[vlad]

If you use 2 compression chambers (2 hydraulic accumulators or 2 sides of cylinder) you can cycle hydraulic oil from one to another. In this case you don't need to start/stop hydraulic pump. You don't need to control gas flow you only need check valves.

Also I didn't get how you want to retract diaphragm? Your gas supply line is max 2 psi. It will take forever to retract diaphragm and push hydraulic fluid back. You can push/pull cylinder but you only can push diaphragm.

Also as you mentioned oxygen is no no. I absolutely agree oxygen is your first enemy. This is why I want to use 1 stage to get 200 Psi. In this case gas will always want to escape and you will never pull air=oxygen in your system. If you use piston to pull gas which is at very low pressure you will get to vacuum and if your seal leaks you get air in....

So, I think 1 stage is kinda unavoidable for less pumping with 2 stage and for keeping air out...