A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantial unused oxygen content, and unused inert content when present, 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 directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, while also eliminating the bubbles otherwise produced by an open circuit system. The latter advantage over other systems is useful for covert military operations by frogmen, as well as for undisturbed observation of underwater wildlife. A rebreather is generally understood to be a portable apparatus carried by the user. The same technology on a vehicle or non-mobile installation is more likely to be referred to as a life-support system.

Rebreather technology may be used where breathing gas supply is limited, such as underwater, in space, where the environment is toxic or hypoxic (as in firefighting), mine rescue, high-altitude operations, or where the breathing gas is specially enriched or contains expensive components, such as helium diluent or anaesthetic gases.

Rebreathers are used in many environments: underwater, diving rebreathers are a type of self-contained underwater breathing apparatus which have provisions for both a primary and emergency gas supply. On land they are used in industrial applications where poisonous gases may be present or oxygen may be absent, firefighting, where firefighters may be required to operate in an atmosphere immediately dangerous to life and health for extended periods, in hospital anaesthesia breathing systems to supply controlled concentrations of anaesthetic gases to patients without contaminating the air that the staff breathe, and at high altitude, where the partial pressure of oxygen is low, for high altitude mountaineering. In aerospace there are applications in unpressurised aircraft and for high altitude parachute drops, and above the Earth's atmosphere, in space suits for extra-vehicular activity. Similar technology is used in life-support systems in submarines, submersibles, atmospheric diving suits, underwater and surface saturation habitats, spacecraft, and space stations, and in gas reclaim systems used to recover the large volumes of helium used in saturation diving.

The recycling of breathing gas comes at the cost of technological complexity and specific hazards, some of which depend on the application and type of rebreather used. Mass and bulk may be greater or less than open circuit depending on circumstances. Electronically controlled diving rebreathers may automatically maintain a partial pressure of oxygen between programmable upper and lower limits, or set points, and be integrated with decompression computers to monitor the decompression status of the diver and record the dive profile.

General concept

As a person breathes, the body consumes oxygen and produces carbon dioxide. Base metabolism requires about 0.25 L/min of oxygen from a breathing rate of about 6 L/min, and a fit person working hard may ventilate at a rate of 95 L/min but will only metabolise about 4 L/min of oxygen.

However, if this is done without removing the carbon dioxide, it will rapidly build up in the recycled gas, resulting almost immediately in mild respiratory distress, and rapidly developing into further stages of hypercapnia, or carbon dioxide toxicity.

A high ventilation rate is usually necessary to eliminate the metabolic product carbon dioxide (CO<sub>2</sub>). The breathing reflex is triggered by CO<sub>2</sub> concentration in the blood, not by the oxygen concentration, so even a small buildup of CO<sub>2</sub> in the inhaled gas quickly becomes intolerable; if a person tries to directly rebreathe their exhaled breathing gas, they will soon feel an acute sense of suffocation, so rebreathers must remove the CO<sub>2</sub> in a component known as a carbon dioxide scrubber.

By adding sufficient oxygen to compensate for the metabolic usage, removing the carbon dioxide, and rebreathing the gas, most of the volume is conserved.

[[File:Physiological effects of carbon dioxide concentration and exposure period.png|thumb|upright=1.3|Relation of physiological effects to carbon dioxide concentration and exposure period.

Breathing hoses are usually long enough to connect the apparatus to the user's head in all attitudes of their head, but should not be unnecessarily long, which will cause additional weight, hydrodynamic drag, risk snagging on things, or contain excess dead space in a pendulum rebreather. Breathing hoses can be tethered down to a diver's shoulders or ballasted for neutral buoyancy to minimise loads on the mouthpiece.

Mouthpiece or facemask

A mouthpiece with bite-grip, an oro-nasal mask, a full-face mask, or a sealed helmet is provided so that the user can breathe from the unit hands-free.

