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The International Temperature Scale of 1990 (ITS-90)
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Summary
The normal hydrogen scale
The International Temperature Scale of 1927
The evolution of ITS-27 and its modifications in 1948
The 1958 4He and 1962 3He vapour pressure scales
The International Practical Temperature Scale of 1968 (IPTS-68)
The 1976 Provisional 0.5 K to 30 K Temperature Scale (EPT-76)
The International Temperature Scale of 1990 (ITS-90)
Direct access

  1990
The International Temperature Scale of 1990 (ITS-90) came into effect on 1 Janurary 1990, replacing the IPTS-68 and the EPT-76.

The ITS-90 differs from the IPTS-68 in a number of important respects:
it uses the triple point of water (273.16 K), rather than the freezing point of water (273.15 K), as a defining point;
it extends to lower temperatures: 0.65 K instead of 13.8 K;
it is in closer agreement with thermodynamic temperatures;
it has improved continuity and precision;
it has a number of overlapping ranges and sub-ranges;
in certain ranges it has alternative but substantially equivalent definitions;
it includes the helium vapour pressure scales;
it includes an interpolating gas thermometer as one of the defining instruments;
the range of the platinum resistance thermometer as defining instrument has been extended from 630 °C up to the silver point, 962 °C;
the Pt/10 % Rh-Pt thermocouple is no longer a defining instrument of the scale;
the range based upon the Planck radiation law begins at the silver point instead of at the gold point, but options exist for using any one of the silver, gold or copper points as reference points for this part of the scale.


For further details please refer to the following BIPM publications:
Preston-Thomas H., The International Temperature Scale of 1990 (ITS-90), Metrologia, 1990, 27, 3-10 and 107;
Techniques for approximating the International Temperature Scale of 1990;
Supplementary information for the International Temperature Scale of 1990.