A practical vapor pressure equation for helium-3 from 0.01K to the critical point

A saturation vapor pressure equation, p(T), is an essential component in the 3He state equation currently under development. The state equation is valid over the range 0.01-20K with pressures from 0 to the melting pressure or 15MPa. The vapor pressure equation consequently must be valid from 0.01K to the critical temperature. This paper surveys available 3He critical temperature and pressure measurements, leading to new recommended critical values of 3.3157K and 114603.91Pa. The ITS-90 temperature scale is defined by the 3He vapor pressure from 0.65 to 3.2K. A new vapor pressure equation is developed for the interval from the upper end of the T90 scale to this newly defined critical point, employing a mathematical form in which the second derivative d2p/dT2 diverges in agreement with scaling laws at the critical point. Below 0.65K, an empirical vapor pressure expression is adopted, consistent with a theoretical expression valid in the limit T- > 0. These two new components are fitted to be piecewise continuous with the EPT-76 p(T) scale rather than the ITS-90 T(p) scale between 0.65 and 3.2K. Probable deviations between this vapor pressure scale and PLTS-2000 melting pressure-temperature scale are recognized, but not reconciled.