Special Report on the International Temperature Scale of 1990: Report on the 17th Session of the Consultative Committee on Thermometry

This article summarizes the results of the 17th Session of the Consultative Committee on Thermometry of the International Committee of Weights and Measures (Comité Consultatif de Thermométrie of the Comité International des Poids et Mesures) that met in Sèvres, France, September 12–14, 1989. That session was devoted exclusively to the completion of the International Temperature Scale of 1990, described herein, and to the implications of its adoption.


Introduction
The Consultative Committee on Thermometry (Comite Consultatif de Thermometrie, CCT) is one of eight Consultative Committees (Comites Consultatifs) of the International Committee of Weights and Measures (Comite International des especially in the temperature region from r68=903.89 K (fss = 630.74-C) to ^8=1337.58 K (^68= 1064.43 °C), the region in which the Pt-10%Rh/Pt thermocouple was the standard interpolating instrument.
The ITS-90 extends upward from 0.65 K and temperatures on this scale are in much better agreement with thermodynamic values than are those on the IPTS-68(75) and the EPT-76. The new scale has subranges and alternative definitions in certain ranges that greatly facilitate its use. Furthermore, its continuity, precision, and reproducibility throughout its range are much improved over the corresponding characteristics of the previous scales. The replacement of the thermocouple with the platinum resistance thermometer at temperatures in the range from 630.74 to 961.93 °C of the IPTS-68(75) resulted in the biggest improvement in reproducibility. Also, improvements in radiometric techniques have allowed using the silver freezing point as the reference point for radiation thermometry. This is a lower temperature reference point than was used in the IPTS-68(75).
The change in the temperature scale affects not only technical interests involved directly in thermometry but also those involved with other reference standards, such as electrical standards, if those standards are sensitive to temperature. As examples, standard resistors and standard cells are sensitive to temperature and generally are maintained in constant-temperature environments, at least in national standards laboratories. At the present time, the temperatures of those environments are normally determined with thermometers that have been calibrated on the IPTS-68(75). A given thermodynamic temperature expressed on the ITS-90, however, has a value that is different from that expressed on the IPTS-68(75), as indicated [2,8] in figure 1. A table of differences between temperatures on the ITS-90, TQO or t^o, and those on the IPTS-68(75), Tes or t^s, and those on the EPT-76, 7^76, is given in the text of the ITS-90. Since temperature values expressed on these scales are different, if the temperature of the environment of a reference standard is adjusted so that its value when expressed on the ITS-90 has the same value as had been used on the IPTS-68(75), there will have been a change of the thermodynamic temperature and the value of the reference standard will usually change. Of course, one may not want to change the thermodynamic temperature of the reference standard. In that case, the thermodynamic temperature, as expressed on the IPTS-68(75), can simply be expressed on the ITS-90 (a numerical value different from that on the IPTS-68(75)) and the reference standards will be unaffected. For more details on the effects of the change of the temperature scale on electrical standards, see National Institute of Standards and Technology (NIST) Technical Note 1263 [4].
In addition to the effect on reference standards for measurements, all temperature-sensitive properties that are presently expressed on the IPTS-68(75) may be affected and may require changes in values.
For details on realizations and approximations of the ITS-90, see NIST Technical Note 1265 [9].

