weight may be critical. good ergonomic design for ease of carrying, comfort, and balance are important where the unit may be worn in confined spaces, while climbing and crawling through small gaps. a spring loaded counterlung pressurisation system may be used to provide slight positive pressure in the breathing loop, to prevent noxious gas entry if the mask seal is not perfect. there are no constraints due to the physiological effects of breathing under pressure. complex gas mixtures are unnecessary. oxygen rebreathers can usually be used, which makes the design considerably simpler, and the mechanism more reliable. a system allowing the user to access drinking water from a bag may be fitted, a bite operated valve van be used to control flow. radio voice communications may be fitted. a constant mass flow systen may be used to ensure a minimum fresh gas supply, with a demand valve to add gas if the loop is emptied. gas supply and absorbent duratio would usually be matched for efficiency insulation may be needed for the breathing loop for use in sub-zero temperatures where freezing of the absorbent would reduce its effectiveness. a water trap may be provided to collect condensed water from the absorbent reaction and from metabolic product. monitoring functions may include: cylinder pressure (analog and digital) estimated remaining duration of use until residual low pressure warning is activated temperature indication automatic recording of warning signals optical and/or acoustic warning signals may be provided for: low gas pressure in the cylinder alarm triggered by lack of user movement manually triggered alarm. mountaineering rebreathers mountaineering rebreathers provide oxygen at a higher concentration than available from atmospheric air in a naturally hypoxic environment. they need to be lightweight and to be reliable in severe cold including not getting choked with deposited frost. 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. the exothermic reaction helps keep the scrubber contents from freezing, and helps reduce heat loss from the user. both chemical and compressed gas oxygen have been used in experimental closed-circuit oxygen systems – the first on mount everest in 1938. the 1953 expedition used closed-circuit oxygen equipment developed by tom bourdillon and his father for the first assault team of bourdillon and evans; with one "dural" 800l compressed oxygen cylinder and soda lime canister (the second (successful) assault team of hillary and tenzing used open-circuit equipment). rebreathers for unpressurised aircraft and high altitude parachuting similar requirement and working environment to mountaineering, but weight is less of a problem. the soviet ida71 rebreather was also manufactured in a high altitude version, which was operated as an oxygen rebreather. anaesthesia systems main article: anaesthetic machine anaesthetic machines can be configured as rebreathers to provide oxygen and anaesthetic gases to a patient during surgery or other procedures that require sedation. an absorbent is present in the machine to remove the carbon dioxide from the loop. both semi-closed and fully closed circuit systems may be used for anaesthetic machines, and both push-pull (pendulum) two directional flow and one directional loop systems are used. the breathing circuit of a loop configured machine has two unidirectional valves so that only scrubbed gas flows to the patient while expired gas goes back to the machine. the anaesthetic machine can also provide gas to ventilated patients who cannot breathe on their own. a waste gas scavenging system removes any gasses from the operating room to avoid environmental contamination. space suits buzz aldrin on the moon on the apollo 11 mission, showing the portable life support system. orlan space suit on an eva from the international space station, with the life-support umbilical in view. one of the functions of a space suit is to provide the wearer with breathing gas. this can be done via an umbilical from the life-support systems of the spacecraft or habitat, or from a primary life support system carried on the suit. both of these systems involve rebreather technology as they both remove carbon dioxide from the breathing gas and add oxygen to compensate for oxygen used by the wearer. space suits usually use oxygen rebreathers as this allows a lower pressure in the suit which gives the wearer better freedom of movement. habitat life-support systems submarines, underwater habitats, bomb shelters, space stations, and other living spaces occupied by several people over medium to long periods on a limited gas supply, are equivalent to closed circuit rebreathers in principle, but generally rely on mechanical circulation of breathing gas through the scrubbers. safety there are several safety issues with rebreather equipment, and these tend to be more severe in diving rebreathers. hazards some of the hazards are due to the way the equipment works, while others are related to the environment in which the equipment is used, as rebreathers are usually used where no breathable ambient atmosphere is available. hypoxia hypoxia can occur in any rebreather which contains enough inert gas to allow breathing without triggering automatic gas addition. in an oxygen rebreather this can occur if the loop is not sufficiently purged at the start of use. purging is the replacement of the original gas content by fresh gas, and may have to be repeated to completely remove inert gas. purging should be done while breathing off the unit so that the inert gas in the user's lungs and body tissues that finds its way into the loop is also removed from the system. carbon dioxide buildup carbon dioxide buildup will occur if the scrubber medium is absent, badly packed, inadequate or exhausted. the normal human body is fairly sensitive to carbon dioxide partial pressure, and a buildup will be noticed by the user. however, there is not often much that can be done to rectify the problem except changing to another breathing gas supply until the scrubber can be repacked. continued use of a rebreather with an ineffective scrubber is not possible for very long, as the levels will become toxic and the user