1) Breathing gas 
A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user`s exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content of each breath. Oxygen is added to replenish the amount metabolised by the user. This differs from open-circuit breathing apparatus, where the exhaled gas is discharged d....
an underwater breathing system that recycles air; divers inhale air from a tank and exhale air into a machine that recycles the air and makes it available for the diver again; used for longer and deeper research and military dives
a closed-circuit system which filters exhaled air and recirculates it for rebreathing by the diver. Using a rebreather requires special training.
The basic idea is
Breathing equipment that captures, cleanses and re-oxygenates exhaled breath so that it can be re-inhaled.