|Acronym||CCUBA (closed circuit underwater breathing apparatus); CCR (closed circuit rebreather), SCR (semi-closed rebreather)|
|Related items||Davis apparatus|
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. 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.
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 applications for 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, and 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 off-planet, in space suits for extra-vehicular activity. Similar technology is used in life-support systems in submarines, submersibles, 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.
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 chemically 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.
|Application and effect|
|<0.08||Coma ultimately leading to death|
|0.08-0.10||Unconsciousness in most people|
|0.09-0.10||Serious signs/symptoms of hypoxia|
|0.14-0.16||Initial signs/symptoms of hypoxia (normal environment oxygen in some very high altitude areas)|
|0.21||Normal environment oxygen (sea level air)|
|0.35–0.40||Normal saturation dive PO2 level|
|0.50||Threshold for whole-body effects; maximum saturation dive exposure|
|1.0–1.20||Common range for recreational closed circuit set point|
|1.40||Recommended limit for recreational open circuit bottom sector|
|1.60||NOAA limit for maximum exposure for a working diver|
Recreational/technical limit for decompression
|2.20||Commercial/military "Sur-D" chamber surface decompression on 100% O2 at 12 msw|
|2.40||40% O2 nitrox recompression treatment gas for use in the chamber at 50 msw (meters of sea water)|
|2.80||100% O2 recompression treatment gas for use in the chamber at 18 msw|
|3.00||50% O2 nitrox recompression treatment gas for use in the chamber at 50 msw|
There are two basic arrangements controlling the flow of breathing gas inside the rebreather, known as the pendulum and loop systems.
In the pendulum configuration, the user inhales gas from the counter-lung through a breathing tube, and exhaled gas returns to the counter lung by flowing back through the same tube. The scrubber is usually between the breathing tube and the counter-lung bag, and gas flow is bi-directional. All of the flow passages between the user and scrubber are dead space.
In the loop configuration, the user inhales gas through one tube, and exhales through a second tube. Exhaled gas flows into the scrubber from one side, and exits at the other side. There may be one large counter-lung, 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. Only the flow passages before the split between inhalation and exhalation tubes is dead space.
This is a loop configuration where the rebreather has two breathing bags, and the gas in circuit goes from the lungs to one bag to the absorbent to the other bag and back to the lungs.
The breathing bag is a bag of strong flexible material that the rebreather's user breathes in and out of.
A cylinder, of various sizes and length/width ratios, sometimes the size of a coffee can, that has an input valve on one side and an output one on the other side. The container is filled with CO2 absorbing material, mostly strong bases like soda lime or slaked lime, water and small percentages, between 1 and 5%, of sodium hydroxide and potassium hydroxide.
An oxygen tank that feeds pure oxygen into the rebreathing mixture, either steadily, or when the user operates the oxygen valve, or (in an automatic rebreather) after an O2 sensor has detected insufficient oxygen levels. In some loop systems it might positioned at the beginning of such loop and another tank, called diluent cylinder, is responsible for adjusting the O2 levels at the end.
Around 1620, in England, Cornelius Drebbel made an early oar-powered submarine. To re-oxygenate the air inside it, he likely generated oxygen by heating saltpetre (potassium nitrate) in a metal pan to emit oxygen. Heating turns the saltpetre into potassium oxide or hydroxide, which absorbs carbon dioxide from the air. That may explain why Drebbel's men were not affected by carbon dioxide build-up as much as would be expected. If so, he accidentally made a crude rebreather more than two centuries before Saint Simon Sicard's patent.
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. 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, and in 1853 by Professor T. Schwann in Belgium. It had a large back mounted oxygen tank with working pressure of about 13.3 bar, and two scrubbers containing sponges soaked in a caustic soda solution.
The first commercially practical closed-circuit scuba was designed and built by the diving engineer Henry Fleuss in 1878, while working for Siebe Gorman in London. His self-contained breathing apparatus consisted of a rubber mask connected to a breathing bag, with (estimated) 50–60% O2 supplied from a copper tank and CO2 scrubbed by rope yarn soaked in a solution of caustic potash; the system giving a duration of about three hours. Fleuss tested his device in 1879 by spending an hour submerged in a water tank, then one week later by diving to a depth of 5.5 m in open water, upon which occasion he was slightly injured when his assistants abruptly pulled him to the surface.