will experience extreme respiratory distress, followed by loss of consciousness and death. the rate at which these problems develop depends on the volume of the circuit and the metabolic rate of the user at the time. carbon dioxide buildup can also occur when a combination of exertion and work of breathing exceeds the capacity of the user. if this occurs where the user cannot reduce exertion sufficiently, it may be impossible to correct. this problem is more likely to occur with diving rebreathers at depths where the density of the breathing gas is severely elevated. leakage of toxic gases into the breathing loop industrial rebreathers are often used where the ambient air is contaminated, and may be toxic. parts of the loop will be at a slightly lower than external ambient pressure during inhalation, and if the circuit is not airtight external gases may leak in. this is a particular issue around the edge of a full-face mask, where the rubber mask skirt must seal against the user's face. fire hazards of high concentration of oxygen high partial pressures of oxygen greatly increase fire hazard, and many materials which are self-extinguishing in atmospheric air will burn continuously in a high oxygen concentration. this is more of a hazard for terrestrial applications such as rescue and firefighting than for diving, where the ignition risk is relatively low. caustic cocktail caused by a loop flood reaching the absorbent canister, so only applicable in immersed applications. scrubber failure the term "break-through" means the failure of the scrubber to continue removing sufficient carbon dioxide from the gas circulating in the loop. this will inevitably happen if the scrubber is used too long, but can happen prematurely in some circumstances. there are several ways that the scrubber may fail or become less efficient: complete consumption of the active ingredient in a "general break through". depending on scrubber design and user workload, this may be gradual, allowing the user to become aware of the problem in time to make a controlled exit or bailout to open circuit, or relatively sudden, triggering an urgent or emergency response. bypassing the absorbent. the absorbent granules must be packed closely so that all exhaled gas comes into contact with the surface of soda lime and the canister is designed to avoid any large spaces or gaps between the absorbent granules or between the granules and the canister walls that would let gas bypass contact with the absorbent. if any of the seals, such as o-rings, or spacers that prevent bypassing of the scrubber, are not present or not fitted properly, or if the scrubber canister has been incorrectly packed or fitted, it may allow the exhaled gas to bypass the absorbent, and the scrubber will be less effective. this failure mode is also called "tunneling" when absorbent settles to form void spaces inside the canister. bypass will cause an unexpected early break-through. when the gas mix is under pressure at depth, the gas molecules are more densely packed, and the carbon dioxide molecules' mean path between collisions is shorter, so they are not so free to move around to reach the absorbent surface, and require a longer dwell time. because of this effect, the scrubber must be bigger for deep diving than is needed for a shallow-water, industrial or high altitude rebreather. carbon dioxide absorbent can be caustic and can cause burns to the eyes, mucous membranes and skin. a mixture of water and absorbent occurs when the scrubber floods and depending on the chemicals used, can produce a chalky taste or a burning sensation if the contaminated water reaches the mouthpiece, which should prompt the diver to switch to an alternative source of breathing gas and rinse their mouth out with water. this is known to rebreather divers as a caustic cocktail. the excessive wetting of the absorbent also reduces the rate of carbon dioxide removal and can cause premature break-through even if no caustic liquid reaches the diver. work of breathing may also increase. many modern diving rebreather absorbents are designed not to produce this caustic fluid if they get wet. in below-freezing operation (primarily mountain climbing), wet scrubber chemicals can freeze when oxygen bottles are changed, while there is a pause in the exothermic reaction of taking up the carbon dioxide, thus preventing carbon dioxide from reaching the scrubber material, and slowing the reaction when used again. other failure modes flooding of the ambient pressure volume can occur in diving rebreathers. gas leakage to or from the ambient pressure volume can occur. since the volume of gas lost or gained is likely to be very small, it is most relevant when the surroundings contain toxic gases or fumes. oxygen monitoring failure can lead to incorrect partial pressure of oxygen in the breathing gas. this is only relevant in mixed gas rebreathers used for diving. gas injection system failure is also mainly a problem of mixed gas diving rebreathrs. oxygen rebreathers gas injection systems are generally robust and reliable and can be manually overridden if they fail, and this form of failure is identifiable by an inappropriate volume of gas in the ambient pressure volume of the rebreather. history early history around 1620, cornelius drebbel discovered that heating saltpetre (potassium nitrate) would generate oxygen. the first basic rebreather based on carbon dioxide absorption was patented in france in 1808 by pierre-marie touboulic from brest, a mechanic in napoleon's imperial navy. this early rebreather design worked with an oxygen reservoir, the oxygen being delivered progressively by the diver and circulating in a closed circuit through a sponge soaked in limewater, a solution of calcium hydroxide in water. touboulic called his invention ichtioandre (greek for 'fish-man'). there is no evidence of a prototype having been manufactured. a prototype rebreather was built in 1849 by pierre aimable de saint simon sicard, in 1853 professor t. schwann presented a rebreather at the belgian academy of science. it had a large back mounted oxygen tank with working pressure of about 13.3 bar, and two scrubbers containing sponges soaked in