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ITS-90 Text
Contents
- Units of Temperature and Introduction from Metrologia 27, 3-10 (1990)
- Principles of the International Temperature Scale of 1990 (ITS-90)
- Definition of the International Temperature Scale of 1990
- Supplementary Information and Differences from Earlier Scales
4. Supplementary Information and Differences from Earlier Scales & Appendix
The apparatus, methods and procedures that will serve to realize the ITS-90 are given in "Supplementary Information for the ITS-90". This document also gives an account of the earlier International Temperature Scales and the numerical differences between successive scales that include, where practicable, mathematical functions for the differences T90 - T68. A number of useful approximations to the ITS-90 are given in "Techniques for Approximating the ITS-90".
These two documents have been prepared by the Comité Consultatif de Thermométrie and are published by the BIPM; they are revised and updated periodically.
The differences T90 -T68 are shown in Fig. 1 and Table 6. The number of significant figures given in Table 6. allows smooth interpolations to be made. However, the reproducibility of the IPTS-68 is, in many areas, substantially worse than is implied by this number.
Appendix
The International Temperature Scale of 1927 (ITS-27)
The International Temperature Scale of 1927 was adopted by the seventh General Conference of Weights and Measures to overcome the practical difficulties of the direct realization of thermodynamic temperatures by gas thermometry, and as a universally acceptable replacement for the differing existing national temperature scales. The ITS-27 was formulated so as to allow measurements of temperature to be made precisely and reproducibly, with as close an approximation to thermodynamic temperatures as could be determined at that time. Between the oxygen boiling point and the gold freezing point it was based upon a number of reproducible temperatures, or fixed points, to which numerical values were assigned, and two standard interpolating instruments. Each of these interpolating instruments was calibrated at several of the fixed points, this giving the constants for the interpolating formula in the appropriate temperature range. A platinum resistance thermometer was used for the lower part and a platinum rhodium/platinum thermocouple for temperatures above 660°C. For the region above the gold freezing point, temperatures were defined in terms of the Wien radiation law: in practice, this invariably resulted in the selection of an optical pyrometer as the realizing instrument.
The International Temperature Scale of 1948 (ITS-48)
The International Temperature Scale of 1948 was adopted by the ninth General Conference. Changes from the ITS-27 were: the lower limit of the platinum resistance thermometer range was changed from -190°C to the defined oxygen boiling point of -182.97°C, and the junction of the platinum resistance thermometer range and the thermocouple range became the measured antimony freezing point (about 630°C) in place of 660°C; the silver freezing point was defined as being 960.8°C instead of 960.5°C; the gold freezing point replaced the gold melting point (1063°C); the Planck radiation law replaced the Wien law; the value assigned to the second radiation constant became 1.438 x 10 -2 m · K in place of 1.432 x 10-2 m · K; the permitted ranges for the constants of the interpolation formulae for the standard resistance thermometer and thermocouple were modified; the limitation of 
T for optical pyrometry (
T<3 x 10-3 m K) was changed to the requirement that "visible" radiation be used.
The International Practical Temperature Scale of 1948 (Amended Edition of 1960) (IPTS-48)
The International Practical Temperature Scale of 1948, amended edition of 1960, was adopted by the eleventh General Conference: the tenth General Conference had already adopted the triple point of water as the sole point defining the kelvin, the unit of thermodynamic temperature. In addition to the introduction of the word "Practical", the modifications to the ITS-48 were: the triple point of water, defined as being 0.01°C, replaced the melting point of ice as the calibration point in this region; the freezing point of zinc, defined as being 419.505°C, became a preferred alternative to the sulphur boiling point (444.6°C) as a calibration point; the permitted ranges for the constants of the interpolation formulae for the standard resistance thermometer and the thermocouple were further modified; the restriction to "visible" radiation for optical pyrometry was removed.
Inasmuch as the numerical values of temperature on the IPTS-48 were the same as on the ITS-48, the former was not a revision of the scale of 1948 but merely an amended form of it.
The International Practical Temperature Scale of 1968 (IPTS-68)
In 1968 the International Committee of Weights and Measures promulgated the International Practical Temperature Scale of 1968, having been empowered to do so by the thirteenth General Conference of 1967-1968. The IPTS-68 incorporated very extensive changes from the IPTS-48. These included numerical changes, designed to bring it more nearly in accord with thermodynamic temperatures, that were sufficiently large to be apparent to many users. Other changes were as follows: the lower limit of the scale was extended down to 13.81 K; at even lower temperatures (0.5 K to 5.2 K), the use of two helium vapour pressure scales was recommended; six new defining fixed points were introduced - the triple point of equilibrium hydrogen (13.81 K), an intermediate equilibrium hydrogen point (17.042 K), the normal boiling point of equilibrium hydrogen (20.28 K), the boiling point of neon (27.102 K), the triple point of oxygen (54.361 K), and the freezing point of tin (231.9681°C) which became a permitted alternative to the boiling point of water; the boiling point of sulphur was deleted; the values assigned to four fixed points were changed - the boiling point of oxygen (90.188 K), the freezing point of zinc (419.58°C), the freezing point of silver (961.93°C), and the freezing point of gold (1064.43°C); the interpolating formulae for the resistance thermometer range became much more complex; the value assigned to the second radiation constant c2, became 1.4388 x 10-2 m · K; the permitted ranges of the constants for the interpolation formulae for the resistance thermometer and thermocouple were again modified.
The International Practical Temperature Scale of 1968 (Amended edition of 1975) (IPTS-68)
The International Practical Temperature Scale of 1968, amended edition of 1975, was adopted by the fifteenth General Conference in 1975. As was the case for the IPTS-48 with respect to the ITS-48, the IPTS-68(75) introduced no numerical changes. Most of the extensive textual changes were intended only to clarify and simplify its use. More substantive changes were: the oxygen point was defined as the condensation point rather than the boiling point; the triple point of argon (83.798 K) was introduced as a permitted alternative to the condensation point of oxygen; new values of the isotopic composition of naturally occurring neon were adopted; the recommendation to use values of T given by the 1958 4He and 1962 3He vapour-pressure scales was rescinded.
The 1976 Provisional 0.5 K to 30 K Temperature Scale (EPT-76)
The 1976 Provisional 0.5 K to 30 K Temperature Scale was introduced to meet two important requirements: these were to provide means of substantially reducing the errors (with respect to corresponding thermodynamic values) below 27 K that were then known to exist in the IPTS-68 and throughout the temperature ranges of the 4He and 3He vapour pressure scales of 1958 and 1962 respectively, and to bridge the gap between 5.2 K and 13.81 K in which there had not previously been an international scale. Other objectives in devising the EPT-76 were "that it should be thermodynamically smooth, that it should be continuous with the IPTS-68 at 27.1 K, and that it should agree with thermodynamic temperature T as closely as these two conditions allow". In contrast with the IPTS-68, and to ensure its rapid adoption, several methods of realizing the EPT-76 were approved. These included: using a thermodynamic interpolation instrument and one or more of eleven assigned reference points; taking differences from the IPTS-68 above 13.81 K; taking differences from helium vapour pressure scales below 5 K; and taking differences from certain well-established laboratory scales. Because there was a certain "lack of internal consistency" it was admitted that "slight ambiguities between realizations" might be introduced. However the advantages gained by adopting the EPT-76 as a working scale until such time as the IPTS-68 should be revised and extended were considered to outweigh the disadvantages.