Rebreather

A fully closed circuit electronic rebreather (AP Diving Inspiration)
Acronym	CCUBA (closed circuit underwater breathing apparatus); CCR (closed circuit rebreather), SCR (semi-closed rebreather)
Uses	Breathing set
Related items	Davis apparatus, Self-contained breathing apparatus, Escape hood

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, 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.^[1] 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.^[2]

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.^[3]

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.^[4]

By adding sufficient oxygen to compensate for the metabolic usage, removing the carbon dioxide, and rebreathing the gas, most of the volume is conserved.^[4]

Effects of different levels of oxygen partial pressure^[1]

PO2 (bar)	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 (meters of sea water)
2.40	40% O2 nitrox recompression treatment gas for use in the chamber at 50 msw
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

Endurance

The endurance of a rebreather, the duration for which it can be safely and comfortably used, is dependent on the oxygen supply at the oxygen consumption rate of the user, and the capacity of the scrubber to remove carbon dioxide at the rate it is produced by the user. These variables are closely linked, as the carbon dioxide is a product of metabolic oxygen consumption, though not the only product. This is independent of depth, except for work of breathing increase due to gas density increase.^[4]

Architecture

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.^[6]

Components

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 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.^[7] 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.^[8] Gas flow through the scrubber may be in one direction in a loop rebreather, or both ways in a pendulum rebreather. The scrubber canister generally has an inlet on one side and an outlet on the other side.

A typical absorbent is Sodalime, which is made up of calcium hydroxide Ca(OH)₂, and potassium hydroxide KOH, 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.^[8]

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:^[6] 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.^[8] 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^[6][8] 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.^[9]

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

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.^[6]

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.^[6]

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.

• Closed circuit rebreather: 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.^{[citation needed]}
• Semi-closed circuit rebreather 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 amount (mass) of gas in the loop to maintain the working volume.^{[citation needed]}

Oxygen rebreathers

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.^[10]

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.^[11] 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.^[12] 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

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

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 use.^[13] This technology may be applied to both oxygen and mixed gas rebreathers, and can be used for diving and other applications. Potassium superoxide reacts vigorously with liquid water, releasing considerable heat and oxygen,and causing a fire hazard, so the more successful applications have been for space-suits, fire-fighting and mine rescue.^[14]

Rebreathers which use liquid oxygen

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^[15]

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.^[16] 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.^[17][18] Rebreathers may also be used as self-contained diver bailout systems for either scuba or surface-supplied diving.^[19]
• 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.^[20]
• 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

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 systemsedit

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:^[21]

• 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.^[22] This is extended to the divers through the diver umbilicals.^[21]

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.^[22]

Atmospheric diving suitsedit

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 rebreathersedit

Zdroj:https://en.wikipedia.org?pojem=Cryogenic_rebreather
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