The ITS-90 (International Temperature Scale of 1990) defines procedures by which certain practical thermometers of the required quality and precision can be calibrated in such a way that the values obtained from them can be precise and reproducible, while at the same time representing the corresponding thermodynamic temperatures as closely as possible.
The ITS-90 specifies certain fixed points (the freezing points of seven metals, the melting point of gallium, the triple points of water, mercury and four cryogenic gases, and the boiling points of hydrogen at two pressures) as references for the calibration of thermometers.
The temperature of these fixed points is precisely defined and internationally agreed. It also specifies the thermometers to be used over a particular temperature range, for example standard platinum resistance thermometers (PRTs, see below) between 13.8 K and 961.78 °C and radiation thermometry above 961.78 °C, as well as the equations used in the ITS-90 definition. For more information read: ‘The International Temperature Scale of 1990’ booklet, HMSO ISBN 0114800596, and Supplementary Information for the ITS-90, both available from NPL.
Standard PRTs can then be used for the calibration of other thermometers in comparison baths, cryostats or furnaces. For greatest assurance, these services should be UKAS-accredited, and in this case the temperature measurements made with them are fully traceable to the ITS-90.