Closed circuit mixed gas rebreathers

Closed circuit rebreathers (CCR) allow long dives and produce no bubbles most of the time. Closed circuit rebreathers supply two breathing gases to the loop: one is pure oxygen and the other is a diluent gas such as air, nitrox, heliox or trimix.

A major function of the closed circuit rebreather is to control the oxygen partial pressure in the loop and to warn the diver if it becomes dangerously low or high. Too low a concentration of oxygen results in hypoxia leading to unconsciousness and ultimately death. Too high a concentration of oxygen results in hyperoxia, leading to oxygen toxicity, a condition causing convulsions which can make the diver lose the mouthpiece when they occur underwater, and can lead to drowning. The gas mixture is controlled by the diver in manually controlled closed circuit rebreathers by adding diluent gas or oxygen. Adding diluent can prevent the loop gas mixture becoming too oxygen rich, and adding oxygen increases oxygen concentration.

In fully automatic closed-circuit systems, an electronically controlled solenoid valve injects oxygen into the loop when the control system detects that the partial pressure of oxygen in the loop has fallen below the required level. Electronically controlled CCRs can be switched to manual control in the event of some control system failures. Addition of gas to compensate for compression during descent is usually done by an automatic diluent valve.