discrepancy. See 
Sec. 4.1. 

Work will continue to develop JNT utilizing the latest 
advances in digital noise processing techniques, in con- 
junction with the advances in the Josephson pulse-quan- 
tized voltage sources, to achieve relative uncertainties of 
1 X 10"' between 83.8 K and 430 K. See Sec. 4.2. 



Table 11. Small SRM fixed-point cells. The expanded uncertainty (k = 2) is denoted by U 











Re-entrant 






SRM 


Sample 


Cell 


Fixed- point 


well i.d. 


U 


Reference 


number 


material 


type^ 


CO 


(mm) 


(mK) 


number 


1968 


gallium 


MP 


29.7646 


3.6 


0.7 


[28] 


1972 


ethylene carbonate 


TP 


36.3143 


4.5 


1.5 


[108] 


1969 


rubidium 


TP 


39.265 


5.0 


10 


[109,110] 


1973 


n-docosane 


TP 


43.879 


4.5 


2.5 


[111] 


1970 


succinonitrile 


TP 


58.0642 


4.5 


1.5 


[112-114] 


1971 


indium 


FP 


156.5985 


4.4 


2 


[115] 


^MP: melting point; FP: freezing point; and TP: triple point. 










Table 12. 


SRM thermometers. The expanded uncertainty (k = 


= 2) is denoted by U 








SRM 


Sample 




Temperature range 




Max t/ 


Reference 


number 


material 




CO 






number 


934 


Hg-in-glass thermometer 
for clinical laboratory 




-0.20 to 0.20 
and 24 to 34 




0.03 K 


[116] 


1967 


Pt thermoelement (Pt-67) 




-197tol768 




2 |jlV 


[117] 


1749 


Au/Pt thermocouple 




to 1000 




14 mK 


[118] 


1750 


Capsule SPRT 




-259.3467 to 156.5985 




0.7 mK 


[119] 



133 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



Thermocouple thermometry is relatively simple to 
perform. Significant improvement in accuracy has been 
realized recently with pure metal thermocouples, i.e., 
Au/Pt and Pt/Pd thermocouples, which do not suffer 
alloy composition variation along the wire or preferen- 
tial oxidation or vaporization. The use of Au/Pt and 
Pt/Pd thermocouples has been accepted but procedures 
for optimal application of them still must be developed. 
See Sec. 5.3. 

An area of work in progress is the development and 
application of thin-film (< 1 fxm) thermocouples which 
have fast response and do not perturb the object being 
measured. A number of thermocouple materials, includ- 
ing intermetallics, have been investigated. The applica- 
tion of the thin- film thermocouples for calibrating other 
thermometers, e.g., light-pipe radiation thermometers, 
is an on-going activity. See Sec. 5.3. 

Work is in progress on developing various cryogenic- 
gas fixed points (triple points) in sealed cells for testing 
the non-uniqueness of the ITS-90 and for the interna- 
tional comparison of the temperature scale through ex- 
change of sealed cells. One of the active areas is the 
international comparison of the triple point of D2, with 
the D2 sample as free of HD and H2 as possible. See Sec. 
5.1. 

The ITS-90 has been realized. The capability below 
83.8 K is being closely monitored to improve the real- 
ization and to be able to calibrate NIST standard refer- 
ence thermometers for calibrating other thermometers 
when the necessity arises. 

As part of an effort to determine the non-uniqueness 
of the ITS-90, HTSPRTs are being compared in the 
range 600 °C to 970 °C and will be continued in lower 
ranges. See Sec. 6.2.2.2. 

The work to improve and maintain the calibration of 
(HT)SPRTs, directly against fixed points in the range 
83 K to 1235 K, and of CSPRTs and RIRTs, by com- 
parison techniques in the range 0.65 to 83.4 K, for the 
nation's thermometry community is an on-going activ- 
ity. This work also supports the active liquid-in-glass, 
IPRT, thermistor, digital thermometer, and low-temper- 



ature thermocouple calibration programs by providing 
calibrated SPRTs for use as the reference thermometer. 
The calibration of thermocouples from °C to the 
Au FP is continuing. 

Semi-annual precision thermometry seminars are 
also an on-going effort of the NIST Thermometry 
Group. 



Part II. Non-Contact (Radiation) 
Thermometry 



8. Introduction 

This part discusses the non-contact method of deter- 
mining temperature from measurements of the radiant 
flux; this technique is often termed radiation thermome- 
try. The quantity realized can be either the thermody- 
namic temperature or a value on the ITS-90, depending 
on the measurement techniques (see Sec. 1 in Part I for 
more on thermodynamic temperature). To realize ther- 
modynamic temperature from measurements of radiant 
flux, the radiation thermometer must be a primary 
device, that is, the equation of state does not depend on 
unknown, temperature-dependent parameters. Exam- 
ples of both of these methods are given in Sees. 9 and 
10. 

