I'm a little surprised CO2 isn't an issue already. The way it works in SH3 is pretty good, but can be improved upon.
If your crew fires lots of torpedoes while submerged, or does a lot of depth-changing, then the CO2 level could rise quicker (presumably the crew are stressed and working hard to do tasks).
Simulate also the engines "breathing" air from inside the sub depending on how hard they're working when the hatch is closed.
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