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Rebreather
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A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where humans cannot safely breathe from the atmosphere. In the armed forces it is sometimes called "CCUBA" (Closed Circuit Underwater Breathing Apparatus).
eather technology is used in many environments:
This article is mainly about diving rebreathers.
As a person breathes, the body consumes oxygen and makes carbon dioxide.
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Encyclopedia
A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where humans cannot safely breathe from the atmosphere. In the armed forces it is sometimes called "CCUBA" (Closed Circuit Underwater Breathing Apparatus).
Basics
Rebreather technology is used in many environments:
- Underwater - where it is sometimes known as CCR = "closed circuit rebreather", "closed circuit scuba", "semi closed scuba", SCR = "semi closed rebreather", or CCUBA = "closed circuit underwater breathing apparatus", as opposed to Aqua-Lung-type equipment, which is known as "open circuit scuba".
- Mine rescue and in industry - where poisonous gases may be present or oxygen may be absent.
- Space suits - outer space is, for all intents and purposes, a vacuum where there is no oxygen to support life.
- Hospital anaesthesia breathing systems - to supply controlled proportions of gases to patients without letting anaesthetic gas get into the atmosphere that the staff breathe.
- Submarines and hyperbaric oxygen therapy chambers - where the gas in the habitat must remain safe. Here the rebreather is big and is connected to the air in the habitat.
This article is mainly about diving rebreathers.
As a person breathes, the body consumes oxygen and makes carbon dioxide. A person with an open-circuit breathing set typically only uses about a quarter of the oxygen in the air that is breathed in. The rest is breathed out along with nitrogen and carbon dioxide.
The rebreather recirculates the exhaled gas for re-use and does not discharge it to the atmosphere or water. It absorbs the carbon dioxide, which otherwise would accumulate and cause carbon dioxide poisoning. It removes the carbon dioxide by a process called scrubbing. The rebreather adds oxygen to replace the oxygen that was consumed. Thus, the gas in the rebreather's circuit remains breathable and supports life.
History of rebreathers
Around 1620 in England, Cornelius Drebbel made an early oar-powered submarine. Records show that, to re-oxygenate the air inside it, he likely generated oxygen by heating saltpetre (sodium or potassium nitrate) in a metal pan to make it emit oxygen. That would turn the saltpetre into sodium or potassium oxide or hydroxide, which would tend to absorb carbon dioxide from the air around. That may explain how 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 nearly three centuries before Fluess and Davis.
In 1853 Professor T. Schwann designed a rebreather in Belgium; he exhibited it in Paris in 1878.
In 1878 Henry Fleuss invented the first certainly known rebreather using stored oxygen and absorption of carbon dioxide by an absorbent (here rope yard soaked in caustic potash solution), to rescue mineworkers who were trapped by water.
The Davis Escape Set was the first rebreather which was practical for use and produced in quantity. It was designed about 1900 in Britain for escape from sunken submarines. Various industrial oxygen rebreathers (e.g. the Siebe Gorman Salvus and the Siebe Gorman Proto, both invented in the early 1900s) were descended from it; shows a Draeger rebreather used for mines rescue in 1907.
In 1903 to 1907 Professor Georges Jaubert, invented Oxylithe, which is a form of sodium peroxide (Na2O2) or sodium dioxide (NaO2). As it absorbs carbon dioxide it emits oxygen. In 1909 Captain S.S. Hall, R.N., and Dr. O. Rees, R.N., developed a submarine escape apparatus using Oxylithe; the Royal Navy accepted it. It was used for shallow water diving but never in a submarine escape; it was used in the first filming (1907) of Twenty Thousand Leagues Under the Sea.
The first known systematic use of rebreathers for diving was by Italian sport spearfishers in the 1930s. This practice came to the attention of the Italian Navy, which developed its frogman unit, which affected World War II.
