I have a relatively simple design problem but my memories of thermodynamics are very rusty and I can't figure it out on my own. To make it short, I want to put a mix of solid, water and air in a 500mL pressure vessel and heat it all up to 250'C for several weeks. T and P are at room conditions initially and the amount of water, air and solid is known. The solid part has a compressibility that is significant. Both its density and compressibility are known. I need to know what will be the final pressure or at least a rough estimate with a safety margin in order to choose the right pressure vessel to use. The pressure vessel will of course be equipped with a pressure relief valve but I need to keep the water inside the vessel the whole time. I only find cases of heated vessels entirely filled with liquid. I guess the presence of air that is much more compressible than water will act as a buffer and I will get lower pressures than equivalent cases of water filled vessels (is that right?). The volume ratio water/air is not imposed and I can play with it to match a target pressure. I've looked at steam tables, water phase diagrams, thermodynamics basics but I'm still stuck. Anyone can help or direct me to the right resources?