There are several quantities in radiometry that will be 
mentioned below, so here we provide a brief introduc- 
tion; a summary is given in Table 13. Radiant flux or 
power is sensed by a detector that operates as a trans- 
ducer using various physical mechanisms. The geomet- 
ric extent of the radiant flux is defined by some optical 
system that includes the source and the detector and 
often includes defining apertures and other optical ele- 
ments. Irradiance is the radiant flux incident on a sur- 
face divided by the area of that surface (i.e., radiant flux 
per area), where the surface can be real (e.g. the aper- 
ture in front of the detector), or arbitrary (e.g., as a 
function of distance from a point source). Radiance is 



Table 13. A brief summary of radiometric quantities as they apply to non-contact thermometry 



Quantity 



Quantity 
symbol 



Typical description 



SI unit 



Power, radiant flux 


</> 


Irradiance 


E 


Radiance 


L 


Exitance 


M 


Radiance temperature 


Tr 



Collected by an optical radiation detector 

Radiant flux per area at the detector 

Radiant flux in a defined beam and a given 

direction per area at the source 

Radiant flux per source area emitted by a 

source into the hemisphere 

Temperature derived from radiant flux with an 

emissivity of unity for the source 



W 

Wm"" 
W m"' sr" 

Wm"' 

K 



134 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



the radiant flux in a beam per area and solid angle of 
that beam. It is often the case that the area in question 
refers to the source of radiant flux. Exitance is the 
radiant flux emitted into the entire hemisphere above the 
radiating surface. All of these quantities exhibit spectral 
dependence, so an additional qualifier is necessary: 
"spectral" refers to the amount of radiation per wave- 
length. 

We begin with a brief historical review, then discuss 
the calibration services and current related research in 
the Optical Technology Division at NIST, and conclude 
with a look to the future. 



9. Historical Developments 

Accurate determinations of material temperatures us- 
ing non-contact, optical methods have long played criti- 
cal roles in history. For example, some of the very early 
practitioners of optical pyrometry were the earliest 
metal workers, who visually determined the correct 
temperature at which to begin forming or tempering 
their metal implements. In general, the radiative method 
of temperature determination is utilized when the tem- 
peratures are too high for the materials used with other 
types of thermometers or the object is remote or other- 
wise inaccessible (in motion, for example). The relation- 
ship between the temperature and the radiative proper- 
ties of the material were not fully understood until 
Planck published his critical papers in 1900 and 1901 
[120]. The importance of Planck's contributions is well 
known. The formation of NIST in 1901 (as the National 
Bureau of Standards) coincided with the beginnings of 
quantum physics that were initiated by Planck's discov- 
eries. 

Before discussion of the historical development of 
non-contact thermometry at NIST, the relationship be- 
tween radiance temperature and temperature, which de- 



pends on the spectral emissivity of the source, ^(A), 
must be explained. Spectrally-selective pyrometers or 
optical radiometers measure some spectral portion of 
the radiance called spectral radiance L(X) which is de- 
scribed by Planck's law. 



L(A) = 



ClL 



«(A) 



M'A'exp[c2/(MA7')]-l 



(5) 



where Cil is the first radiation constant for spectral radi- 
ance, C2 is the second radiation constant, ^(A) is the 
index of refraction of air, and T the thermodynamic 
temperature of the source. If the spectral emissivity of 
the source, ^(A), is set to one, then the temperature 
derived from Eq. (5) becomes the radiance temperature, 
Tr, of the source. Thus, it is critical to have knowledge 
of the spectral emissivity of the material before the 
thermodynamic temperature can be inferred from the 
measurement of radiance temperatures. 

The radiant power per area emitted by a source at all 
wavelengths into the hemisphere is the total exitance M. 
For an ideal blackbody (^=1) at temperature T, the 
exitance is derived from integration of Eq. (5): 



M=n^o-T\ 



(6) 



where a is the Stefan-Boltzmann constant, and Eq. (6) 
is known as the Stefan-Boltzmann law. Spectrally-flat 
radiometers, with uniform response over a broad range 
of wavelengths, are used to measure M. In Eq. (6), the 
spectral dependence of the index of refraction was ne- 
glected, n(X)^n; for air, it is often sufficient to approx- 
imate further, with n^ ~ 1. If the emissivity is not unity, 
but is independent of wavelength, then the exitance of 
this graybody follows Eq. (6) but is reduced in propor- 
tion to the emissivity s. The values for Cil, C2, and cr are 
given in Table 14. 



Table 14. Values for the constants encountered in radiometry, the standard uncertainties, and the relationship to basic fundamental constants (here 
k is the Boltzmann constant, ^ = h/(27T), where h is the Planck constant, and c is the speed of light in vacuum; ci = lirhc^ is the first radiation 
constant) 



Quantity 



Symbol 



Expression 



Value 



SI unit 



Rel. Stand. 
Uncert. 



Stefan- 
Boltzmann 
constant 

First radiation 
constant for 
radiance 

Second 

radiation 

constant 



ClL 



7T K 

60 ^'c^ 


5.670 400 X 10"' 


W m"' K"^ 


7.0 X 10 




1.191 042 722 X 10"^' 


W m' sr"' 


7.8 X 10 


he 

k 


0.014 387 752 


m-K 


1.7 X 10 



135 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



In either the spectral or total mode of measurement, 
the ideal blackbody functions as a primary standard 
because the equations of state are known in terms of the 
underlying physics, and there are no parameters that 
depend on temperature. Practically speaking, the inac- 
curacy of the measurements is determined by the degree 
to which the blackbody source and the radiometer per- 
form in ways that can be modeled. Examples of in- 
evitable effects include temperature gradients in the 
blackbody, instability of the reflectance of the cavity 
walls, diffraction and scatter of the radiant flux, and the 
spectral, temporal, spatial response functions of the ra- 
diometer. 

