Security valves with two connectors open if the pressure exceed a value x +/- tolerance on one side. I'd like to know if there's a mechanical valve which opens if the pressure on one side sinks below a certain value.
The use case is a system with a desired pressure d (e.g. 2 bar), but irregular pressure during operation (because it's based on bio mass fermentation) and a gas storage which stores extranous gas if the pressure is above 2 bar. This storage needs a value which can release pressure from the storage into the system if the pressure in the system sinks below the desired pressure.