Guidelines for realizing the International Temperature Scale of 1990 (ITS-90)

This Technical Note describes the International Temperature Scale of 1990 (ITS-90) that became the official international temperature scale on 1 January 1990, superseding the previous scales, and provides information on how the scale may be realized at different levels of accuracy. The ITS-90 extends upward from 0.65 K, is in close agreement with the Kelvin Thermodynamic Temperature Scale, has much improved continuity, precision and reproducibility throughout its ranges over that of previous scales, and has subranges and alternative definitions in certain ranges that greatly facilitate its use. In addition to a description of the ITS-90 and how it can be realized, there are included in this document reproductions of some articles and excerpts from documents concerned with the ITS-90. The composition of the Comite Consultatif de Thermometrie (CCT) of the Comite International des Poids et Mesures (CIPM) at the time of the adoption of the ITS-90 is given. The differences between the temperatures on the ITS-90 and those on the International Practical Temperature Scale of 1968, Amended Edition of 1975, [IPTS-68(75)] and those on the 1976 Provisional 0.5 K to 30 K Temperature Scale (EPT-76) are tabulated. Measurement procedures for realizing the ITS-90 throughout the various ranges of the scale are given. Also, for the most important temperature region, the region of the platinum resistance thermometer (PRT) , computational examples are given for determining the coefficients of the relevant deviation equations for PRTs calibrated at various sets of fixed points. The effects of the introduction of the ITS-90 on electrical reference standards are addressed also.