Why do we need both thermodynamic and practical temperature scales?
Temperatures can’t be added together or divided up, like metres or kilograms or amps. A temperature scale has to be derived from fundamental principles.
Physical laws governing the properties of gases or thermal radiation relate observable properties (pressure and radiant intensity, etc.) to thermodynamic temperature and it follows that these laws can be used to produce a thermodynamic scale. However, the experiments to achieve this are very difficult and time consuming. Therefore, a substitute practical scale, the International Temperature Scale of 1990 (ITS-90) provides precise and reproducible values of temperature, with close approximations to the corresponding thermodynamic values, that can be used in science, technology and everyday life.
The National Physical Laboratory maintains the ITS-90 in the UK, and compares this with the ITS-90 maintained in other national laboratories. In this way temperature standards around the world can be accurately equivalent.
ITS-90 makes use of ‘fixed points’ whose temperatures have been established in careful experiments and which can be used as reference values up and down the scale.
Fixed points are temperatures at which a substance changes its phase, for example from liquid to vapour (boiling) or solid to liquid (melting).
While this is happening the temperature remains almost constant, i.e. it is ‘fixed’.
The text of ITS-90 is available from www.bipm.org
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