All submarines which are going to carry humans need to have a life support system aboard. Dry-type pressure hull submarines have very complex life support systems which must scrub contaminants and carbon dioxide from the air inside the hull, as well as replenishing the oxygen, and maintaining humidity and temperature levels.
In order to conform with ISR guidelines, our submarine has a wet hull, ie., it is flooded with water from the surroundings. This reduces the complexity of the life support system that must be carried on board. We still breath air, but we do it from a SCUBA rig. This means that all operators must be SCUBA certified and be comfortable operating in a confined, flooded, dark, underwater space.
ISR rules state that all breathing apparatus must be standard open circuit SCUBA. Open Circuit means that all the air that is breathed out is expelled into the water and is not recirculated. Therefore, there is no scrubbing or oxygen replacement system to worry about. This keeps the system simple. Open circuit also means that the air supply doesn't last as long, since it is being exhausted into the water with every exhale. We're using a standard 80 cubic foot SCUBA tank, which will last an average person, breathing at an at-rest rate, about 2 hours on the surface, or 1 hour at 33 feet. Even though the basin is only 22 feet deep, we have designed for 33 feet. This gives us a good margin of safety, and it represents one atmosphere of added pressure.