W. W. Coblentz [121] and G. K. Burgess [122] per- 
formed the earliest work at NIST on blackbody radiation 
and the accurate measurement of radiance temperature. 
Coblentz was especially prolific, working to measure 
the Stefan-Boltzmann constant [123] using an early pre- 
cursor for absolute detectors [121]. Later, this work was 
directed to spectroradiometry, with accurate determina- 
tions of radiance temperatures required for calibration 
of carbon-filament lamps [124]. The transfer standards 
were used, just as at present, to calibrate spectrora- 
diometers to measure spectral irradiance [125]. Years 
later, D. C. Ginnings and M. L. Reilly developed a NIST 
cryogenic electrical substitution radiometer with the 
goal of determining the freezing-point temperature of 
gold from the ratio of the exitance to that from a black- 
body at the triple point of water [126]. Characterization 
of the system revealed unacceptably large systematic 
effects from diffraction and scatter but no steps were 
taken at that time to rebuild the apparatus (but see be- 
low). 

From Eq. (6), it is clear that absolute determination of 
the exitance from a blackbody at a known temperature 
provides a measure of the Stefan-Boltzmann constant. T. 
J. Quinn and J. E. Martin, at the National Physical 
Laboratory in the United Kingdom, used a blackbody at 
the triple point of water and a cryogenic electrical sub- 
stitution radiometer to determine a with a relative stan- 
dard uncertainty of 0.0134 % [127]. This radiometri- 
cally-measured value agreed to within the combined 
uncertainties with the value derived from fundamental 
constants (Ref. [128] and Table 14), but the uncertainty 
in this measured value is about 1 9 times larger than that 
derived from the fundamental constants. A significant 
outcome of performing these and other difficult temper- 
ature experiments (see Ref. [ 129] and references 
therein) was refinements in the field of cryogenic elec- 
trical substitution radiometry. Often termed absolute 
cryogenic radiometers (ACRs), these devices now func- 
tion as the primary standard, by way of stabilized laser 
radiation, for spectral flux calibration of detector stan- 
dards (see for example Ref. [130]). 



As mentioned previously, human vision served as the 
first radiometer for radiometric temperature determina- 
tions. At the turn of the 20th century, it became possible 
to quantify the results with the invention of the disap- 
pearing-filament pyrometer [131]. This device has an 
incandescent filament that is viewed through a red filter 
in conjunction with the material under test. The pyrom- 
eters are calibrated using a blackbody by adjusting the 
electrical current in the filament until the human ob- 
server cannot distinguish it from the blackbody radia- 
tion. The resulting current-to-temperature relationship is 
critically dependent on the training and experience of 
the operator. Disappearing-filament pyrometers are still 
in use today; they are calibrated at NIST from 800 °C to 
4200 °C. 

In the middle of the 20th century, researchers began 
to utilize photomultiplier tubes (PMTs) in optical py- 
rometers. These photoelectric pyrometers, which gener- 
ally include spectrally-selective components to limit the 
wavelength range to a few nanometers, are calibrated 
using a fixed-point blackbody at a known temperature 
(see below). The temperature of the test blackbody is a 
function of the ratio of the spectral radiance of the test 
to the fixed-point blackbody. Above the freezing point 
of silver, 961.78 °C, the ITS-90 defines temperature in 
terms of these spectral radiance ratios, where the refer- 
ence blackbody can be that at the freezing-point temper- 
ature of silver, gold, or copper [1]. The first NIST pho- 
toelectric pyrometer, described by Lee [132], has 
features common with the device used today [133]. 

A fixed-point blackbody source operates at the 
unique temperature of the equilibrium liquid to solid 
phase transition in pure metals. The blackbody cavity is 
surrounded by the metal ingot, which in turn is con- 
tained in a mechanical structure (the crucible) that is 
sealed to the front portion of the blackbody cavity. The 
assembled crucible, shown in Fig. 17, is operated in a 
resistively-heated furnace. For indium, tin, aluminum, 
silver, gold, or copper reference blackbodies, the cavity 
and the crucible are machined from high-purity, high- 
density graphite. Lee has described the early NIST im- 
plementation [134]; the current implementation includes 
an alkali-metal heat-pipe furnace liner to limit the tem- 
perature gradients along the crucible [135]. The metal is 
kept pure during assembly using a vacuum processing 
technique [136]. 

Because the fixed-point blackbody sources are te- 
dious to operate and are not easily portable, radiance 
temperature lamps are used as secondary standards. The 
vacuum tungsten-strip lamp of the Quinn-Lee design 
[137] was developed at NIST and used in the interna- 
tional temperature comparison of the IPTS-68 [138]. 
The vacuum strip lamps were stable, drifting less 
than 0.1 K per 1000 h of operation, but they cannot be 



136 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



Gi^Keec 



\\, 



^^^^^^^ ffi > j iN j i ti * ' Cavliy 




Mct33 mgoL 



Fig. 17. Schematic of a crucible of a freezing-point blackbody. 

Operated at high currents or the tungsten will evaporate. 
At 655 nm the maximum radiance temperature is about 
1700 °C. Gas-filled tungsten-strip lamps can be oper- 
ated at higher currents than the vacuum strip lamps; at 
655 nm, the maximum radiance temperature is about 
2300 °C. 



10. Current Work at NIST in Non-Contact 
Thermometry 

10.1 Calibration Capabilities 

The effort in radiance temperature at NIST can be 
divided into routine calibrations and radiance tempera- 
ture research. Both detectors and sources are calibrated 
over the temperature range from 60 K to 3000 K. Three 
main facilities exist for calibrations: The Low-Back- 
ground InfraRed (LBIR) facility, the Low-Level Tem- 
perature (LLT) facility, and the NIST Radiation Temper- 
ature Calibration Laboratory (RTCL). The LBIR 
calibration facility is capable of measuring blackbody 
temperatures from 60 K to about 1000 K in a low back- 
ground 20 K environment [139]. The LLT facility, with 
variable-temperature water- and oil-bath blackbodies 
and cesium and sodium heat-pipe blackbodies, has been 
developed for calibration of pyrometers and sources in 
the temperature range from 288 Kto 1223 K [140-142]. 
The LLT facility also has various fixed-point black- 
bodies ranging from gallium (302.9146 K) to gold 
(1337.33 K). The third facility is the NIST RTCL for the 
dissemination of the ITS-90 [133]. Each of these facili- 
ties establishes traceability to SI quantities in different 
ways. The LBIR facility relates the quantity of radiant 
flux to electrical standards; the LLT and RTCL facilities 
relate the quantity of radiant flux to the ITS-90. 



