Redefinition of the Kelvin and Thermodynamic Temperature Measurement

Abstract

The unit for thermodynamic temperature in the International System of Units (SI), kelvin, was redefined in May 2019. In this article, an overview is given of the redefinition, its background and implications to future thermometry. The redefinition has no immediate impact: the temperature that we measure daily is on the International Temperature Scale of 1990 (ITS-90), which has its own definition independent from the definition of the unit kelvin. The efforts within the international metrology community towards the redefinition led to improvement in primary thermometry, and the difference between thermodynamic temperature T and ITS-90 temperature T₉₀ has become available with improved accuracy. This will enable converting T₉₀, obtained by secondary thermometers, to T. In Japan, traceability for radiation thermometers in high-temperature range has already been implemented utilizing primary standards of high-temperature fixed points with assigned thermodynamic temperature, and the industry now has access to thermodynamic temperature above 1000 ºC.