In World War II captured Italian frogmen's rebreathers influenced design of British frogman's rebreathers. Many British frogmen's breathing sets' oxygen cylinders were German pilot's oxygen cylinders recovered from shot-down German Luftwaffe planes. Those first breathing sets may have been modified Davis Submarine Escape Sets; 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. One version had a flip-up single window for both eyes to let the user get binoculars to his eyes when on the surface. They used bulky thick diving suits called Sladen suits. Early British frogman's rebreathers had rectangular breathing bags on the chest like Italian frogman's rebreathers; later British frogman's rebreathers had a square recess in the top so they could extend further up onto his shoulders; in front they had a rubber collar that was clamped around the absorbent canister, as in the illustration below.
In the early 1940s US Navy rebreathers were developed by Dr. Christian J. Lambertsen for underwater warfare. Dr. Lambertsen, who currently works at the University of Pennsylvania, is considered by the US Navy as "the father of the Frogmen".
Advantages of rebreather diving
Efficiency advantages
The main advantage of the rebreather over other breathing equipment is the rebreather's economical use of gas. With open circuit scuba, the entire breath is expelled into the surrounding water when the diver exhales. A breath inhaled from an open circuit scuba system whose cylinders are filled with ordinary air is about 21% oxygen. When that breath is exhaled back into the surrounding environment, it has an oxygen level in the range of 15 to 16% when the diver is at atmospheric pressure. This leaves the available oxygen utilization at about 25%; the remaining 75% is lost.
At depth, the advantage of a rebreather is even more marked. Since the generation of CO2 is directly related to the body's consumption of O2 (about ~99.5% of O2 is converted to CO2 on exhalation), the amount of O2 consumption doesn't change, therefore CO2 generation doesn't change. This means that at depth, the diver is not using any more of the O2 gas supply than when shallower. This is a marked difference from open circuit where the amount of gas used is directly proportional to the depth.
Feasibility advantages
Long or deep dives using open circuit equipment may not be feasible as there are limits to the number and weight of diving cylinders the diver can carry. The economy of gas consumption is also useful when the gas mix being breathed contains expensive gases, such as helium. In normal use only oxygen is consumed: small volumes of expensive inert gases can be reused for many dives. For example, a closed circuit rebreather diver effectively doesn't use any of their diluent gas once they've reached the bottom phase of the dive; they could turn off their diluent. On ascent, no diluent is used. A very small amount of trimix would then last for many dives. It is not uncommon for a 3 litre (19 cubic foot) diluent cylinder to last for eight dives.
Other advantages
Except on ascent, closed circuit rebreathers produce no bubbles and make no bubble noise and much less gas hissing, unlike open-circuit scuba; this can conceal military divers and allow divers engaged in marine biology and underwater photography to avoid alarming marine animals and thereby get closer to them. The fully closed circuit rebreather is able to minimise the proportion of inert gases in the breathing mix, and therefore minimise the decompression requirements of the diver, by maintaining a specific and relatively high oxygen partial pressure (ppO2) at all depths. The breathing gas in a rebreather is warmer and more moist than the dry and cold gas from open circuit equipment making it more comfortable to breathe on long dives and causing less dehydration in the diver.
Most modern rebreathers have a system of very sensitive oxygen sensors, which allow the diver to adjust the partial pressure of oxygen. This can offer a dramatic advantage at the end of deeper dives, where a diver can raise the partial pressure of oxygen somewhat at shallower depth, in order to shorten decompression times. Care must be taken that the ppO2 is not set to a level where it can become toxic though. Research has shown that a ppO2 of 1.6 bar is toxic with extended exposure
Main rebreather design variants
Oxygen rebreather
This is the oldest type of rebreather and was commonly used by navies from the early twentieth century. The only gas that it supplies is oxygen. As pure oxygen is toxic when inhaled at pressure, oxygen rebreathers are limited to a depth of 6 meters (20 ft); some say 9 meters (30 ft). Oxygen rebreathers are also sometimes used when decompressing from a deep open-circuit dive, as breathing pure oxygen makes the nitrogen diffuse out of the blood more rapidly.
In some rebreathers, e.g. the Siebe Gorman Salvus, the oxygen cylinder has two first stages in parallel. One is constant flow; the other is a plain on-off valve called a bypass; both feed into the same exit pipe which feeds the breathing bag. In the Salvus there is no second stage and the gas is turned on and off at the cylinder. Some simple oxygen rebreathers had no constant-flow valve, but only the bypass, and the diver had to operate the valve at intervals to refill the breathing bag as he used the oxygen.
