THERMODYNAMIC BASIS OF THE ITS-90

The International Temperature Scale of 1990, ITS-90 [1], came into effect on 1 January 1990, replacing the International Practical Temperature Scale of 1968, IPTS-68 [2], and the "Echelle Provisoire de Temperature de 1976 entre 0,5 K et 30 K", EPT-76 [3]. The present paper reviews the various experimental measurements of thermodynamic temperature T (often expressed as differences (T - T68)) in the range 0,65 K to 1 357,77 K, which were available prior to September 1989, and on the basis of which values of temperature were adopted in the ITS-90. Estimates are given for the uncertainties in the derivation of these values.