thebfs:
pressure sensor inside compared to one outside if they differ cap is not on
Fair idea. The issues i would have would be that the pressure could also potentially be the same if the cap is on... and the fact that the sensor would have to be rated for the depth of water in order for it to be outside of the housing.
robtillaart:
It could even compensate for the pressure from outside. At 10 M depth the pressure is 2 Atm, 1ATm for every 10 meter IIRC..
we have already build the walls of the house to withstand the pressure. Would take a little too much air to compensate for 4,000 meters ( ~17,500 feet )....... 
T_Lex:
So, can you have the wires connect to terminals on either side of that plunger-thing at the "top", with the plunger under spring tension pushing it into place to keep the connection. Then a smaller radius pin in the cap can push down the plunger when the cap is place, breaking the connection.
If i understand you properly this would be similar to a standard hood switch on most alarm systems. It was also discussed at one point and we are looking at how to get it into the housing in the most efficient way possible.
be80be:
This would be a simple fix let a spring hit the microswitch The spring would let you over tighten the cap and not break the microswitch.And could easily be setup to insure you have tighten the cap
Also a very good idea, we would have to get the spring sized just right and make sure to keep it locked into the bottom so it wouldn't fall out of the housing when the cap was removed but that shouldn't be too bad.