Mixed-Gas Closed-Circuit Rebreathers
Mixed-gas closed-circuit rebreathers (CCR) improve upon SCR designs. CCRs are the most advanced rebreather systems and require the most upkeep and maintenance. They are also the most expensive and complex to operate and require the most extensive training. CCRs offer the best gas economy, reduced decompression obligations, and provide extended depth capabilities.
They operate by injecting oxygen, as needed, into the loop to compensate for your metabolic oxygen consumption rate. To do this, they must monitor the PPO2 in the breathing loop. When the PPO2 drops, additional oxygen is injected to keep the PPO2 constant.
Since the PPO2 within the loop must be monitored at all times, CCRs use an electronic control device and oxygen sensors. CCRs were designed to deliver a gas mixture with a constant PPO2 no matter what the depth, which means you’re always breathing the best possible mix for the depth you’re at.
CCRs also use two gas cylinders — one for oxygen and one for a diluent that is used to dilute the oxygen in the breathing loop so that a constant PPO2 is maintained throughout the dive. The most common diluent is air, but trimix and heliox are also used in CCRs for deep diving. Whatever diluent is used, it must be capable of sustaining life if you would have to use it alone.
The electronics in CCRs are responsible for controlling the addition of oxygen through an electrically operated valve. This valve keeps the PPO2 constant no matter what your metabolic consumption rate or changes in depth. The electronics are either analog or computer-controlled and three redundant oxygen sensors are used to maintain a constant PPO2. In addition to electronic valve, manual addition valves allow you to manually add oxygen or diluent to the breathing loop in case of an emergency. Because maintaining an accurate and safe PPO2 is required throughout a dive, most systems have a display of some sort to notify you what the actual PPO2 is within the breathing loop.
Source:
Boxanic, Jeffrey E. Understanding Rebreathers. Flagstaff, AZ: Best Publishing Company, 2002.