Importance of Third Virial Coefficients for Representing the Gaseous Phase Based on Measuring PVT-Properties of 1,1,1-Trifluoroethane (R143a)

For a reliable derivation of the thermodynamic properties in the gaseous phase from thermodynamic equations of state, it has been pointed out that third virial coefficients significantly affect calculations of heat capacities. Among existing equations of state including internationally accepted equations, there is a large discrepancy, sometimes more than 5%, in calculated heat-capacity values near saturation. Two different approaches have been conducted in addressing this problem. One is for providing the third virial coefficient from intermolecular-potential models based on speed-of-sound measurements with a spherical resonator, and another is for confirming the effect of the third virial coefficient on density values near saturation by measuring the density precisely with a magnetic suspension densimeter. This report is focused on the latter case, i.e., precise measurements of density for 1,1,1-trifluoroethane, R143a, near saturation and some important evidence for the necessity of considering third virial coefficients for calculating reliable thermodynamic properties in the gaseous phase.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.