ITS-90 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 low part and a platinum rhodium/platinum
thermocouple for temperatures above 660 șC. 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 platinum resistance
thermometer range was changed from -190 șC to the defined oxygen boiling point of -182.97
șC, and the junction of the platinum resistance thermometer range and the thermocouple
range became the measured antimony freezing point (about 630 șC) in place 660 șC; the
silver freezing point was defined as being 960.8 șC instead of 960.5 șC; the gold
freezing point replaced the gold melting point (1063 șC); 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 formula for the standard resistance thermometer and
thermocouple were modified; the limitation on lT for optical pyrometry (lambda·T<3x10-3
m · K) was changed on 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 șC,
replaced the freezing point of zinc, defined as being 419.505 șC, became a preferred
alternative to the sulfur boiling point (444.6 șC) 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 to 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 vapor 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 șC) which became a permitted alternative to the
boiling point of water; the boiling point of sulfur 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 șC), the freezing point of silver (961.93 șC), and the freezing
point of gold (1064.43 șC): 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 textural 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 vapor-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 vapor 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 ETP-76 were "that it should be
thermodynamically smooth, that it should be continuous with the IPTS-68 at 27.1 K, and
that is 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 ETP-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 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.
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