Ok here is the deal. I am looking at constructing a reservior (say 3L in volume) that can act as a constant pressure source (say 100mbar). The theory is that the reservoir will be filled with air with a pump then used as needed. While the volume of air would decrease when a tap was opened, the air pressure would be kept constant by way of a constant force (weight) applied to a moving boundary, either: a) a flexible membrane or b) a rigid disc (imagine a piston being the moving boundary in a cylinder for example) Therefore the air pressure in the reservoir would depend on the weight used. Are these ideas feasible? I realise that the idea (b) would require some good seals to prevent leakage, but this would not be too problematic as long as the pressure was kept constant and the leakage was a small percentage of the total volume. Any thoughts? Any advances on these ideas (reservoir, not pumping solutions)? Cheers.