Definition of the ITS-90
The ITS-90 was designed by the CCT in such a manner that temperature values obtained on it do not deviate from the Kelvin thermodynamic temperature values by more than the uncertainties of the latter values at the time the ITS-90 was adopted. Thermodynamic temperature is indicated by the symbol T and has the unit known as the kelvin, symbol K. The size of the kelvin is defined to be 1/273.16 of the thermodynamic temperature of the triple point of water.
Tempera;tures on the ITS-90 can be expressed in terms of the International Kelvin Temperatures, with the symbol Tgo, or in terms of the International Celsius Temperatures, with the symbol fgo. The unit of the temperature 7^90 is the kelvin, symbol K, and the unit of the temperature /90 is the degree Celsius, symbol °C. The relation between T^go and ^90 is: The ITS-90 has alternative definitions of T<^ in certain temperature ranges and they have equal status. In measurements of the highest precision made at the same temperature, the alternative definitions may yield detectably different temperature values. Also, at any given temperature between defining fixed points, different interpolating thermometers that meet the specifications of the ITS-90 may indicate different temperature values. The magnitude of the differences resulting from these two sources, however, is sufficiently small to be negligible for all practical purposes.
Temperatures on the ITS-90 are defined in terms of equilibrium states of pure substances (defining fixed points), interpolating instruments, and equations that relate the measured property to T^^. The defining equilibrium states of the pure substances and the assigned temperatures are listed in table 1. 2.1.1 Temperature Range From 0.65 to 5.0 K Between 0.65 and 3.2 K, the ITS-90 is defined by the vapor pressure-temperature relation of ^He, and between 1.25 and 2.1768 K (the X point) and between 2.1768 and 5.0 K by the vapor pressure-temperature relations of *He. T'go is defined by the vapor pressure equations of the form: value of which is determined by using either ^He or ""He vapor pressure thermometry.
For a ^He CVGT used between 4.2 K and the triple point of neon (24.5561 K), T'90 is defined by the equation: where/j is the CVGT pressure and a, b, and c are coefficients that are determined by calibration at the three specified temperatures, but with the additional requirement that the calibration with the vapor pressure thermometer be made at a temperature between 4.2 and 5.0 K.
For a "He CVGT used between 3.0 and 4.2 K, and for a 'He CVGT used from 3.0 to 24.5561 K, the non-ideality of the gas must be taken into account, using the appropriate second virial coefficient, 54(3%) or 53(7^50)-3^90 is defined in this range by the equation: ■'90with the values of the coefficients A/, and of the constants AQ, B, and C of the equations being specified, and given in table 2.
2.1.2 Temperature Range From 3.0 to 24.5561 K Between 3.0 and 24.5561 K, the ITS-90 is defined in terms of the ""He or *He constant volume gas thermometer (CVGT). The thermometer is calibrated at three temperatures-at the triple point of neon (24.5561 K), at the triple point of equilibrium hydrogen (see footnote a in table 1) (13.8033 K), and at a temperature between 3.0 and 5.0 K, the a+bp+cp^ \+B,(T^)N/V' where p is the CVGT pressure; a, b, and c are coefficients that are determined from calibration at the three defining temperatures; B^(T^) refers to Bi(T^) or B4{Tgo); and N/V is the gas density in moles per cubic meter in the CVGT bulb. The values of the second virial coefficients at any given temperature are to be calculated according to equations specified in the official document of the ITS-90 (and also in the NIST Technical Note 1265 [9]). , the ITS-90 is defined in terms of the specified fixed points given in table 1, by resistance ratios of platinum resistance thermometers (PRTs) obtained by calibration at specified sets of the fixed points, and by reference functions and deviation functions of resistance ratios which relate to T^o between the fixed points.
Temperatures on the ITS-90 are expressed in terms of the ratio W(Tgo) of the resistance R (Tso) at temperature Tgo and the resistance i? (273.16 K) at the triple point of water, i.e., For a PRT to be an acceptable instrument of the ITS-90, its coil must be made from pure platinum and be strain-free. Additionally, the finished PRT must meet one of the following criteria: An acceptable PRT that is to be used to the freezing point of silver must meet the following requirement also: >iF(1234.93 K) > 4.2844.
The temperature Tgo is calculated from the resistance ratio relation: where W(T^o) is the observed value, W^X^M) is the value calculated from the reference function, and AW(Tso) is the deviation of the observed }V(T^) value of the particular PRT from the reference function value at T^.
There are two reference functions W^iTgo), one for the range 13.8033 to 273.16 K and the second for the range 273.15 to 1234.93 K. The deviation A WiTgo) is obtained as a function of 7^90 for various ranges by calibration at specified fixed points. The form of the deviation function depends upon the temperature range of calibration.
The specified inverse of this equation, equivalent to within ±0.000 1 K, is: The values of the constants AQ and BQ and values of the coefficients Aj and B/ of the two equations are listed in table 3. Table 3. Values of the coefficients A,, Bi, C" and A and of the constants Ar^ Ba, Co, and Da in the reference functions, eqs (10) and (18), and in the inverse functions approximating them, given by eqs (11)  If the PRT is to be used throughout the range from 13.8033 to 273.16 K, it must be calibrated at the triple points of equilibrium hydrogen (13.8033 K), neon (24.5561 K), oxygen (54.3584 K), argon (83.8058 K), mercury (234.3156 K), and water (273.16 K) and at two additional temperatures very close to 17.0 and 20.3 K. The temperatures of calibration at 17.0 and 20.3 K may be determined by using either a CVGT or the vapor pressure-temperature relation of equilibrium hydrogen. When the CVGT is used, the two temperatures must be within the ranges 16.9 to 17.1 K and 20.2 to 20.4 K, respectively. When the equilibrium hydrogen vapor pressure thermometer is used, the two temperatures must be within the ranges 17.025 to 17.045 K and 20.26 to 20.28 K, respectively. The temperatures of the equilibrium hydrogen vapor pressure thermometer are determined from the values of the hydrogen vapor pressure,/?, and the equations: The deviation function for calibration in the range 13.8033 to 273.16 K is given by the relation:

