Don’t know if you are still looking for a low pressure regulator, but I had some thoughts. If you need a small finite volume of air (or gas) for an experiment, you could simply use a thin plastic bag large enough to hold the required amount of gas, with a weight on top to set the pressure. If you need more gass than conveniently fits in a bag, add a supply port and a switch and solenoid valve that feeds in more gas when the weight falls too far. Basically a low pressure version of a hydraulic accumulator.