In the LBIR facility, the blackbody radiance tempera- 
tures are determined using an ACR at 2 K with a preci- 
sion aperture set at a known distance from a blackbody 
that has a second precision aperture. The measurements 
are done in a vacuum chamber with a pressure of 
1.3 X 10~^ Pa or less when the chamber is at 270 K. 
During operation, cryoshrouds that form the interior 
walls are cooled to 20 K using He gas. The radiance 
temperature is found from Eq. (6), the Stefan-Boltz- 
mann law, and a geometric factor that accounts for the 
less-than-hemispherical collection geometry: 



r ( ^ V 



(7) 



where (j) is the optical power (radiant flux), 
Fi = - iz — (z^ — ^x^y^y^ j is the configuration factor 

with X = — , y = — , and z = 1 + 1 + x^ y^, s is the dis- 

tance between the apertures, and ri and r2 are the radii 
of the blackbody and the ACR apertures, respectively. 
Since the blackbodies are characterized in the range of 
temperatures and wavelengths where diffraction effects 
become significant, the optical power in Eq. (7) is cor- 
rected for diffraction. Accurate diffraction corrections 
were determined at NIST by E. L. Shirley [143]; the 
computer code is available on diskette for the general 
use of the radiometry community [144]. Because the 
uncertainties in the diffraction corrections and the un- 
certainties caused by geometric alignment depend on 
the aperture sizes, the final radiance temperature uncer- 
tainty assignments depend on aperture size (Table 15). 
For the smallest apertures at the lowest temperatures, 
the large uncertainties in the measured signals preclude 
measurements of radiance temperatures. 

In the LLT facility, the temperatures of the water- and 
the oil-bath blackbodies are determined using a plat- 
inum resistance thermometer or a thermistor that is im- 
mersed in the stirred bath [140, 141]. The temperatures 
of the cesium and the sodium heat-pipe blackbodies are 
determined using Au/Pt thermocouples that are inserted 
into wells along the outside of the cavities [142]. The 
emissivity of the blackbodies under isothermal condi- 
tions is calculated using a Monte Carlo analysis [145] 
and verified experimentally by measurement with dif- 
ferent source apertures. Preliminary comparisons be- 
tween the Ag and the Al fixed-point blackbodies and the 
cesium heat-pipe blackbody agree to within 60 mK 
[146]. The capabilities of the LLT facility are summa- 
rized in Table 16 and the uncertainty budget for the 
pressure-controlled heatpipe-blackbody source is given 
in Table 17. 



137 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



Table 15. The expanded uncertainties {k = 2),m kelvin, for radiance 
temperature determinations of the blackbodies in the LBIR facility. 
As the aperture in front of the blackbody becomes smaller, the relative 
uncertainties become larger due to greater uncertainties in the diffrac- 
tion correction and geometric alignment uncertainties 



Temperature, T 
(K) 



0.204 



Aperture diameter, 2ri (mm) 



0.284 0.405 3.214 6.407 



60 








0.16 


0.11 


70 








0.16 


0.12 


80 








0.16 


0.12 


100 








0.17 


0.13 


125 








0.17 


0.15 


145 








0.18 


0.16 


170 






1.4 


0.19 


0.18 


195 




2.11 


1.38 


0.21 


0.2 


225 


2.53 


2.1 


1.36 


0.22 


0.22 


250 


2.56 


2.1 


1.35 


0.24 


0.25 


275 


2.6 


2.12 


1.35 


0.26 


0.27 


315 


2.69 


2.16 


1.36 


0.29 


0.3 


400 


2.95 


2.31 


1.43 


0.35 


0.38 



Table 16. The types of variable -temperature blackbodies available in 
the LLT facility 



Blackbody 



Temperature range 

CO 



Type of 
thermometer 



Water bath 


15 to 90 


PRT 


Oil bath 


90 to 200 


PRT 


Cs heatpipe 


350 to 700 


Au/Pt TC 


Na heatpipe 


600 to 950 


Au/Pt TC 



In the NIST RTCL, illustrated in Fig. 1 8, the radiance 
temperature scale above 800 °C is disseminated using 
the NIST photoelectric pyrometer (PEP) with a gold- 
point blackbody, a vacuum strip lamp of the Quinn-Lee 
design, and a variable temperature blackbody (VTBB). 
The temperature of the Quinn-Lee strip lamp is deter- 
mined by spectral radiance ratios to the gold-point 
blackbody, and the temperature of the VTBB is deter- 
mined by ratios of its spectral radiance to that of the 
Quinn-Lee lamp. The spectral response of the PEP is 
centered at 655.3 nm with a full-width-half-maximum 
of 1.1 nm using two interference filters. The field stop 
in the PEP limits the target area to a rectangle 0.6 mm 
by 0.8 mm. 



