Let me just add that when there is no fluid (such as silicon oil) used to transfer the ambient pressure to the HP diaphragm, such as in the case of dry chamber, it then uses a mechanical pushing rod with a button that sits just under the outer sealing diaphragm. The working area of the outer and inner diaphragm needs to be very close to equal.
IMHO, if the designer can’t get the effective area of both diaphragms to be equal then he lets marketing advertise it as an overpressure first stage or some cockamamie scheme like that…lol. When he does that then he always want to match it to a balanced second stage so that the change in IP doesn’t really affect performance. Otherwise it couldn’t really be adjust it to work at its best at all depth.
The dry chambers with the pushing device are much easier to work with without the mess of pressure transferring fluid. Opening the chamber to adjust the IP is very easy without any mess.
A non environmentally sealed diaphragm first stage is fairly good in cold water since the very cold air doesn’t directly flow near the external moving parts. Similar to non-balanced flow by piston first stage, but better since the only part where ice can build up is the external spring and the rubber diaphragm has some thermal insulating properties. The opposite is a balanced flow through piston where the very cold air flows through the moving piston and environmental chamber, where the spring and moving piston are exposed to ambient water.
