We consider chemical equilibrium, so we have equality of chemical potential at the boundary of the vapour, then we have also equality of the fugacity, the boundary and the vapour, fugacity is equivalent to pressures, and also the equality of the fugacity of saturation.
Above the critical point of water the entity of absorption is given by this expression, and the absorbed water potential is given here. If we combine this we obtain this equation, which is very similar to the Kelvin-Laplace's law, below the critical point, we have just to identify. So if we make this identification we can expand the model also beyond the critical point, we can go up to 1.000, to 1.200 degrees centrigrade, which have been measured in tunnel fires for instance.
