rebreather rebreather 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, 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, and, for covert military use by frogmen or observation of underwater life, eliminating the bubbles produced by an open circuit system and in turn not scaring wildlife being filmed. a rebreather is generally understood to be a portable unit 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. rebreathera fully closed circuit electronic rebreather (ap diving inspiration)acronymccuba (closed circuit underwater breathing apparatus); ccr (closed circuit rebreather), scr (semi-closed rebreather)usesbreathing setrelated itemsdavis apparatus, self-contained breathing apparatus, escape hood rebreather technology may be used where breathing gas supply is limited, such as underwater or in space, where the environment is toxic or hypoxic, as in firefighting, mine rescue and 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 the oxygen metabolised is generally about 4% to 5% of the inspired volume at normal atmospheric pressure, or about 20% of the available oxygen in the air at sea level. exhaled air at sea level contains roughly 13.5% to 16% oxygen. the situation is even more wasteful of oxygen when the oxygen fraction of the breathing gas is higher, and in underwater diving, the compression of breathing gas due to depth makes the recirculation of exhaled gas even more desirable, as an even larger proportion of open circuit gas is wasted. continued rebreathing of the same gas will deplete the oxygen to a level which will no longer support consciousness, and eventually life, so gas containing oxygen must be added to the breathing gas to maintain the required concentration 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 (co2). the breathing reflex is triggered by co2 concentration in the blood, not by the oxygen concentration, so even a small buildup of co2 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 co2 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. relation of physiological effects to carbon dioxide concentration and exposure period. effects of different levels of oxygen partial pressure po2(bar) application and effect <0.08coma ultimately leading to death 0.08-0.10unconsciousness in most people 0.09-0.10serious signs/symptoms of hypoxia 0.14-0.16initial signs/symptoms of hypoxia (normal environment oxygen in some very high altitude areas) 0.21normal environment oxygen (sea level air) 0.35–0.40normal saturation dive po2 level 0.50threshold for whole-body effects; maximum saturation dive exposure 1.0–1.20common range for recreational closed circuit set point 1.40recommended limit for recreational open circuit bottom sector 1.60noaa limit for maximum exposure for a working diverrecreational/technical limit for decompression 2.20commercial/military "sur-d" chamber surface decompression on 100% o2 at 12 msw (meters of sea water) 2.4040% o2 nitrox recompression treatment gas for use in the chamber at 50 msw 2.80100% o2 recompression treatment gas for use in the chamber at 18 msw 3.0050% o2 nitrox recompression treatment gas for use in the chamber at 50 msw architecture comparison of pendulum and loop rebreather architecture1 mouthpiece with dive/surface valve2 two way breathing hose2a exhalation hose with non-return valve2b inhalation hose with non-return valve3 scrubber4 counterlung5 automatic make-up valve6 manual bypass valve7 breathing gas storage cylinder8 cylinder valve9 regulator first stage10 cylinder pressure gauge11 over-pressure valve there are two basic arrangements controlling the flow of breathing gas inside the rebreather, known as the pendulum and loop systems. pendulum in the pendulum configuration, the user inhales gas from the counterlung through a breathing hose, and exhaled gas returns to the counter lung by flowing back through the same hose. the scrubber is usually between the breathing hose and the counterlung bag, and gas flow is bi-directional. all of the flow passages between the user and the active absorbent in the scrubber are dead space – volume containing gas which is rebreathed without modification by the rebreather. the dead space increases as the absorbent is depleted. breathing hose volume must be minimised to limit dead space. loop in the loop configuration, the user inhales gas through one hose, and exhales through a second hose. exhaled gas flows into the scrubber from one side, and exits at the other side. there may be one large counterlung, on either side of the scrubber, or two smaller counterlungs, one on each side of the scrubber. flow is in one direction, enforced by non-return valves, which are usually in the breathing hoses where they join the mouthpiece. only the flow passage in the mouthpiece before the split between inhalation and exhalation hoses is dead space, and this is not affected by hose volume. components oxygen rebreather kip-8 