Since temperatures are not additive and can’t be divided up, like metres or amps, a temperature scale has to be derived from fundamental principles. This is very difficult to do, and in practice it is convenient to make use of ‘fixed points’ whose temperatures, once established, can be used as reference values up and down the scale. A ‘practical’ scale is then derived by choosing a thermometer which can be calibrated at the fixed points and subsequently used as a standard for the measurement of temperature throughout the range.

Thus the purpose of the International Temperature Scale is to define procedures by which certain specified practical thermometers can be calibrated in such a way that the values of temperature obtained from them are precise and reproducible, while at the same time approximating the corresponding thermodynamic values as closely as possible.

The ITS-90 is defined using Standard Platinum Resistance Thermometers to interpolate between fixed points in various parts of the range from the triple point of hydrogen, 13.8033 K, to the freezing point of silver, 961.78 °C. At higher temperatures it specifies the use of a (spectral) radiation thermometer calibrated at the freezing point of silver (or gold or copper). In this case the Planck law is used to extrapolate the calibration to indefinitely high temperatures.

The ITS-90 extends to 0.65 K, using appropriate methods, and the Provisional Low Temperature Scale of 2000, PLTS-2000, provides an agreed scale for the range from 0.9 mK to 1 K.

The text of the ITS-90 is published by HMSO and is available through NPL, together with related documents and guidance on its realisation. They are also available from the BIPM.

From time to time, as better knowledge is gained, the values and prescriptions of the International Temperature Scale are revised. At present work is in progress to develop new fixed points, melting points of metal-carbon alloys, which may in future be used to redefine the ITS-90 above the silver point and considerably reduce the uncertainties.