Recommendation 5 of the 78th CIPM (1989)
The International Temperature Scale of 1990
The Comité International des Poids et Mesures (CIPM) acting in accordance with Resolution 7 of the 18th Conférence Générale des Poids et Mesures (1987) has adopted the International Temperature Scale of 1990 (ITS-90) to supersede the International Practical Temperature Scale of 1968 (IPTS-68).
The CIPM notes that, by comparison with the IPTS-68, the ITS-90
- extends to lower temperatures, down to 0.65 K, and hence also supersedes the EPT 76,
- is in substantially better agreement with corresponding thermodynamic temperatures,
- has much improved continuity, precision and reproducibility throughout its range and
- has subranges and alternative definitions in certain ranges which greatly facilitate its use.
The CIPM also notes that, to accompany the text of the ITS-90 there will be two further documents, the Supplementary Information for the ITS-90 and Techniques for Approximating the ITS-90. These documents will be published by the BIPM and periodically updated.
The CIPM recommends
- that on 1 January 1990 the ITS-90 come into force and
- that from this same date the IPTS-68 and the EPT-76 be abrogated.
The kelvin was redefined by the 26th CGPM in 2018 (Resolution 1).