His apparatus was first used under operational conditions in 1880 by Alexander Lambert, the lead diver on the Severn Tunnel construction project, who was able to travel 1000 feet in the darkness to close several submerged sluice doors in the tunnel; this had defeated his best efforts with standard diving dress due to the danger of the air supply hose becoming fouled on submerged debris, and the strong water currents in the workings.
Fleuss continually improved his apparatus, adding a demand regulator and tanks capable of holding greater amounts of oxygen at higher pressure. Sir Robert Davis, head of Siebe Gorman, improved the oxygen rebreather in 1910 with his invention of the Davis Submerged Escape Apparatus, the first practical rebreather to be made in quantity. While intended primarily as an emergency escape apparatus for submarine crews, it was soon also used for diving, being a handy shallow water diving apparatus with a thirty-minute endurance, and as an industrial breathing set.
The rig comprised a rubber breathing/buoyancy bag containing a canister of barium hydroxide to scrub exhaled CO2 and, in a pocket at the lower end of the bag, a steel pressure cylinder holding approximately 56 litres of oxygen at a pressure of 120 bar. The cylinder was equipped with a control valve and was connected to the breathing bag. Opening the cylinder's valve admitted oxygen to the bag and charged it to the pressure of the surrounding water. The rig also included an emergency buoyancy bag on the front of to help keep the wearer afloat. The DSEA was adopted by the Royal Navy after further development by Davis in 1927. Various industrial oxygen rebreathers such as the Siebe Gorman Salvus and the Siebe Gorman Proto, both invented in the early 1900s, were derived from it.
Professor Georges Jaubert invented the chemical compound Oxylithe in 1907. It was a form of sodium peroxide (Na2O2) or sodium superoxide (NaO2). As it absorbs carbon dioxide in a rebreather's scrubber it emits oxygen. This compound was first incorporated into a rebreather design by Captain S.S. Hall and Dr. O. Rees of the Royal Navy in 1909. Although intended for use as a submarine escape apparatus, it was never accepted by the Royal Navy and was instead used for shallow water diving.
In 1912 the German firm Dräger began mass production of their own version of standard diving dress with the air supply from a rebreather. The apparatus had been invented some years earlier by Hermann Stelzner, an engineer at the Dräger company, for mine rescue.
In the 1930s, Italian sport spearfishers began to use the Davis rebreather; Italian manufacturers received a licence from the English patent holders to produce it. This practice soon came to the attention of the Italian Navy, which developed an extensively upgraded model designed by Teseo Tesei and Angelo Belloni that was used by its frogman unit Decima Flottiglia MAS with good results during World War II.
During the Second World War, captured Italian frogmen's rebreathers influenced improved designs for British rebreathers. Many British frogmen's breathing sets used aircrew breathing oxygen cylinders salvaged from shot-down German Luftwaffe aircraft. The earliest of these breathing sets may have been modified Davis Submerged Escape Apparatus; their fullface masks were the type intended for the Siebe Gorman Salvus, but in later operations different designs were used, leading to a fullface mask with one big face window, at first circular or oval and later rectangular (mostly flat, but the sides curved back to allow better vision sideways). Early British frogman's rebreathers had rectangular counterlungs on the chest like Italian frogman's rebreathers, but later designs had a square recess in the top of the counterlung so it could extend further up toward the shoulders. In front they had a rubber collar that was clamped around the absorbent canister. Some British armed forces divers used bulky thick diving suits called Sladen suits; one version of it had a flip-up single faceplate for both eyes to let the user get binoculars to his eyes when on the surface.
The Dräger rebreathers, especially the DM20 and DM40 model series, were used by the German helmet divers and German frogmen during World War II. Rebreathers for the US Navy were developed by Dr. Christian J. Lambertsen for underwater warfare. Lambertsen held the first closed-circuit oxygen rebreather course in the United States for the Office of Strategic Services maritime unit at the Naval Academy on 17 May 1943.
