This equation computes the specific volume of a two phase system, νν from the specific volume of the saturated liquid, the specific volume of the saturated vapor, and the mass fraction of the vapor phase.
vfvf = specific volume of the saturated liquid
vgvg = specific volume of the saturated vapor
xx = mass fraction (mgmg+mfmgmg+mf)of the vapor phase, which is a function of the mass of vapor, mgmg, and the mass of liquid, mfmf.
This equation, Specific Volume of Two-Phase System, references 1 page