3He Vapor Pressure near Its Critical Point

Between 0.65 K and 3.2 K, the temperature dependence of the vapor pressure P of 3He is defined by the International Temperature Scale of 1990 (ITS-90). However, the ITS-90 vapor pressure equation was not designed to be consistent with the scaling law required for the second temperature derivative of the vapor pressure in the vicinity of the liquid-vapor critical point. In this paper, two scaling-type equations are used to describe the 3He vapor pressure in the region near the critical point. The first scaling equation contains two unknown coefficients which are obtained by taking as reference the temperature T̄ at which the product (Tc − T )P presents a maximum (T̄ = 2.56736 K). The second scaling equation contains three unknown coefficients which are obtained by using as references T̄ and Tup = 3.2 K, the upper value of the ITS-90 interval. In both equations we take for the critical temperature and pressure the values Tc = 3.31554 K and Pc = 114 632.7 Pa. The proposed equations, specially the second one, are satisfactorily compared with experimental data for P and dP/dT within the temperature range (Tc − T )/Tc ≤ 0.065 and with semiempirical data for d2P/dT 2 within the temperature range 0.0001 ≤ (Tc − T )/Tc ≤ 0.03.