Thanks Hugh & Jon for the vote of confidance.
I did qualify this with " In an emergency situation". I think I have been misunderstood.
You take your sub down to its 250ft limit with several extra dive tanks on board.
At this point the outside of your hull is experiencing round 125psi.
For every 33ft further you go down you add 14.7psi. The hull would always be experiencing
125psi from the outside as the pressure you're adding would be countering the additional
pressure from additional depth.
So no pressure as such from the inside would be pushing your view ports out.
The reason I said to increase the O2 flow into the hull is the "bellows add" system based on
sensing a drop in pressure wouldn't work. In that system the O2 flow is set below the users
normal O2 consumption & then topped up from the bellows add system because if the O2
was set too high there would be a continual pressure build up & 02 % increase.
You could safely bump up the O2 a tad as it would be safer to have too much O2 than not
enough. This, as Jon said wouldn't matter much because your time would be constrained by
decompression tables, & there would be enough air in the hull to breath from.
So in a life or death emergency you could take several air tanks into your hull & a set of
dive timetables, open the tanks by hand at the 250ft mark & go to 400ft.
Alan