I have a closed pressure vessel that is partially filled with water and the head space has been purged with He and charged to about 100 psi. What I wish to know is how to calculate the overall pressure in this vessel after heating it to a given temperature (e.g., 300 or 400 C). I know the volume of water and the head space at ambient temperature. I think I am having difficulties because I keep wanting to apply an equation of state such as the Redlich-Kwang, BWR, or Soave but these are for gases and in my case I have a liquid and a gas. It is likely that all of the liquid will exist in the vapor state at an end temperature of 400 C but likely it will be a saturated vapor/liquid mix at 300 C.