Basics

Rebreather technology is used in many environments:

• Underwater - where it is sometimes known as CCR = "closed circuit rebreather", "closed circuit scuba" or "semi closed scuba", or CCUBA = "closed circuit underwater breathing apparatus", as opposed to Aqua-Lung-type equipment, which is known as "open circuit scuba".
• Mine rescue and in industry - where poisonous gases may be present or oxygen may be absent.
• Space suits - outer space is, for all intents and purposes, a vacuum where there is no oxygen to support life.
• Hospital anaesthesia breathing systems - to supply controlled proportions of gases to patients without letting anaesthetic gas get into the atmosphere that the staff breathe.
• Submarines and hyperbaric oxygen therapy chambers - where the gas in the habitat must remain safe. Here the rebreather is big and is connected to the air in the habitat.

This article is mainly about diving rebreathers.

As a person breathes, the body consumes oxygen and makes carbon dioxide. A person with an open-circuit breathing set typically only uses about a quarter of the oxygen in the air that is breathed in. The rest is breathed out along with nitrogen and carbon dioxide.

The rebreather recirculates the exhaled gas for re-use and does not discharge it to the atmosphere or water. It absorbs the carbon dioxide, which otherwise would accumulate and cause carbon dioxide poisoning. It removes the carbon dioxide by a process called scrubbing. The rebreather adds oxygen to replace the oxygen that was consumed. Thus, the gas in the rebreather's circuit remains breathable and supports life.

Nearly always, the oxygen comes from a gas cylinder, and the carbon dioxide is absorbed in a canister full of some suitable absorbent such as a form of soda lime. Some absorbent chemical designed for diving applications are Sofnolime, Dragersorb, or Sodasorb. Some systems use a prepackaged Reactive Plastic Curtain (RPC) based cartridge: one brand of these RPC cartridges is ExtendAir.

Pure oxygen is not considered to be safe for recreational diving below 6 meters, so recreational rebreathers and many professional diving rebreathers also have a cylinder of diluent gas. This reduces the percentage of oxygen breathed and enables the unit to be used at greater depths. This diluent cylinder may be filled with compressed air or another diving gas mix such as nitrox or trimix.