Oxygen Closed-Circuit Rebreathers
Oxygen closed-circuit rebreathers are oxygen-only breathing rebreathers and are the most simple design and most inexpensive type of rebreather. Because there is no helium or nitrogen to breathe, there are never any decompression requirements.
Oxygen closed-circuit rebreather systems use a valve to add the oxygen into the breathing loop. The valve is designed to keep the oxygen supply constant, meaning that as you metabolize oxygen or descend deeper, the valve adds oxygen to compensate. With this system, there is no way to vent over pressurized gas (no over pressurization valve), which means that an oxygen closed-circuit rebreather is the only true fully closed-circuit rebreather.
Oxygen closed-circuit rebreathers can be operated with zero bubbles as long as you maintain a certain depth or ascend slowly enough to make up for gas expansion. When using this type of rebreather, you must not ascend faster than your metabolic consumption of oxygen if you want to maintain bubble-free diving or risk pulmonary embolism. If you do ascend to quickly, you have to vent gas from your mask to avoid injury.
Breathing oxygen greater than 20 feet (6 m) deep is risky due to an oxygen partial pressure of 1.6 ATM and CNS oxygen toxicity. This means that an oxygen closed-circuit rebreather must be used at depths shallower than 20 feet (6 m). If you want to maintain the recommended 1.4 ATM, you cannot exceed 13 feet (4 m).
The military is the biggest users of oxygen closed-circuit rebreathers. Combat swimmers use this type of rebreather for shallow water operations requiring complete cover. Recreational use of oxygen closed-circuit rebreathers is very limited and thus is not used very often.
Source:
Boxanic, Jeffrey E. Understanding Rebreathers. Flagstaff, AZ: Best Publishing Company, 2002.<>