I have 2 sets for twinsets: 1 with G250V for both primary and secondairy. And the other with XTX40 turned into XTX50 (adjustable screw), bot for primary and backup reg. My recreational set has 2 mk17 first stages and then an R395 as backup and X650 as primary reg. For stages I use apeks and aqualung Titan LX supreme. I have used the Titans first on my twinset.
I have taken my regs on dives over 110m, and it doesn't matter if you breathe from the G250V or from an XTX50. The regs must give gas. The intermediate pressure of the first stages is 9 bar, I screwed it down from 9.5 to 9.
With the scubapro ones, I pull out the pin to hold the cover of the second stage. I want to clean them at the divesite if I want. The covers of Apeks and Aqualung are not soo tight I cannot get them off myself by hand.
Last weekend I had a student with a leaking second stage (seat in the second stage), we took it off (only the second stage), screwed one of my regs on and we did the dive. The second stage does not know if the the first stage if from another brand. There is just an hose between it, so apeks can be used with scubapro etc. It is officially not CE, but the intermediate pressure has to be ok. And 9.5 bar Apeks is 9.5 bar Aqualung or Scubapro.
A second stage can normally have some overpressure before freeflow. So normally a first stage that moves from 9.5 bar to 10.5 bar will not directly give a freeflow. An balanced first stage gives at surface 9.5 bar, but at 100m it is not that the first stage does not work anymore because it still has 9.5 bar, but has an pressure balanced to the ambient pressure, so there is still an overpressure of about 9.5 bar. That was an question I got when I did my first 100m dive, where my regs still working? Yes. But the 9.5 bar intermediate pressure and you was at 11 bar ambient pressure, so did you still get some gas to breathe? Good question
