The Analysis of4He Isotherms: Density and Dielectric Virial Coefficients, and the Accuracy of NPL-75

The highly accurate conventional isotherms of Berry and dielectric constant isotherms of Gugan and Michel have been analysed in a mutually consistent way between 2.6 K and 27 K by the method of surface-fitting. Semi-empirical expressions are presented for the variation of the virial coefficients C ( T ) and D ( T ). A variety of expressions which conform to the expected theoretical constraints have been examined for B ( T ): the best of these agrees closely down to about 9 K with values recently calculated by Feltgen et al. using a potential derived from scattering experiments. The second dielectric virial coefficient b ( T ) shows a temperature dependence reasonably close to that calculated from the pair-polarizability given by Dacre, and the internal consistency of these results suggests that the values obtained for B ( T ) have an uncertainty of about ± 0.05 cm 3 mol -1 down to 6 K, rising to perhaps ± 0.2 cm 3 mol -1 below this. The temperature scale NPL-75 is assessed against several recent experiments: while there are indications of minor discrepancies the evidence is not conclusive and NPL-75 appears to be accurate at very close to the level originally claimed.