During and after WWII, needs arose in the armed forces to dive deeper than allowed by pure oxygen. That prompted, at least in Britain, design of simple constant-flow "mixture rebreather" variants of some of their diving oxygen rebreathers (= what is now called "nitrox"): SCMBA from the SCBA (Swimmer Canoeist's Breathing Apparatus), and CDMBA from the Siebe Gorman CDBA, by adding an extra gas supply cylinder. Before a dive with such a set, the diver had to know the maximum or working depth of his dive, and how fast his body used his oxygen supply, and from those to calculate what to set his rebreather's gas flow rate to.
Due to the military importance of the rebreather, amply demonstrated during the naval campaigns of the Second World War, most governments were reluctant to issue the technology into the public domain. In Britain rebreather use for civilians was negligible, and the BSAC formally prohibited rebreather use by its members. The Italian firms Pirelli and Cressi-Sub at first each sold a model of sport diving rebreather, but after a while discontinued those models. Some home made rebreathers were used by cave divers to penetrate cave sumps.
Eventually the Cold War ended, and in 1989 the Communist Bloc collapsed, and as a result the perceived risk of sabotage attacks by combat divers dwindled, and Western armed forces had less reason to requisition civilian rebreather patents, and automatic and semi-automatic recreational diving rebreathers with ppO2 sensors started to appear.
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.
This is the earliest type of rebreather and was commonly used by navies and for mining rescue and in industry from the early twentieth century. Oxygen rebreathers can be remarkably simple designs, and they were invented before open-circuit scuba. They only supply oxygen, so there is no requirement to control the gas mixture other than removing the carbon dioxide.
In some rebreathers, e.g. the Siebe Gorman Salvus, 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. In the Salvus there is no second stage and the gas is turned on and off at the cylinder.
Others such as the USN Mk25 UBA 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.
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.
SCRs are almost exclusively used for underwater diving, as they are bulkier, heavier, and more complex than closed circuit oxygen rebreathers, and applications at atmospheric pressure and below do not require the oxygen to be diluted to avoid acute toxicity. Military and recreational divers use these because they provide better underwater duration than open circuit, have a deeper maximum operating depth than oxygen rebreathers and can be fairly simple and cheap. They do not rely on electronics for control of gas composition, but may use electronic monitoring for improved safety and more efficient decompression. An alternative term for this technology is "gas extender".
Semi-closed circuit equipment generally supplies one breathing gas such as nitrox or trimix at a time. The gas is injected into the loop at a rate to replenish oxygen consumed from the loop by the diver. Excess gas must be vented from the loop as necessary to make space for fresh, oxygen-rich gas. As some oxygen remains in the vented gas, semi-closed circuit is wasteful of both oxygen and inert components.
A gas mix which has a maximum operating depth that is safe for the depth of the dive being planned, and which will provide a breathable mixture at the surface must be used, or it will be necessary to change mixtures during the dive. As the amount of oxygen required by the diver increases with work rate, the gas injection rate must be carefully chosen and controlled to prevent unconsciousness in the diver due to hypoxia. A higher gas addition rate reduces the likelihood of hypoxia and provides a more stable loop gas composition, but wastes more gas.
This type of rebreather works on the principle of adding fresh gas to compensate for reduced volume in the breathing circuit. A portion of the respired gas is discharged that is in some way proportional to usage. Generally it is a fixed volumetric fraction of the respiratory flow, but more complex systems have been developed which exhaust a close approximation of a ratio to the surface respiratory flow rate. These are described as depth compensated or partially depth compensated systems. Gas addition is triggered by low counterlung volume.
An active addition system adds feed gas to the breathing circuit and excess gas is dumped to the environment via an over-pressure valve. These rebreathers tend to operate near maximum volume.
