How does a helium thermometer work?
There are two types of helium thermometers, both used in the definition and realisation of the International Temperature Scale of 1990 (ITS-90) at very low temperatures. The first uses the variation of the vapour pressure of liquid He3 and He4 to define the scale between 0.65 and 5 K. The second is a gas thermometer with helium as the working fluid. This operates over the range from 3.0 to 24.55 K, and is calibrated at three fixed points in this range whose assigned temperatures also form part of the definition of ITS-90. In general these instruments are constructed by the national standards laboratories in the major technological countries, and the realisations of ITS-90 thus obtained circulated to users through low temperature resistance thermometers which act as secondary standards. Although the basic operating principles of each type are relatively simple, the actual instruments themselves are quite complex. Detailed information and references may be obtained by application to the
