International Temperature Scale of 1990

The International Temperature Scale of 1990 (ITS-90) is an equipment calibration standard specified by the International Committee of Weights and Measures (CIPM) for making measurements on the Kelvin and Celsius temperature scales. It is an approximation of thermodynamic temperature that facilitates the comparability and compatibility of temperature measurements internationally. It defines fourteen calibration points ranging from 0.65 K to 1357.77 K (−272.50 °C to 1084.62 °C) and is subdivided into multiple temperature ranges which overlap in some instances. ITS-90 is the most recent of a series of International Temperature Scales adopted by the CIPM since 1927.^[1] Adopted at the 1989 General Conference on Weights and Measures, it supersedes the International Practical Temperature Scale of 1968 (amended edition of 1975) and the 1976 "Provisional 0.5 K to 30 K Temperature Scale". The CCT has also published several online guidebooks to aid realisations of the ITS-90. The lowest temperature covered by the ITS-90 is 0.65 K. In 2000, the temperature scale was extended further, to 0.9 mK, by the adoption of a supplemental scale, known as the Provisional Low Temperature Scale of 2000 (PLTS-2000).^[2]

In 2019, the kelvin was redefined. However, the alteration was very slight compared to the ITS-90 uncertainties, and so the ITS-90 remains the recommended practical temperature scale without any significant changes. It is anticipated that the redefinition, combined with improvements in primary thermometry methods, will phase out reliance on the ITS-90 and the PLTS-2000 in the future.^[3]

^ "Beginning in 1927, the CIPM, acting under the authority of the General Conference on Weights and Measures (CGPM) and, since 1937, on the advice of the Consultative Committee for Thermometry (CCT), has adopted a series of International Temperature Scales. Subsequent to the 1927 scale, new scales have been adopted in 1948, 1968, and 1990, with occasional minor revisions in intervening years."
^ "Considerable research has been conducted on establishing a temperature scale extending to temperatures lower than 0.65 K; the PLTS-2000 is the resulting outcome, defining temperature from 1 K down to 0.9 mK. The PLTS-2000 is explicitly a provisional scale, recognizing that the data sets comprising the basis of the scale were somewhat inconsistent below 10 mK. In the temperature range 0.65 K to 1 K, temperature may be defined on either the ITS-90 or the PLTS-2000. Either scale is acceptable; the choice of scale typically is dictated by convenience or the attainable uncertainty of realization. In rare cases where use of either scale is convenient, T₂₀₀₀ is a better approximation of thermodynamic temperature than T₉₀ in the region of overlap." Consultative Committee for Thermometry, "Mise en pratique for the definition of the kelvin", 2011.
^ "Mise en pratique for the definition of the kelvin in the SI" BIPM, May 2019.

[1] "Beginning in 1927, the CIPM, acting under the authority of the General Conference on Weights and Measures (CGPM) and, since 1937, on the advice of the Consultative Committee for Thermometry (CCT), has adopted a series of International Temperature Scales. Subsequent to the 1927 scale, new scales have been adopted in 1948, 1968, and 1990, with occasional minor revisions in intervening years."

[2] "Considerable research has been conducted on establishing a temperature scale extending to temperatures lower than 0.65 K; the PLTS-2000 is the resulting outcome, defining temperature from 1 K down to 0.9 mK. The PLTS-2000 is explicitly a provisional scale, recognizing that the data sets comprising the basis of the scale were somewhat inconsistent below 10 mK. In the temperature range 0.65 K to 1 K, temperature may be defined on either the ITS-90 or the PLTS-2000. Either scale is acceptable; the choice of scale typically is dictated by convenience or the attainable uncertainty of realization. In rare cases where use of either scale is convenient, T₂₀₀₀ is a better approximation of thermodynamic temperature than T₉₀ in the region of overlap." Consultative Committee for Thermometry, "Mise en pratique for the definition of the kelvin", 2011.

[mepk2019-3] "Mise en pratique for the definition of the kelvin in the SI" BIPM, May 2019.

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