GPL 

WSL 

TL 

VTBB 

TL 

GPBB 
TL 



© 
© 



© 



© 



OL 



't 



FS AS DL 




CPD 



CL IF 



LEGEND 



t 



AS 


Aperture stop 


CL 


Collimating lens 


CPD 


Cooled photomultiplier detector 


DL 


Diverging lens 


FS 


0.6 mm X 0.8 mm rect. field stop 


GPBB 


Gold-point blackbody 


GPL 


Gold-point lamp 


IF 


Interference filters 


OL 


Objective lens 


TL 


Test lamp 


VTBB 


Variable temperature blackbody 


WSL 


Working standard lamp 



Fig. 18. Schematic of the NIST Radiation Temperature Calibration 
Laboratory, with the various sources mounted on a translation table. 
The PEP consists of the objective lens, field stop, collimating lens, 
aperture stop, interference filter, diverging lens, and cooled photo- 
multiplier detector. 



Table 17. Expanded uncertainty (k = 2) in radiance temperature for the Cs or Na pressure- 
controlled heatpipe blackbody source at 800 ^C. The uncertainty is given for two different 
target diameters of the radiation thermometer 



Factor 



Radiation thermometer target diameter (mm) 
1.0 10 



BB Emissivity 
Au/Pt thermocouple 
Digital voltmeter & ice bath 
Pressure stability 
RT noise 

BB radiance uniformity 
Total 



0.02 


0.02 


0.01 


0.01 


0.00 


0.00 


0.02 


0.02 


0.02 


0.02 


0.06 


0.57 


0.07 


0.57 



138 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



The simplified measurement equation for the calibra- 
tion of the Quinn-Lee lamp, which has a radiance tem- 
perature of about 1530 K, is 



U(W _ 1 



expl C2/(nArAu) -1 



expl C2/(nArR) 1 — 1 



(8) 



where Tau is the temperature of the gold freezing-point, 
Tr is the radiance temperature of the Quinn-Lee lamp, 
and A is the mean effective wavelength of the PER 
Evaluation of the index of refraction n as a function of 
wavelength is given as Eq. (7) in Ref. [133]; that refer- 
ence also gives sau = 0.9999. A similar, but not identi- 
cal, version of Eq. (8) applies for the calibration of the 
VTBB using the Quinn-Lee lamp. 

For temperatures above the freezing point of silver at 
961.78 °C, the NIST procedure is an implementation of 
ITS-90, with a subtle difference regarding the determi- 
nation of the value for the temperature at which gold 
freezes. The ITS-90 value for the freezing point of gold 
(or silver or copper) was achieved by a critical compila- 
tion of experimental values, but no uncertainties were 
included. It is possible, by direct measurement of the 
radiant flux, to determine the freezing-point tempera- 
ture radiometrically, without reference to gas ther- 
mometry. Using a room-temperature electrical substitu- 
tion radiometer and a calibrated p-n silicon photodiode, 
the freezing temperature of gold was determined to be 
1337.33 K with an expanded uncertainty (^=2) of 
0.23 K [135]. This value is in agreement with the ITS-90 
assignment, and the uncertainty is included in the uncer- 



tainty budget for the NIST radiance temperature and 
spectroradiometric scales. The result is termed the 1990 
NIST Radiance Temperature Scale [147]. 

The standard measurements in the RTCL are calibra- 
tions of tungsten-ribbon filament lamps, disappearing- 
filament optical pyrometers, and portable radiation ther- 
mometers. Other tests can be done as a special request 
depending on the capabilities of the RTCL. Tungsten- 
ribbon filament lamps are calibrated at 655.3 nm by 
comparison to the Quinn-Lee lamp. The lamps to be 
measured are either vacuum tungsten- strip lamps, 
which can be calibrated from 800 °C to 1700 °C, or 
argon-filled lamps, which can be calibrated from 800 °C 
to 2300 °C. Typical uncertainties for the latter are given 
in Table 18 [133]. The disappearing-filament optical 
pyrometers and the portable radiation thermometers are 
calibrated using the VTBB, which has an experimen- 
tally-determined emissivity of 0.9995. Typical uncer- 
tainties for a radiation thermometer are given in Table 
19. 

10.2 Research in the Field of Radiance 
Temperature 

NIST uses a variable temperature blackbody as a stan- 
dard of spectral radiance to provide its spectral radiance 
and irradiance calibration services [148, 149]; the black- 
body temperature is also determined using a gold-point 
blackbody and spectral radiance ratios. Therefore, the 
uncertainty in the temperature determinations affects 
the uncertainty of the spectroradiometric quantities, and 
a critical objective at NIST is to reduce both of these 
uncertainties. The standard lamps of spectral irradiance. 



Table 18. Expanded uncertainty (k = 2) in radiance temperature for an argon-filled ribbon filament lamp in the RTCL. The uncertainty, in °C, 
given for different radiance temperatures (also in °C) 



Source of Uncertainty 



Type 



800 



Radiance temperature (°C) 
1100 1500 1900 



2300 



Calibration of the reference radiance 
temperature lamp relative to the 1990 
NIST Radiance Temperature Scale 

Test lamp temperature determination 

Lamp current measurement 

Mean effective wavelength measurement for 
the NIST PEP 

Test lamp alignment 

1990 NIST Radiance Temperature Scale 
relative to thermodynamic temperature 
scale 

Overall uncertainty of test lamp calibration 
with respect to SI units 



0.12 



0.6 



0.19 



0.4 



0.32 



0.7 



0.48 



1.0 



0.67 



A 


0.42 


0.17 


0.29 


0.43 


0.60 


B 


0.29 


0.19 


0.15 


0.14 


0.14 


B 


0.10 


0.04 


0.09 


0.28 


0.54 


B 


0.09 


0.14 


0.24 


0.36 


0.51 


B 


0.15 


0.24 


0.40 


0.61 


0.85 



1.5 



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Journal of Research of the National Institute of Standards and Technology 