Oxygen supply

A store of oxygen, usually as compressed gas in a high pressure cylinder, but sometimes as liquid oxygen, that feeds gaseous oxygen into the ambient pressure breathing volume, either continuously, or when the user operates the oxygen addition valve, or via a demand valve in an oxygen rebreather, when the volume of gas in the breathing circuit becomes low and the pressure drops, or in an electronically controlled mixed gas rebreather, after a sensor has detected insufficient oxygen partial pressure, and activates a solenoid valve.

Valves

Valves are needed to control gas flow in the breathing volume, and gas feed from the storage container. They include:

  • Non-return valves in the breathing loop of loop rebreathers, which enforce one-directional flow to minimise dead space,
  • Dive/surface valves on diving rebreathers, which prevent water from entering the breathing volume when the mouthpiece is removed, or the user elects to breathe ambient air at the surface.
  • Gas supply valves, including a cylinder valve, to allow high pressure gas to flow from the cylinder. This may be manually operated by the user to directly supply make-up gas, or may provide the gas to a pressure regulator which reduces the pressure to a few bar above ambient pressure, and supplies this intermediate pressure gas to the gas feed system, which may contain one or more of:
  • Manually operated feed valve,
  • Constant mass flow orifice or needle valve, to provide a continuous feed,
  • Demand valve which automatically adds gas when the volume of the counterlung(s) is too low, and pressure in the breathing volume drops below ambient pressure.
  • Overpressure valve, to release excess gas. This is mainly used in diving rebreathers to compensate for expansion during ascent. Excess gas may also be vented past the skirt seal of a full-face mask, or through the nose when a mouthpiece is used.

Oxygen sensors

Oxygen sensors may be used to monitor partial pressure of oxygen in mixed gas rebreathers to ensure that it does not fall outside the safe limits, but are generally not used on oxygen rebreathers, as the oxygen content is fixed at 100%, and its partial pressure varies only with the ambient pressure.

System variants

Rebreathers can be primarily categorised as diving rebreathers, intended for hyperbaric use, and other rebreathers used at pressures from slightly more than normal atmospheric pressure at sea level to significantly lower ambient pressure at high altitudes and in space. Diving rebreathers must often deal with the complications of avoiding hyperbaric oxygen toxicity, while normobaric and hypobaric applications can use the relatively trivially simple oxygen rebreather technology, where there is no requirement to monitor oxygen partial pressure during use providing the ambient pressure is sufficient.

Rebreathers can also be subdivided by functional principle as closed circuit and semi-closed circuit rebreathers.

  • : A closed circuit rebreather adds oxygen to the loop gas to make up for oxygen used by metabolic processes. These processes do not use diluent gas, so none is added unless the volume of the loop is reduced for other reasons, such as intentional dumping, flushing, or an ambient pressure change. Gas is dumped from the loop when it expands during a pressure reduction, or too much is added.
  • , also known as a gas extender: A semi-closed circuit rebreather either dumps some loop gas nearly constantly or constantly adds gas to the loop, and consequently needs an inflow of both diluent and oxygen to make up the volume. Changes in ambient pressure also require changes in the number (mass) of gas in the loop to maintain the working volume.

Oxygen rebreathers

<!-- target from redirect ]] Closed circuit oxygen rebreather]] -->

thumb|upright|Siebe Gorman Proto 1 mine rescue rebreather, a simple oxygen rebreather.

<!-- thumb|upright=1.3|Schematic diagram of a closed circuit oxygen rebreather with a pendulum configuration and radial flow scrubber -->

<!-- thumb|upright=1.3|Schematic diagram of a closed circuit oxygen rebreather with a loop configuration and axial flow scrubber -->

This is the earliest type of rebreather and was commonly used by navies for submarine escape and shallow water diving work, for mine rescue, high altitude mountaineering and flight, and in industrial applications from the early twentieth century. Oxygen rebreathers can be remarkably simple and mechanically reliable, and they were invented before open-circuit scuba. They only supply oxygen, so there is no requirement to control the gas composition other than removing the carbon dioxide.

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  • Anesthesia breathing systems
  • NIOSH Docket # 123, titled "Reevaluation of NIOSH limitations on and precaution for safe use of positive-pressure closed-circuit SCBA" is available at web.archive.org