An Analysis of the ITS-90 Relations for the Non-Ideality of³He and⁴He: Recommended Relations Based on a New Interatomic Potential for Helium

Quantum calculations for the second virial coefficients of 3 He and 4 He based on an interatomic potential have been refined to the point where the accuracy of the calculations is at level which is an order of magnitude better than the experimental error associated with the measured virials. A new accurate potential has been constructed for the ground state of He 2 and new relations for the non-ideality of 3 He and 4 He based on this potential are recommended. These relations, which are more physically based, appear preferable to those proposed for the International Temperature Scale of 1990 (ITS-90). For 4 He the suggested relation for the second virial coefficients is almost identical with the corresponding relation of the ITS-90 scale in the region of applicability (3-25 K) but is more realistic below that range. The suggested relation for 3 He second virial coefficients agrees more closely with the experimental data (1,47 to 20,1 K) after a correction has been made for the presence of third virial coefficients.