Between the critical point and the triple point is the saturation region of fluids. In this region, the gaseous phase coexists with the liquid phase under saturation pressure P sat , and the saturation temperature T sat . Under the saturation pressure, the liquid phase has a molar volume of v l , and the gaseous phase has a molar volume of v g . The corresponding molar densities are ρ ρ l and ρ ρ g . These are the saturation properties needed to compute second and third virial coefficients.