Semi-closed circuit rebreather
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 are fairly simple and cheap.
Semi-closed circuit equipment generally supplies one breathing gas such as air or nitrox or trimix. The gas is injected into the loop at a constant rate to replenish oxygen consumed from the loop by the diver. Excess gas must be constantly vented from the loop in small volumes to make space for fresh, oxygen-rich gas. As the oxygen in the vented gas cannot be separated from the inert gas, semi-closed circuit is wasteful of oxygen.
The diver must fill the cylinders with gas mix that has a maximum operating depth that is safe for the depth of the dive being planned.
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 injection rate reduces the likelihood of hypoxia but consumes more gas and wastes more oxygen.
Fully closed circuit rebreather
Military, photographic and recreational divers use these because they allow long dives and produce no bubbles. Closed circuit rebreathers generally supply two breathing gases to the loop: one is pure oxygen and the other is a diluent or diluting gas such as air or trimix.
The major task of the fully closed circuit rebreather is to control the oxygen concentration, known as the oxygen partial pressure, in the loop and to warn the diver if it is becoming dangerously low or high. The concentration of oxygen in the loop depends on two factors: depth and the proportion of oxygen in the mix. Too low a concentration of oxygen results in hypoxia leading to sudden 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.
In fully automatic closed-circuit systems, a mechanism injects oxygen into the loop when it detects that the partial pressure of oxygen in the loop has fallen below the required level. Often this mechanism is electrical and relies on oxygen sensitive electro-galvanic fuel cells called “ppO2 meters” to measure the concentration of oxygen in the loop.
The diver may be able to manually control the mixture by adding diluent gas or oxygen. Adding diluent can prevent the loop's gas mixture becoming too oxygen rich. Manually adding oxygen is risky as additional small volumes of oxygen in the loop can easily raise the partial pressure of oxygen to dangerous levels.
Rebreathers whose absorbent releases oxygen
There have been a few rebreather designs (e.g. the Oxylite) which had an absorbent canister filled with potassium superoxide, which gives off oxygen as it absorbs carbon dioxide: 4KO2 + 2CO2 = 2K2CO3 + 3O2; it had a very small oxygen cylinder to fill the loop at the start of the dive. This system is dangerous because of the explosively hot reaction that happens if water gets on the potassium superoxide. The Russian IDA71 military and naval rebreather was designed to be run in this mode or as an ordinary rebreather.
Tests on the IDA71 at the United States Navy Experimental Diving Unit in Panama City, Florida showed that the IDA71 could give significantly longer dive time with superoxide in one of the canisters than without.
Rebreathers which store liquid oxygen
If used underwater, the liquid-oxygen tank must be well insulated against heat coming in from the water. As a result, 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. They include these types:
- Aerophor.
- Aerorlox. See http://www.healeyhero.co.uk/rescue/glossary/aerorlox.htm .
- Cryogenic rebreather: see below.
Cryogenic rebreather
There have been plans for a "cryogenic rebreather". It has a tank of liquid oxygen and no absorbent canister. The carbon dioxide is frozen out in a "snow box" by the cold produced as the liquid oxygen expands to gas as the oxygen is used and is replaced from the oxygen tank.
Such a rebreather called the S-1000 was built around or soon after 1960 by Sub-Marine Systems Corporation. It had a duration of 6 hours and a maximum dive depth of 200 meters of salt water. Its ppO2 could be set to anything from 0.2 bar to 2 bar without electronics, by controlling the temperature of the liquid oxygen, thus controlling the equilibrium pressure of oxygen gas above the liquid. The diluent could be either liquid nitrogen or helium depending on the depth of the dive. The set could freeze out 230 grams of carbon dioxide per hour from the loop, corresponding to an oxygen consumption of 2 liters per minute. If oxygen was consumed faster (high workload), a regular scrubber was needed. See:
- Fischel H., Closed circuit cryogenic SCUBA, "Equipment for the working diver" 1970 symposium, Washington, DC, USA. Marine Technology Society 1970:229-244.
