An important note
The International Temperature Scale of 1990 (ITS-90)
President of the Comité Consultatif de Thermométrie and Vice-President of the Comité International des Poids et Mesures Division of Physics, National Research Council of Canada, Ottawa, K1A OS1 Canada
The official French text of the ITS-90 is published by the BIPM as part of the Procès-verbaux of the Comité International des Poids et Mesures (CIPM). However, the English version of the text reproduced here has been authorized by the Comité Consultatif de Thermométrie (CCT) and approved by the CIPM.
The International Temperature Scale of 1990 was adopted by the International Committee of Weights and Measures at its meeting in 1989, in accordance with the request embodied in Resolution 7 of the 18th General Conference of Weights and Measures of 1987. This scale supersedes the International Practical Temperature Scale of 1968 (amended edition of 1975) and the 1976 Provisional 0.5 K to 30 K Temperature Scale.
1. Units of Temperature
The unit of the fundamental physical quantity known as thermodynamic temperature, symbol T, is the kelvin, symbol K, defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water1.
Because of the way earlier temperature scales were defined, it remains common practice to express a temperature in terms of its difference from 273.15 K, the ice point. A thermodynamic temperature, T, expressed in this way is known as a Celsius temperature, symbol t, defined by:
t / °C = T/K - 273.15 . (1)
The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin. A difference of temperature may be expressed in kelvins or degrees Celsius.
The International Temperature Scale of 1990 (ITS-90) defines both International Kelvin Temperatures, symbol T90, and International Celsius Temperatures, symbol t90. The relation between T90 and t90, is the same as that between T and t, i.e.:
t90 / °C = T90/K - 273.15 . (2)
The unit of the physical quantity T90 is the kelvin, symbol K, and the unit of the physical quantity t90, is the degree Celsius, symbol °C, as is the case for the thermodynamic temperature T and the Celsius temperature t.
1 Comptes Rendus des Séances de la Treizième Conférence Générale des Poids es Mesures (1967-1968), Resolutions 3 and 4, p. 104
Continues: 2 Principles of the International Temperature Scale of 1990 (ITS-90)
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