Hi Guys, I've been lurking a bit around here lately, but this is my first post. I do not have an ROV yet, but am considering it.I just wanted to contribute with something I was thinking of as a simple solution to be able to go deeper... theoretically, very deep. This is just an idea and may be utter rubbish, but maybe not. You decide.
First, a little background of the idea. I am a rebreather diver and one of my simple rebreather kits in particular uses a principle that could be a simple solution to this problem. The unit is a SCR or semi closed rebreather that operates on the principle of a leaky valve with an over-pressure dump valve. The breathing loop is kept replenished by a small trickle of breathing gas fed at a constant rate. When a slight overpressure occurs in the breathing loop, a shoulder dump valve lets the excess leak out. Simple, and works great.
So... for an ROV, rather than fighting the pressure differential, why not just compensate it on a similar principle. An ROV could carry a very small air tank that would gently leak into the pressure compartments; the paintball crowd have some very small, very high pressure tanks. A release valve that is calibrated to dump at a very slight overpressure would keep the internal pressure just a tiny bit above ambient at all times.
One of the considerations in such a setup is that pressure varies very quickly with very minor ascents and descents. For a diver this means that depending on your orientation in the water, the dump valve can require adjusting during the dive. In my mind, an ROV is meant to keep a fairly stable horizontal position, so this is probably a non-issue. Another consideration is that of buoyancy. As an air tank slowly empties, its buoyancy will change quite significantly. This would make the neutrality of the ROV would change during the dive from negative at the beginning to positive during the end. The last consideration, is the sensitivity of electronics to pressure. I really don't know if this is an issue, but don't expect it would be unless some components have sealed air cavities inside.