Vapour pressure thermometry

The most interesting liquids for low-temperature thermometry are 3He and 4He, especially for the calibration of resistance thermometers in the range from 0.5 to 4.2 K. Vapour pressure of H2 is also interesting to realize vapour pressure-fixed points included in ITS-90. The measure of He vapour pressure has been carried out with great accuracy [42,43] to establish the ITS-90 (see Section 8.3). There are several experimental precautions to be observed in order to obtain reliable measurements [2],... 

A detailed discussion of the problems encountered in vapour pressure measurements at low temperature is given in ref. [46,47], where also the use of an in situ manometer is described. Vapour pressure gas thermometry with other liquids besides He is discussed in ref. [43, p. 49],... 

Not all of them are equally suitable as fixed points for thermometry. In (b) and (c), the points are characterized by a high equilibrium pressure and may be better considered as pressure fixed points (see, for example, ref. [9]). Types (d) and (e) are not generally referred to as triple points, but as solid-solid and liquid-liquid transitions, respectively. At low temperature, the last solid-liquid-vapour triple point is that of hydrogen (para 99.996%) at 13.8033 K. 

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