The most common system of active addition of make-up gas in semi-closed rebreathers is by use of a constant mass flow injector, also known as choked flow. This is easily achieved by using a sonic orifice, as provided the pressure drop over the orifice is sufficient to ensure sonic flow, the mass flow for a specific gas will be independent of the downstream pressure. The mass flow through a sonic orifice is a function of the upstream pressure and the gas mixture, so the upstream pressure must remain constant for the working depth range of the rebreather to provide a reliably predictable mixture in the breathing circuit, and a modified regulator is used which is not affected by changes in ambient pressure. Gas addition is independent of oxygen use, and the gas fraction in the loop is strongly dependent on exertion of the diver – it is possible to dangerously deplete the oxygen by excessive physical exertion.
The principle of operation is to add a mass of oxygen that is proportional to the volume of each breath. This approach is based on the assumption that the volumetric breathing rate of a diver is directly proportional to metabolic oxygen consumption as a proxy for carbon dioxide production, which experimental evidence indicates is close enough to work within reasonable tolerances.
Closed circuit rebreathers (CCR) allow long dives and produce no bubbles most of the time. Closed circuit rebreathers supply two breathing gases to the loop: one is pure oxygen and the other is a diluent gas such as air, nitrox, heliox or trimix.
A major function of the closed circuit rebreather is to control the oxygen partial pressure in the loop and to warn the diver if it becomes dangerously low or high. Too low a concentration of oxygen results in hypoxia leading to unconsciousness and ultimately death. Too high a concentration of oxygen results in hyperoxia, leading to oxygen toxicity, a condition causing convulsions which can make the diver lose the mouthpiece when they occur underwater, and can lead to drowning. The gas mixture is controlled by the diver in manually controlled closed circuit rebreathers by adding diluent gas or oxygen. Adding diluent can prevent the loop gas mixture becoming too oxygen rich, and adding oxygen increases oxygen concentration.
In fully automatic closed-circuit systems, an electronically controlled solenoid valve injects oxygen into the loop when the control system detects that the partial pressure of oxygen in the loop has fallen below the required level. Electronically controlled CCRs can be switched to manual control in the event of some control system failures. Addition of gas to compensate for compression during descent is usually done by an automatic diluent valve.
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.
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:
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.
This may be compared with some applications of open-circuit breathing apparatus:
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.
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.
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:
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.
Different design criteria apply to SCBA rebreathers for use only out of the water:
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).
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.
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.
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.
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.
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.
There are several safety issues with rebreather equipment, and these tend to be more severe in diving rebreathers.
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.
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 should be done while breathing off the unit so that the inert gas in the user's lungs is also removed from the system.
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.
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.
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.
Caused by a loop flood reaching the absorbent canister, so only applicable in immersed applications.
The term "break-through" means the failure of the scrubber to continue removing suffient 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:
A breathing tube is a flexible tube for breathing through, as part of a scuba set or other breathing apparatus or a medical oxygen apparatus or anaesthetic apparatus (Here they are distinguished from the medium-pressure hoses which are often found as parts of modern breathing apparatus.)
They are wide, and usually corrugated to let the user's head move about without the tube pinching at kinks.
Each end has an airtight connection. They may contain a one-way valve to keep the air or gas flowing the right way.
According to the expected usage, they may be pure rubber, or rubber reinforced with canvas (outside or embedded) or similar. If the canvas layer is outside (as in the Siebe Gorman Salvus and the IDA71 and some old gasmasks, it protects the rubber from damage from scrapes but makes it harder to wash all the salt off after a saltwater dive.
Breathing tubes have to be long enough to connect the apparatus to the diver's head in all attitudes of his head, but should not be unnecessarily long, which will cause additional hydrodynamic drag or risk snagging on things, or contain excess dead space in a pendulum rebreather. The looping over-the-shoulders breathing tubes can be tethered down to the diver's shoulders or ballasted for neutral buoyancy to minimise loads on the mouthpiece.
Some early rebreathers had one breathing tube, which the breathed gas went through both ways: this is called the pendulum system; others have two breathing tubes, with one-way valves keeping the gas flowing the right way.
In some rebreathers, parts of the apparatus are connected to each other by lengths of breathing tube, resulting in more lengths of tube in the apparatus:
Aerorlox rebreather in a coal mining museum
Mine rescue rebreathers in museum
Mannequin wearing Finnish Navy combat diver equipment. The chest rebreather is likely Viper S-10.