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 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. oxygen rebreathers siebe gorman proto 1 mine rescue rebreather, a simple oxygen rebreather. 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. oxygen feed options in some rebreathers the oxygen cylinder has oxygen supply mechanisms in parallel. one is constant flow; the other is a manual on-off valve called a bypass valve; both feed into the same hose which feeds the counterlung. others are supplied via a demand valve on the counterlung. this will add gas at any time that the counterlung is emptied and the diver continues to inhale. oxygen can also be added manually by a button which activates the demand valve. some simple oxygen rebreathers had no automatic supply system, but only the manual feed valve, and the diver had to operate the valve at intervals to refill the breathing bag as the volume of oxygen decreased below a comfortable level. mixed gas rebreathers mixed gas electronically controlled closed circuit diving rebreather all rebreathers other than oxygen rebreathers may be considered mixed gas rebreathers, as the breathing gas is a mixture of oxygen and metabolically inactive diluent gas. these can be divided into semi-closed circuit, where the supply gas is a breathable mixture containing oxygen and inert diluents, usually nitrogen and helium, and which is replenished by adding more of the mixture as the oxygen is used up, sufficient to maintain a breathable partial pressure of oxygen in the loop, and closed circuit rebreathers, where two parallel gas supplies are used: the diluent, to provide the bulk of the gas, and which is recycled, and oxygen, which is metabolically expended. carbon dioxide is considered a waste product, and in a correctly functioning rebreather, is effectively removed when the gas passes through the scrubber. rebreathers using an absorbent that releases oxygen russian multipurpose rebreather ida-71 with lid of casing opened showing interior – one of the scrubber canisters could optionally be filled with superoxide. there have been a few rebreather designs (e.g. the oxylite) which use potassium superoxide, which gives off oxygen as it absorbs carbon dioxide, as the carbon dioxide absorbent: 4ko2 + 2co2 = 2k2co3 + 3o2. a small volume oxygen cylinder is needed to fill and purge the loop at the start of the dive. this technology may be applied to both oxygen and mixed gas rebreathers, and can be used for diving and other applications. rebreathers which use liquid oxygen aerorlox liquid oxygen rebreather in a coal mining museum a liquid oxygen supply can be used for oxygen or mixed gas rebreathers. if used underwater, the liquid-oxygen container must be well insulated against heat transfer from the water. industrial sets of this type may not be suitable for diving, and diving sets of this type may not be suitable for use out of water due to conflicting heat transfer requirements. the set's liquid oxygen tank must be filled immediately before use. examples of the type include: blackett's aerophor aerorlox cryogenic rebreather a cryogenic rebreather removes the carbon dioxide by freezing it out in a "snow box" by the low temperature produced as liquid oxygen evaporates to replace the oxygen used. fields of application underwater – as a self-contained breathing apparatus, where it is sometimes known as "closed circuit scuba" as opposed to "open circuit scuba" where the diver exhales breathing gas into the surrounding water. 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 gas extender carried by the diver. rebreathers may also be used as self-contained diver bailout systems for either scuba or surface-supplied diving. mine rescue and other industrial applications – where poisonous gases may be present or oxygen may be absent. fire fighting, where personnel may be required to operate in an atmosphere immediately dangerous to life and health for longer periods than open-circuit self-contained breathing apparatus (scba) can provide air. crewed spacecraft and space suits – outer space is, effectively, a vacuum without oxygen to support life. himalayan or high-altitude mountaineering. high altitude reduces the partial pressure of oxygen in the ambient air, which reduces the ability of the climber to function effectively. mountaineering rebreathers are closed-circuit oxygen sets that provide a higher partial pressure of oxygen to the climber than the ambient air. medical anaesthesia breathing systems – to supply controlled concentrations of anaesthetic gases to patients without contaminating the air that the staff breathe, and to conserve anaesthetic gas. life-support systems of submarines, underwater habitats, and saturation diving systems use a scrubber system working on the same principles as a rebreather. this may be compared with some applications of open-circuit breathing apparatus: the oxygen enrichment systems primarily used by medical patients, high altitude mountaineers and commercial aircraft emergency systems, in which the user breathes ambient air which is enriched by the addition of pure oxygen, open circuit breathing apparatus used by firefighters, underwater divers and some mountaineers, which supplies fresh gas for each breath, which is then discharged into the environment. gas masks and respirators which filter contaminants from ambient air which is then breathed. diving rebreathers siva underwater mine countermeasures (mcm) diving apparatus (oxygen rebreather) the widest variety of rebreather types is used in diving, as the consequences of breathing under pressure complicate the requirements, and a large range of options are available depending on the specific application and available budget. a diving rebreather is safety-critical life-support equipment – some modes of failure can kill the diver without warning, others can require immediate appropriate response for survival. surface supplied diving gas reclaim systems a helium reclaim system (or push-pull system) is used to recover helium based breathing gas after use by the diver when this is more economical than losing it to the environment in open circuit systems. the recovered gas is passed through a scrubber system to remove carbon dioxide, filtered to remove odours, and pressurised into storage containers, where it may be mixed with oxygen to the required composition for re-use, either immediately, or at a later date. saturation diving life-support systems the life support system provides breathing gas and other services to support life for the personnel under pressure in the accommodation chambers and closed diving bell. it includes the following components: breathing gas supply, distribution and recycling equipment: scrubbers, filters, boosters, compressors, mixing, monitoring, and storage facilities chamber climate control system - control of temperature and humidity, and filtration of gas instrumentation, control, monitoring and communications equipment fire suppression systems sanitation systems the life support system for the bell provides and monitors the main supply of breathing gas, and the control station monitors the deployment and communications with the divers. primary gas supply, power and communications to the bell are through a bell umbilical, made up from a number of hoses and electrical cables twisted together and deployed as a unit. this is extended to the divers through the diver umbilicals. the accommodation life support system maintains the chamber environment within the acceptable range for health and comfort of the occupants. temperature, humidity, breathing gas quality, sanitation systems, and equipment function are monitored and controlled. atmospheric diving suits us navy atmospheric diving suit an atmospheric diving suit is a small one-man articulated submersible of roughly anthropomorphic form, with limb joints which allow articulation under external pressure while maintaining an internal pressure of one atmosphere. breathing gas supply may be surface supplied by umbilical, or from a rebreather carried on the suit. an emergency gas supply rebreather may also be fitted to a suit with either surface supply or rebreather for primary breathing gas. as the internal pressure is maintained at one atmosphere, there is no risk of acute oxygen toxicity. this is an underwater diving application, but has more in common with industrial applications than with ambient pressure scuba rebreathers. industrial and rescue self-contained rebreathers mine rescue rebreathers in a czech museum different design criteria apply to scba rebreathers for use only out of the water: there is no variation in ambient pressure on the components. the counterlung may be placed for comfort and convenience. cooling of the gas in the breathing loop may be desirable, as the absorbent produces heat as it reacts with carbon dioxide, and the warming of the gas is not welcome in hot industrial situations such as firefighting and deep mines. absorbent containers may in some cases rely on gravity for preventing channeling. if a full-face mask is used, it may have viewports designed for convenience or improved field of vision, and they do not need to be flat and parallel to prevent visual distortion as when underwater. in firefighting rebreathers, consideration must be given to making the set reasonably flame-proof and protecting it from heat and debris impacts. the need to ditch the set quickly may not arise, and harness straps may not need a quick-release. buoyancy is not a consideration, but