^W,{T^)= W{T,o)-W,{T,^)=a,[W(T^)-\] ^b,[W{T,,)-\f+'2c^[\^W(T,o)r'',
with n =2. The coefficients fli, b\, and the five c,'s of the deviation function are obtained by calibration at all of the above eight temperatures, including that at the triple point of water. The values of W^{Tg^ are obtained from the reference function for this range. Although the official text of the ITS-90 does not assign subscripts to the coefficients a and b, nor does it designate the deviation equations by the symbols LWJJ',),^, where in eq (14) m = 1, these designations will be used in this paper for clarity and for ease of reference. In any case, some such terminology must be used in PRT calibration reports and this was chosen for convenience. The values of the constants Co and Do and of the coefficients C, and £>, for these equations are listed in table 3. If the PRT is to be used over this entire subrange (273.15 to 1234.93 K), it must be calibrated at the triple point of water (273.16 K) and at the freezing points of tin (505.078 K), zinc (692.677 K), aluminum (933.473 K), and silver (1234.93 K).

Subrange
The deviation function is given by the relation:

Recommendations of the CCT
Three recommendations were adopted by the CCT at its 17th Session. These recommendations were considered by the CIPM and Recommendation Tl (1989) of the CCT was adopted as Recommendation 5 (CI-89) of the CIPM. Recommendations T2 (1989) and T3 (1989) of the CCT were noted by the CIPM as CCT recommendations. The CCT recommendations were as follows:

Recommendation Tl (1989)
The International Temperature Scale of 1990 The Comite Consultatif de Thermometrie (CCT) acting in accordance with Resolution 7 of the 18'^ CGPM has generated the International Temperature Scale of 1990 (ITS-90) in order to supersede the International Practical Temperature Scale of 1968 (IPTS-68).
The CCT notes that, by comparison with the IPTS-68, the ITS-90 -extends to lower temperatures, down to 0.65 K, and hence also supersedes the EPT-76, -is in substantially better agreement with corresponding thermodynamic temperatures, -has much improved continuity, precision, and reproducibility throughout its range and -has subranges and alternative definitions in certain ranges which greatly facilitate its use.
The CCT also notes that, to accompany the text of the ITS-90 there will be two further documents, the Supplementary Information for the ITS-90 and Techniques for Approximating the ITS-90. These documents will be published by the BIPM and periodically updated.
The CCT recommends -that on January 1, 1990 the ITS-90 come into force and -that from this same date the IPTS-68 and the EPT-76 be abrogated.

Reference Tables for Thermocouples and Industrial Platinum Resistance Thermometers
The Comite Consultatif de Thermometrie, considering -that the introduction of the International Temperature Scale of 1990 (ITS-90) will lead to an urgent requirement for new reference tables for both thermocouples and industrial platinum resistance thermometers, requests its Working Group 2 -to collaborate with national laboratories in the rapid preparation of new reference tables taking into account not only the change from IPTS-68 to ITS-90 but also new information on the behavior of thermocouples and industrial platinum resistance thermometers, recommends -that these new tables be used as the basis for new national and international reference tables for thermocouples and industrial platinum resistance thermometers and -that meanwhile the existing reference tables based upon IPTS-68 should be used in conjunction with the table of differences T^o-T^i which appears in the ITS-90.
(Note: the table of differences T^Q-T^S referred to here may be obtained also from NIST from the author of this article).

References
The Comite Consultatif de Thermometrie, considering the requirement for assigning an uncertainty to the numerical value of any temperature on the International Temperature Scale of 1990 (ITS-90), encourages national laboratories to a) quantify the uncertainties in the fixed point realizations, b) quantify the uncertainties resulting from the use of the specified interpolating instruments of ITS-90, c) develop the mathematical procedures describing the propagation of these uncertainties to any intermediate temperature.
Although the uncertainties in the values of thermodynamic temperatures above 100 °C used in the definition of the ITS-90 were larger than desired and larger than had been anticipated a few years ago, the agreement of temperatures on the ITS-90 with thermodynamic temperatures is nevertheless a significant improvement over that of previous scales. The replacement of the thermocouple with the platinum resistance thermometer as the standard instrument of the scale at temperatures in the IPTS-68(75) range from 630.74 to 961.93 "C has improved the reproducibility over that of the IPTS-68(75) significantly. Also, advances in radiometric techniques have improved the precision of measurements in radiation thermometry. The precision of the scale, or what has been called the nonuniqueness of the scale, is significantly improved over that of the IPTS-68(75), as is also the scale's continuity. The extension of the scale downward in temperature to 0.65 K and the use of subranges over which thermometers may be calibrated make the ITS-90 more useful and much more flexible than were the previous scales.