Section: 1 | International Temperature Scale of 1990 (ITS-90) |

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INTERNATIONAL TEMPERATURE SCALE OF 1990 (ITS-90)

B. W. Mangum

A new temperature scale, the International Temperature Scale of 1990 (ITS-90), was officially adopted by the Comité International des Poids et Mesures (CIPM), meeting 26-28 September 1989 at the Bureau International des Poids et Mesures (BIPM). The ITS-90 was recommended to the CIPM for its adoption following the completion of the final details of the new scale by the Comité Consultatif de Thermométrie (CCT), meeting 12-14 September 1989 at the BIPM in its 17th Session. The ITS-90 became the official international temperature scale on 1 January 1990. The ITS-90 supersedes the previous scales, the International Practical Temperature Scale of 1968 (IPTS-68) and the 1976 Provisional 0.5 to 30 K Temperature Scale (EPT-76).

The ITS-90 (Refs. 1,2) 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 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 ranges are much improved over that of the previous scales. The replacement of the thermocouple with the platinum resistance thermometer at temperatures below 961.78 °C resulted in the biggest improvement in reproducibility.

The ITS-90 is divided into four primary ranges. Table 1 summarizes the fixed points used in these ranges.

Between 0.65 and 3.2 K, the ITS-90 is defined by the vapor pressure-temperature relation of ³He, and between 1.25 and 2.1768 K (the λ point) and between 2.1768 and 5.0 K by the vapor pressure–temperature relations of ⁴He. T₉₀ is defined by the vapor pressure equations of the form:
$T_{90} / K = A_{0} + \sum_{i = 1}^{9} A_{i} {[(\ln (p / Pa) - B) / C]}^{i}$
The values of the coefficients A_i , and of the constants A₀, B, and C of the equations are given in Table 2.
Between 3.0 and 24.5561 K, the ITS-90 is defined in terms of a ³He or ⁴He 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 (13.8033 K), and at a temperature between 3.0 and 5.0 K, the value of which is determined by using either ³He or ⁴He vapor pressure thermometry.
Between 13.8033 K (–259.3467 °C) and 1234.93 K (961.78 °C), the ITS-90 is defined in terms of the specified fixed points given below, by resistance ratios of platinum resistance thermometers obtained by calibration at specified sets of the fixed points, and by reference functions and deviation functions of resistance ratios which relate to T₉₀ between the fixed points.
Above 1234.93 K, the ITS-90 is defined in terms of Planck’s radiation law, using the freezing-point temperature of either silver, gold, or copper as the reference temperature.

Since the adoption of ITS-90, the isotopic composition of the water and hydrogen, whose fixed points appear in Table 1, has been specified (Ref. 3). A Provisional Low Temperature Scale (PLTS-2000) has been developed, as shown in Table 2, covering the region from 0.9 mK to 1 K (Refs. 2, 4, 5). This scale is based on the melting temperature of ³He.

A more extensive list of reference points on the ITS-90, including secondary reference points, can be found in Section 15.

The International Temperature Scale of 1990, Metrologia 27, 3, 1990; errata in Metrologia 27, 107, 1990. [https://doi.org/10.1088/0026-1394/27/2/010]
Mise en pratique for definition of the kelvin, <www.bipm.org/utils/en/pdf/MeP_K.pdf>, 2011.
Technical Annex for the International Temperature Scale of 1990, <www.bipm.org/utils/en/pdf/MeP_K_Technical_Annex.pdf>, 2005.
The Provisional Low Temperature Scale from 0.9 mK to 1 K, <www.bipm.org/utils/en/pdf/PLTS-2000.pdf>, 2000.
Supplementay Information for the Realization of the PLTS-2000, <www.bipm.org/documents/20126/41773843/Guide-PLTS-2000.pdf>, 2014.

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TABLE 1. Defining Fixed Points of the ITS-90

Material^a	Equilibrium state^b	T₉₀/K	t₉₀/°C
He	VP	3 to 5	-270.15 to -268.15
e-H₂	TP	13.8033	-259.3467
e-H₂ (or He)	VP (or CVGT)	≈17	≈-256.15
e-H₂ (or He)	VP (or CVGT)	≈20.3	≈-252.85
Ne^c	TP	24.5561	-248.5939
O₂	TP	54.3584	-218.7916
Ar	TP	83.8058	-189.3442
Hg^c	TP	234.3156	-38.8344
H₂O	TP	273.16	0.01
Ga^c	MP	302.9146	29.7646
In^c	FP	429.7485	156.5985
Sn	FP	505.078	231.928
Zn	FP	692.677	419.527
Al^c	FP	933.473	660.323
Ag	FP	1234.93	961.78
Au	FP	1337.33	1064.18
Cu^c	FP	1357.77	1084.62

^ae-H₂ indicates equilibrium hydrogen, that is, hydrogen with the equilibrium distribution of its ortho and para states. Normal hydrogen at room temperature contains 25% para hydrogen and 75% ortho hydrogen.
^bVP indicates vapor pressure point; CVGT indicates constant volume gas thermometer point; TP indicates triple point (equilibrium temperature at which the solid, liquid, and vapor phases coexist); FP indicates freezing point; and MP indicates melting point (the equilibrium temperatures at which the solid and liquid phases coexist under a pressure of 101 325 Pa, one standard atmosphere). The isotopic composition is that naturally occurring.
^cPreviously, these were secondary fixed points.

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TABLE 2. Values of Coefficients in the Vapor Pressure Equations for Helium

Coeff. or constant	³He 0.65–3.2 K	⁴He 1.25–2.1768 K	⁴He 2.1768–5.0 K
A₀	1.053447	1.392408	3.146631
A₁	0.980106	0.527153	1.357655
A₂	0.676380	0.166756	0.413923
A₃	0.372692	0.050988	0.091159
A₄	0.151656	0.026514	0.016349
A₅	-0.002263	0.001975	0.001826
A₆	0.006596	-0.017976	-0.004325
A₇	0.088966	0.005409	-0.004973
A₈	-0.004770	0.013259	0
A₉	-0.054943	0	0
B	7.3	5.6	10.3
C	4.3	2.9	1.9

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