The International Practical Temperature Scale of 1968

Abstract

The Comité International des Poids et Mesures (CIPM) atits meeting in October 1968 agreed to adopt the InternationalPractical Temperature Scale of 1968 (IPTS-68) in accordancewith the decision of the 13e Conférence Générale des Poidset Mesures, Resolution 8, of October 1967. This resolution gavethe CIPM permission to introduce a new Scale to replace theexisting IPTS-48 as soon as the Comité Consultatif de Thermométrie (CCT) made its recommendations.There were two important reasons for revising the IPTS-48.The first of these was the need to extend the Scale to lowertemperatures to achieve a unification of the existing nationalscales in the region 10 K–90 K. Secondly the Scale had notbeen revised significantly over the range from −183 °C to1063 °C since its inception in 1927, and modern gas thermometer measurements showed that the Scale gave values considerably different from thermodynamic temperatures.The CCT agreed on the basic data and methods of interpolation for a new definition of the IPTS at its 8th meetingheld in Washington and Ottawa in September 1967. Provisionwas made to extend the Scale down to 13.81 K, the triple pointof equilibrium hydrogen, and for the use of the best knownvalues of thermodynamic temperatures. The value of c2 usedin the Planck equation was also revised as a result of a betterknowledge of the values of fundamental physical constants.The CCT appointed a small sub-committee to prepare a textof the definition of the IPTS-68 in accordance with its recommendations, with instructions to resolve some points of detail.The final draft of the English text, which is reproduced below,received the approval of all members of the CCT before it wassubmitted to the CIPM. The official text of the IPTS-68 isthe French text which was prepared by the Bureau International des Poids et Mesures from the English text, and ispublished as an annexe to Comptes rendus des séances de laTreizième Conférence Générale des Poids et Mesures.