Hello Physics Forum. I'm looking to use the vapor pressure of a liquid to apply a constant pressure to a piston. The vapor pressure of the liquid is higher than the pressure I'm looking to exert (vapor pressure is ~16psi and I need to push with a pressure of 5.5psi). I'm thinking of using an elastomer that is fairly stiff so that it will effectively reduce the pressure from 16psi down to 5.5 due to it's elasticity. Does anyone know what the governing equations would be? I'm thinking of this like an expanding balloon, but as the piston is moving, the pressure needs to remain constant and I'm not even sure that's possible because the starting volume of the liquid is 0.01 mL and the piston needs to move a total volume of 3 mL. What do you folks think, will something like this work?