Hi. This is in response to the diver weight compensation question.
Every dive trip in your sub requires careful measurement of what you bring on-board.
If the dive plan calls for a diver to exit the sub while at depth, I would think that it would be better to add buoyancy to cover the divers extra weight while on-board, and then dump that buoyancy ( air bubble ) when the diver exits the sub, there-by maintaining neutral buoyancy while the diver is outside.
When the diver is back on-board, blow the required amount of air back into the trim tank ( a measured amount of water to compensate for exactly how much the diver weighs ) and again maintaining neutral buoyancy.
A sub with a diver lock-out function would need a bigger trim tank than a sub without that capability.
Don't forget to calculate how much air the diver displaces with his body, as this will figure into the required additional buoyancy too.
You don't need a separate pump system, nor would that be desirable, to let a diver exit and re-enter the sub. The lock-out chamber is controlled by air pressure, and any small amount of water that remained within would be easily compensated for by the additional trim tank volume.
You WILL need some means of dumping the air from the lock-out chamber once the water is blown out and the hatch is closed so the diver is once again at one atmosphere. This needs to be monitored closely so a diver has time to dump the excess nitrogen absorbed by his body tissues and blood. A diver lock-out chamber is really much like a de-compression chamber, but is located inside another chamber ( the sub's pressure hull ) so a compressor to drive out the additional air is required.
Frank D.