THE THERMODYNAMICS OF THE HE-4 AND HE-3 VAPOR-PRESSURE EQUATIONS IN THE ITS-90 AND THE EPT-76

The vapour-pressure versus temperature equations for liquid He-4 and H e-3 that define the International Temperature Scale of 1990 (ITS-90) i n the range 0,65 K to 5,0 K have been derived from earlier equations i n the Echelle Provisoire de Temperature de 1976 (EPT-76), which span t he wider range 0,2 K to 5,2 K. This paper investigates the extent to w hich, i.e. to within what uncertainty, the equations can be obtained f rom thermodynamic calculations using the most recent pVT and thermal d ata for the liquid and gas phases of He-4 and He-3. Regarding the He-3 vapour-pressure equation, the paper is an extension to higher tempera tures of the paper by Fellmuth and Schuster which treated the importan t region below 1,5 K. The paper also deals with vapour pressures near the He-4 lambda-transition and near the critical points.