interior showing the basic components russian firefighting rebreather kip-8 exterior there are some components that are common to almost all personal portable rebreathers. these include the ambient pressure breathing volume components, usually called the breathing loop in a circulating flow rebreather, and the make-up gas supply and control system. counterlung the counterlung is an airtight bag of strong flexible material that holds the volume of the exhaled gas until it is inhaled again. there may be a single counterlung, or one on each side of the scrubber, which allows a more even flow rate of gas through the scrubber, which can reduce work of breathing and improve scrubber efficiency by a more consistent dwell time. scrubber the scrubber is a container that has an inlet on one side and an outlet on the other side, filled with carbon dioxide absorbent material, mostly strong bases, through which the exhaled gas passes to remove the carbon dioxide. the absorbent may be granular or in the form of a moulded cartridge. granular sorb may be manufactured by breaking up lumps of lime and sorting the granules by size, or by moulding granules at a consistent size and shape. gas flow through the scrubber may be in one direction in a loop rebreather, or both ways in a pendulum rebreather. a typical absorbent is sodalime, which is made up of calcium hydroxide ca(oh)2, potassium hydroxide koh, and/or sodium hydroxide naoh. (either or both of these may be present). the main component of soda lime is calcium hydroxide, which is relatively cheap and easily available. other components may be present in the absorbent. sodium hydroxide is added to accelerate the reaction with carbon dioxide. other chemicals may be added to prevent unwanted decomposition products when used with standard halogenated inhalation anaesthetics. an indicator may be included to show when carbon dioxide has dissolved in the water of the soda lime and formed carbonic acid, changing the ph from basic to acid, as the change of colour shows that the absorbent has reached saturation with carbon dioxide and must be changed. the carbon dioxide combines with water or water vapor to produce a weak carbonic acid: co2 + h2o –> h2co3. this reacts with the hydroxides to produce carbonates and water in an exothermic reaction: in the intermediate reaction, the carbonic acid reacts exothermically with sodium hydroxide to form sodium carbonate and water: h2co3 + 2naoh –> na2co3 + 2h2o + heat. in the final reaction, the sodium carbonate reacts with the slaked lime (calcium hydroxide) to form calcium carbonate and sodium hydroxide: na2co3 + ca(oh)2 –> caco3 + 2naoh. the sodium hydroxide is then available again to react with more carbonic acid. 100 grams (3.5 oz) of this absorbent can remove about 15 to 25 litres (0.53 to 0.88 cu ft) of carbon dioxide at standard atmospheric pressure this process also heats and humidifies the air, which is desirable for diving in cold water, or climbing at high altitudes, but not for working in hot environments. other reactions may be used in special circumstances. lithium hydroxide and particularly lithium peroxide may be used where low mass is important, such as in space stations and space suits. lithium peroxide also replenishes the oxygen during the scrubbing reaction. another method of carbon dioxide removal occasionally used in portable rebreathers is to freeze it out, which is possible in a cryogenic rebreather which uses liquid oxygen. the liquid oxygen absorbs heat from the carbon dioxide in a heat exchanger to convert the oxygen to gas, which is sufficient to freeze the carbon dioxide. this process also chills the gas, which is sometimes, but not always, desirable. breathing hoses draeger ray dive/surface valve with bite grip scuba mouthpiece, non-return valves, and breathing hoses a breathing hose or sometimes breathing tube on a rebreather is a flexible tube for breathing gas to pass through at ambient pressure. they are distinguished from the low-, intermediate-, and high-pressure hoses which may also be parts of rebreather apparatus. they have a wide enough bore to minimise flow resistance at the ambient pressure in the operational range for the equipment, are usually circular in cross section, and may be corrugated to let the user's head move about without the tube collapsing at kinks. each end has an airtight connection to the adjacent component, and they may contain a one-way valve to keep the gas circulating the right way in a loop system. depending on the service, they may be made of a flexible polymer, an elastomer, a fibre or cloth reinforced elastomer, or elastomer covered with a woven fabric for reinforcement or abrasion resistance. if the woven layer is bonded to the outside surface it protects the rubber from damage from scrapes but makes it more difficult to wash off contaminants. 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