Table 19. Expanded uncertainty {k = 2) in radiance temperature for a typical radiation thermometer. The uncertainty, in °C, is given for different 
radiance temperatures (also in °C) 



Source of Uncertainty 



Type 



800 



Radiance temperature (°C) 
1100 1500 1900 



2300 



Calibration of the variable temperature 

blackbody relative to the 1990 NIST 

Radiance Temperature Scale 
Mean effective wavelength measurement for 

the NIST PEP 
Blackbody uniformity 

Test thermometer temperature determination 
1990 NIST Radiance Temperature Scale 

relative to thermodynamic temperature scale 

Overall uncertainty of test radiometer 
calibration with respect to SI units 



0.2 



0.1 



0.7 



0.3 



0.0 



0.8 



0.4 



0.1 



0.9 



0.6 



0.3 



1.1 



0.8 



0.5 



0.2 


0.2 


0.2 


0.2 


0.2 


0.6 


0.6 


0.6 


0.6 


0.6 


0.1 


0.2 


0.4 


0.6 


0.9 



1.5 



1000 W FEL lamps^, have spectral shapes similar to a 
3000 K blackbody source in the ultraviolet and visible 
wavelength regions. Due to the spectral mismatch, five 
intermediate steps are necessary to calibrate the spectral 
irradiance standards with concomitant increase in the 
total uncertainties at each step. A method to reduce the 
uncertainties of the spectral irradiance is to directly 
determine the radiance temperature of a large-area, 
high-temperature blackbody (HTBB) at 3000 K and 
then use the output of the blackbody to calibrate the 
1000 W FEL lamps directly. 

The radiance temperature of a HTBB can be deter- 
mined in two different ways. The first, called the ITS-90 
method, is to follow ITS-90 and use spectral radiance 
ratios of the HTBB and the gold-point blackbody (or 
silver or copper). The radiance temperature uncertainty 
u for the HTBB is related to the radiance temperature 
uncertainty of the fixed-point blackbody by 






(9) 



where TV? is the temperature of the fixed-point black- 
body and Tbb is the temperature of the HTBB and their 
standard uncertainties are u{T^^) and u{Tb^ respec- 
tively. Therefore the limiting uncertainty for the ITS-90 
method is u{T^^) - 5u(T^^) for Tbb = 3000 K. 

A second, more direct approach, is to determine the 
radiance temperatures of the blackbody with transfer 
radiometers that are calibrated using ACRs. In the LBIR 
calibration facility, this approach is direct, in that there 
are no transfer radiometers and both the source and 
detector are in the same vacuum chamber. For the 



HTBBs in the LLT and RTCL facilities, transfer ra- 
diometers that function as secondary standards are used. 
In principle, the HTBB could be calibrated with an 
independent (and dedicated) ACR, but NIST has 
adopted the approach of using the High Accuracy Cryo- 
genic Radiometer (HACR) [150] as a central device and 
silicon photodiode trap detectors or other detectors to 
disseminate the spectral flux responsivity scale. The 
candela (SI base unit for luminous intensity) and detec- 
tor spectral flux responsivity are realized this way 
[130], and it is the goal of the radiation temperature 
research to do the same for the radiance temperature, 
spectral radiance, and spectral irradiance scales [151- 
153]. 

One type of transfer radiometer is a filter radiometer 
for spectral irradiance that is calibrated against the 
HACR; a schematic is given in Fig. 19. If the filter 
radiometer is calibrated for spectral power responsivity, 
/?e(A), in units of AAV, then the radiance temperature 
and measured signal are related through the measure- 
ment equation 






:(A)£(A)dA 



Girrf F, /?E(A)TrL(A,rR)dA, 



(10) 



where E{k) is the spectral irradiance at the radiometer 
aperture, G is the gain in V/A, and Te is the signal from 
the radiometer in volts. Since all the geometric factors 
in Fi can be measured, Eq. (10) can be solved iteratively 
for the radiance temperature that produces the measured 
signal. 



^ FEL is a designation, not an acronym. 



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Journal of Research of the National Institute of Standards and Technology 



Filter 



Silicon photcdicde 




Aperture 



TE cooler 



Heat absorbing glass 



Fig. 19. A schematic of the filter radiometers for measurements of spectral irradi- 
ance. The main components are the precision aperture, which is about 4 mm in 
diameter, various filters to limit the spectral bandpass, and a silicon photodiode. 
The bandpass filters are either interference or absorbing glass design. The electron- 
ics are not shown. 



The agreement between the radiance temperatures of 
a HTBB determined using the ITS-90 method and the 
absolute radiometric method has recently been estab- 
lished. The radiance temperature of the VTBB in the 
RTCL was determined with the PEP using radiance 
ratios against the gold fixed-point blackbody. At the 
same time, six filter radiometers, with center wave- 
lengths between 350 nm and 950 nm, measured the flux 
from the VTBB. The agreement between the two deter- 
minations in the temperature range between 2200 K and 
2800 K was found to be closer than 0.5 K [151]; see Fig. 
20. 

The level of agreement between the current NIST 
spectral radiance scale and the spectral radiance as- 
signed to a large area HTBB was also established using 
filter radiometers [152]. The radiance temperature of 
the HTBB was determined using filter radiometers by 
direct measurement of the radiant flux. At the same 
time, the radiance temperature was determined by spec- 
tral radiance ratios using an argon-filled tungsten-strip 
lamp. The lamp was calibrated for spectral radiance in 
the NIST Facility for Spectroradiometric Calibrations 
(PASCAL) using the method of ITS-90 and a gold freez- 
ing-point blackbody. The two independent methods of 
determining spectral radiances were found to agree to 
within 0.5 % from 250 nm to 1050 nm, which is within 
the combined uncertainties of the comparison [152]. 
These results are illustrated in Pig. 21. 