- Cushman, L., Cryogenic Rebreather, Skin Diver magazine, June 1969, and reprinted in Aqua Corps magazine, N7, 28, 79.
Other designs
- In the Siebe Gorman Proto the absorbent was in a flexible-walled compartment in the bottom of the breathing bag and not in a canister.
- describes an experimental drysuit (with built-in hood and fullface mask) and rebreather combination where the drysuit acts as the breathing bag, like in an old Draeger standard diving suit variant which had a rebreather pack attached.
- Some British naval rebreathers (e.g. the Siebe Gorman CDBA) had a backpack weight pouch instead of the diver having a separate weight belt.
Parts of a rebreather
The loop
Although there are several design variations of diving rebreather, all types have a gas-tight loop that the diver inhales from and exhales into. The loop consists of components sealed together. The diver breathes through a mouthpiece or a fullface mask (or with industrial breathing sets, sometimes a mouth-and-nose mask). This is connected to one or more tubes bringing inhaled gas and exhaled gas between the diver and a counterlung or breathing bag. This holds gas when it is not in the diver's lungs. The loop also includes a scrubber containing carbon dioxide absorbent to remove from the loop the carbon dioxide exhaled by the diver. Attached to the loop there will be at least one valve allowing injection of gases, such as oxygen and perhaps a diluting gas, from a gas source into the loop. There may be valves allowing venting of gas from the loop.
Most modern rebreathers have a twin hose mouthpiece or breathing mask where the direction of flow of gas through the loop is controlled by one-way valves. Some have a single pendulum hose, where the inhaled and exhaled gas passes through the same tube in opposite directions. The mouthpiece often has a valve letting the diver take the mouthpiece from the mouth while underwater or floating on the surface without water getting into the loop. Many rebreathers have "water traps" in the counterlungs, to stop large volumes of water from entering the loop if the diver removes the mouthpiece underwater without closing the valve, or if the diver's lips get slack letting water leak in. Regardless of whether the rebreather in question has the facility to trap any ingress of water, any training on a rebreather will feature procedures for removing any excess water.
Gas sources
A rebreather must have a source of oxygen to replenish that consumed by the diver. Nearly always, this oxygen is stored in a gas cylinder. Depending on the rebreather design variant, the oxygen source will either be pure or a breathing gas mixture.
Pure oxygen is not considered to be safe for recreational diving deeper than 6 meters, so recreational rebreathers and many professional diving rebreathers also have a cylinder of diluent gas. This diluent cylinder may be filled with compressed air or another diving gas mix such as nitrox or trimix. The diluent reduces the percentage of oxygen breathed and increases the maximum operating depth of the rebreather. It is important that the diluent is not an oxygen-free gas, such as pure nitrogen or helium, and is breathable; it may be used in an emergency either to flush the loop with breathable gas or as a bailout.
Carbon dioxide scrubber
The exhaled gases are directed through the chemical scrubber, a canister full of some suitable carbon dioxide absorbent such as a form of soda lime, which removes the carbon dioxide from the gas mixture and leaves the oxygen and other gases available for re-breathing.
Some absorbent chemical designed for diving applications are Sofnolime, Dragersorb, or Sodasorb. Some systems use a prepackaged Reactive Plastic Curtain (RPC) based cartridge: one brand of these RPC cartridges is .
The carbon dioxide passing through the scrubber absorbent is removed when it reacts with the absorbent in the canister; this chemical reaction is exothermic. This reaction occurs along a "front" which is a cross section of the canister, of the unreacted soda lime that is exposed to carbon dioxide-laden gas. This front moves through the scrubber canister, from the gas input end to the gas output end, as the reaction consumes the active ingredients. However, this front would be a wide zone, because the carbon dioxide in the gas going through the canister needs time to reach the surface of a grain of absorbent, and then time to penetrate to the middle of each grain of absorbent as the outside of the grain becomes exhausted.
In larger environments, such as recompression chambers, a fan is used to pass gas through the canister.
Scrubber failure
The term "break through" means the failure of the "scrubber" to continue removing carbon dioxide from the exhaled gas mix.
