Rebreather

A rebreather is a closed-circuit 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. Fresh oxygen is added to replenish the amount metabolised by the user. This differs from open-circuit breathing apparatus, such as:

• the oxygen enrichment systems primarily used by patients and by high altitude mountaineers, in which the user breathes ambient air which is enriched by a small amount (2-5 l/min) of pure oxygen,
• open circuit breathing apparatus used by firefighters and scuba divers, which supplies fresh gas for each breath, which is then discharged into the environment.

A fit person working hard may ventilate at a rate of 100 l/min but will only metabolise a maximum of 6l/min of oxygen (Sleeping requires ½ to 1 l/min). A high ventilation rate is usually necessary to eliminate the metabolic product carbon dioxide (CO₂). The breathing reflex is triggered by CO₂ concentration in the blood, not by the oxygen concentration, therefore even a small buildup of CO₂ in the inhaled gas quickly becomes intolerable; if a person tries to rebreathe their exhaled breathing gas, they will quickly feel an acute sense of suffocation, therefore rebreathers must absorb the CO₂, and add the small amount of oxygen used to maintain the required concentration of oxygen.

Rebreather technology is used in many environments:

• Underwater – where it is variously known as "closed circuit scuba", "closed circuit rebreather" (CCR), "semi-closed scuba", "semi-closed rebreather" (SCR), "closed circuit underwater breathing apparatus" (CCUBA - a military term), or just "rebreather", as opposed to "open circuit scuba" where the diver exhales breathing gas into the surrounding water.
• Mine rescue and in industry – where poisonous gases may be present or oxygen may be absent.
• Crewed spacecraft and space suits – outer space is, effectively, a vacuum with no oxygen to support life.
• Hospital anaesthesia breathing systems – to supply controlled concentrations of anaesthetic gases to patients without contaminating the air that the staff breathe.
• Himalayan mountaineering. Both chemical and compressed oxygen has been used in experimental closed-circuit oxygen systems—the first on Mt. Everest in 1938. A high rate of system failures due to extreme cold has not been solved. Breathing pure oxygen results in an elevated partial pressure of oxygen in the blood. A climber breathing pure oxygen at the summit of Mt. Everest has a greater oxygen partial pressure than breathing air at sea level. This results in being able to exert greater physical effort at altitude.
• Submarines, underwater habitats and saturation diving systems use a scrubber system working on the same principles as a rebreather.
• Surface supplied diving equipment may incorporate rebreather technology either as a gas reclaim system, where the surface supplied breathing gas is returned and scrubbed at the surface, or as a self-contained diver bailout system.