11. Future Directions in Non- Contact 
Thermometry 

To date, filter radiometers such as the ones described 
above could be calibrated for spectral flux responsivity 
only by using the NIST Spectral Comparator Facility 
[154]. With this facility, absolute spectral flux respon- 
sivity determinations require that the entrance pupil of 
the filter radiometer be underfilled. Por the spectral 
irradiance responsivity, determination of the area of the 
aperture is necessary from ancillary measurements 
[155]. The Spectral Comparator Facility uses 
monochromators to spectrally select the output of con- 
tinuum sources, and the response of the test detector is 
determined by comparison to a standard transfer detec- 
tor [154]. The instrumental bandwidth of the monochro- 
mator can be varied, but settings below 1 nm generally 
result in inadequate signal-to-noise ratios. The wave- 
length uncertainty is about 0.1 nm. The combined un- 
certainties of 7?e(A) depend on wavelength, and increase 
rapidly below 450 nm and above 900 nm [154]. 

In an effort to reduce the uncertainties in absolute 
spectral irradiance response and to provide a new facil- 
ity for spectral radiance response, the Optical Technol- 
ogy Division at NIST has developed a Spectral Irradi- 
ance and Radiance Responsivity Calibrations with a 
Uniform Source (SIRCUS) facility that is based on tun- 
able lasers [156-158]. The lasers are coupled to integrat- 
ing spheres that have high throughput. The laser-illumi- 
nated sphere source provides a high flux, lambertian. 



141 



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Journal of Research of the National Institute of Standards and Technology 



p 



I 




-0.5 - 



-LO - 



1900 



2000 



2100 



2200 



2300 



2400 



2500 



2600 



Temperature [''C] 



Fig. 20. The differences in the temperature of a high- temperature blackbody determined using ITS- 90 
(labeled TVep) and three irradiance filter radiometers (labeled Tf^). The centroid wavelengths of the filter 
radiometers are 405 nm, 469 nm, and 560 nm, with effective bandwidths of 57 nm, 114 nm, and 107 
nm, respectively. The solid lines represent the expanded (k = 2) component of uncertainty from the 
uncertainty in the freezing point of gold; the dashed lines represent the total expanded uncertainty 
(k = 2) in the ITS-90 realization. 





1.5 


1— I 






1.0 


(U 




Q 




U 


0.5 


• 1—* 








S 


0.0 


.a 




a> 




o 


-0.5 


w 




Ih 




^ 


-1.0 



-1.5 



1 » 1 ' 1 ' r— « 1 ' 




n 






D 













n 




° □ ° 

D 


D 


D 




a u D 


D 












y^--"^ 


1 . 1 . 1 . 1 . 1 . 



200 400 600 800 1000 1200 

Wavelength [ nm ] 



Fig. 21. The percent difference, as a fiinction of wavelength, in spectral radiance of a high-tem- 
perature blackbody determined using a tungsten- strip lamp and the irradiance filter radiometers. 
The solid lines represent the expanded uncertainty in the NIST spectral radiance scale. 



142 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



tunable, monochromatic light source, and the test detec- 
tor is calibrated by direct comparison to a standard irra- 
diance detector. The experimental components are illus- 
trated in Fig. 22. The response of this detector is known 
from measurements of its aperture area [155] and spec- 
tral flux calibration with the HACR. SIRCUS results to 
date compare favorably with the previous method, and 
the uncertainty was improved by a factor of two or more 
for a narrowband radiance filter radiometer [159]. Re- 
sults from the SIRCUS calibrations of the irradiance- 
measuring filter radiometers used for the spectral irradi- 
ance scale realization are under investigation, and it is 
anticipated that the uncertainties in the NIST spectral 
irradiance scale will be reduced by up to a factor of five, 
depending on wavelength. 

SIRCUS will result in a new generation of pyrome- 
ters that are calibrated without dependence on black- 
body radiation. Such absolute pyrometers would deter- 
mine the radiance temperature of blackbodies using 



./« 



r^ = G\Ri,(X)L(X,T^)dX, 



(11) 



where /?l(A ) is the absolute radiance response and Tl is 
the signal from the radiometer. The radiance tempera- 



ture of the blackbody is solved iteratively using Eq. (1 1) 
and the measurements for /?l(A) and Tl. The uncertain- 
ties in radiance temperature that result from the uncer- 
tainty in the spectral radiances follow from the deriva- 
tive of Planck's law in the Wien approximation, 



dL_ 
L " 



A n ' 



(12) 



Future measurements with SIRCUS should result in a 
relative uncertainty in the spectral radiance response of 
about 0.05 % (k = 2) from 400 nm to 2000 nm. If this is 
achieved, the uncertainty component in radiance tem- 
perature arising from the uncertainty in radiance would 
be 23 mK (k = 2) at 1000 K and 650 nm (Table 20). At 
the gold point, this uncertainty component would be 
about 40 mK at 650 nm, which is about five times 
smaller than the combined expanded uncertainty of the 
previous NIST measurement [135]. Of course, the con- 
tributions from the other uncertainty components for 
these new pyrometers (such as measurement repeatabil- 
ity, non-linearity, size-of-source effect, and temporal 
stability) must be < 20 mK for the results to be an 
improvement over the previous work. 



Laser 



_ Intensity 
stabilizer 



Transfer 
standard 



D 



Integrating 
sphere 



Detector 
P acka ge 




Lens 



^ 



"^ 



V\ 



Spectrum 
analyzer 



Wavemeter 



Computer 



^1 



Motor- 
driven 
rotating 
mirror 



Monitor 
photodiode 
(output to 
stabilizer) 

Fig. 22. A schematic of SIRCUS, illustrating the flux stabilized laser sources that are input to an 
integrating sphere to create a uniform, monochromatic source of spectral radiance. The spectral 
radiance is determined from the transfer standard, the area of its precision aperture, the area of the 
precision aperture at the exit port of the sphere, and the distance between the two apertures. The 
radiometer, labeled "detector package," can be calibrated either for radiance or irradiance mea- 
surements. 



143 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



Table 20. The component of uncertainty in radiance temperature due to the uncertainty 
in spectral radiance as a function of wavelength and temperature. A relative expanded 
uncertainty (k = 2) of 0.05% in spectral radiance was taken for the entire range of 
parameters. The results are stated in mK (k = 2); for parameters that would result in low 
levels of spectral radiance, no results are given because the measurement precision would 
be unacceptable 



Temperature 






Wavelength 






(K) 














400 nm 


650 nm 


900 nm 


1500 nm 


2000 nm 


500 








13.0 


17.4 


1000 




22.6 


31.3 


52.1 


69.5 


1500 


31.3 


50.8 


70.4 


117 


156 


2000 


55.6 


90.4 


125 


209 


278 


2500 


86.9 


141 


195 


326 


434 


3000 


125 


203 


281 


469 


626 



The development of absolute pyrometers and the wide 
application of absolute radiometry to measure radiance 
temperatures will also affect future agreements on tem- 
perature scales. The new, absolute radiometric tech- 
niques have been demonstrated to be at least equivalent 
to the ITS-90 techniques [151, 160], and have the poten- 
tial to be more accurate, especially at the higher temper- 
atures required in spectroradiometry. The future inter- 
national temperature scale could include 
recommendations for absolute radiometric determina- 
tions of temperature. Each NMI could maintain a set of 
standard, absolute pyrometers and the fixed-point black- 
body sources would serve as check standards. For this to 
occur, however, it must be established that such a set of 
standard pyrometers is practical for the intended mode 
of operation. Based on the results with the irradiance 
filter radiometers, the absolute radiometric method 
should result in lower uncertainties for calibration of the 
typical spectral irradiance and radiance standards com- 
pared to the method that utilizes ITS-90 for the temper- 
ature determination of the variable-temperature, high- 
temperature blackbody. With much work, the reduction 
in the radiance temperature uncertainties at lower tem- 
peratures should follow. 



12. References 

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Volume 106, Number 1, January-February 2001 

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146 



Volume 106, Number 1, January-February 2001 

Journal of Research of the National Institute of Standards and Technology 



[85] The NBS-55 was a simple numerical modification to the NBS- [100] 

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Fowler, V. I. Sapritsky, and G. Dezsi, A method of realizing 



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spectral irradiance based on an absolute cryogenic radiometer, 

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[154] T. C. Larason, S. S. Bruce, and A. C. Parr, Spectroradiometric 

Detector Measurements, National Institute of Standards and 

Technology Special Publication 25 0-4 1 , U.S. Government 

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[155] J. B. Fowler, R. S. Durvasula, and A. C. Parr, High-accuracy 

aperture-area measurement facilities at the National Institute of 

Standards and Technology Metrologia 35, 497-500 (1998). 
[156] K. R. Lykke, R-S. Shaw, L. M. Hanssen, and G. R Eppeldauer, 

Development of a monochromatic, uniform source facility for 

calibration of radiance and irradiance detectors from 0.2 |jLm to 

18 |jLm, Metrologia 35, 479-484 (1998). 
[157] S. W. Brown, G. R Eppeldauer, and K. R. Lykke, NIST facility 

for spectral irradiance and radiance response calibrations with 

a uniform source, submitted to Metrologia; Proceedings of the 

NEWRAD'99 Conference, Madrid, Spain. 
[158] G. R Eppeldauer, S. W. Brown, T. C. Larason, M. Racz, and K. 

R. Lykke, Realization of a spectral radiance response scale with 

a laser- illuminated source and Si radiance meters, submitted to 

Metrologia; Proceedings of the NEWRAD'99 Conference, 

Madrid, Spain. 
[159] B. C. Johnson, S. W Brown, G. R Eppeldauer, and K. R. Lykke, 

System-level calibration of a transfer radiometer used to vali- 
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About the authors: B. W. Mangum, C. W. Meyer, M. R. 
Moldover, D. C. Ripple, G. F. Strouse, and W. L Tew 
are physicists in the Process Measurements Division 
(PMD) of the Chemical Science and Technology Labo- 
ratory; K. G. Kreider is a metallurgist in the PMD and 
G. T Furukawa (retired from NBS) is a Guest Re- 
searcher in the PMD. With the exception of Moldover, 
all are members of the Thermometry Group and 
Mangum is Leader of that Group. Moldover is Leader 
of the Fluid Sciences Group. B. Carol Johnson, H. W. 
Yoon, C E. Gibson, and R. D. Saunders are physicists 
in the Optical Temperature and Source Group of the 
Optical Technology Division of the NIST Physics Labo- 
ratory. Saunders is the Leader of that Group. The Na- 
tional Institute of Standards and Technology is an 
agency of the Technology Administration, U.S. Depart- 
ment of Commerce. 



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