There are several ways that the scrubber may fail or become less efficient:
- Complete consumption of the active ingredient ("break through").
- The scrubber canister has been incorrectly packed or configured. This allows the exhaled gas to bypass the absorbent. In a rebreather, the soda lime must be packed tightly so that all exhaled gas comes into close contact with the granules of soda lime and the loop is designed to avoid any spaces or gaps between the soda lime and the loop walls that would let gas avoid contact with the absorbent. If any of the seals, such as o rings, or spacers that prevent bypassing of the scrubber, are not cleaned or lubricated or fitted properly, the scrubber will be less efficient, or outside water or gas may get in circuit.
- When the gas mix is under pressure caused by depth, the inside of the canister is more crowded by other gas molecules (oxygen or diluent) and the carbon dioxide molecules are not so free to move around to reach the absorbent. In deep diving with a nitrox or other gas-mixture rebreather, the scrubber needs to be bigger than is needed for a shallow-water or industrial oxygen rebreather, because of this effect. Among British naval rebreather divers, this type of carbon dioxide poisoning was called shallow water blackout.
- A Caustic Cocktail - Soda lime is caustic and can cause burns to the eyes and skin. A "caustic cocktail" is a mixture of water and soda lime that occurs when the "scrubber" floods. It gives rise to a chalky taste, which should prompt the diver to switch to an alternative source of breathing gas and rinse his or her mouth out with water. Many modern diving rebreather absorbents are designed not to produce "cocktail" if they get wet.
Failure prevention
- An indicating dye in the soda lime. It changes the colour of the soda lime after the active ingredient is consumed. For example, a rebreather absorbent called "Protosorb" supplied by Siebe Gorman had a red dye, which was said to go white when the absorbent was exhausted. Color indicating dye was removed from US Navy fleet use in 1996 when it was suspected of releasing chemicals into the circuit. With a transparent canister, this may be able to show the position of the reaction "front". This is useful in dry open environments, but is not useful on diving equipment, where:
- A transparent canister would likely be brittle and easily cracked by knocks.
- Opening the canister to look inside would flood it with water or let unbreathable external gas in.
- The canister is usually out of sight of the user, e.g. inside the breathing bag or inside a backpack box.
- Temperature monitoring. As the reaction between carbon dioxide and soda lime is exothermic, temperature sensors, most likely digital, along the length of the scrubber can be used to measure the position of the front and therefore the life of the scrubber.
- Diver training. Divers are trained to monitor and plan the exposure time of the soda lime in the scrubber and replace it within the recommended time limit. At present, there is no effective technology for detecting the end of the life of the scrubber or a dangerous increase in the concentration of carbon dioxide causing carbon dioxide poisoning. The diver must monitor the exposure of the scrubber and replace it when necessary.
- Carbon dioxide gas sensors exist, but they are not sensitive enough to be used in a rebreather - the scrubber "break through" occurs quite suddenly and the diver shows symptoms before the sensor indicates a dangerous build-up of carbon dioxide. Even if a sensitive carbon dioxide sensor is developed, it may not be useful as the primary tool for monitoring scrubber life when underwater, because mixed gas rebreathers allow very long dives where long decompression stops may be needed: knowing that the rebreather will begin to deliver a poisonous breathing gas in five minutes may not be useful to a diver needing to carry out an hour or more of decompression stops.
Effectiveness
In rebreather diving, the typical effective duration of the scrubber will be half an hour to several hours of breathing, depending on the granularity and composition of the soda lime, the ambient temperature, the design of the rebreather, and the size of the canister. In some dry open environments, such as a recompression chamber or a hospital, it may be possible to put fresh absorbent in the canister when break through occurs.
Controlling the mix
A basic need with a rebreather is to keep the amount of oxygen in the mix, or more technically known as the partial pressure of oxygen or ppO2, from getting too low (causing anoxia or hypoxia) or too high (causing oxygen toxicity).
With humans, the urge to breathe is caused by a build-up of carbon dioxide rather than lack of oxygen. Rebreathers remove exhaled carbon dioxide with the scrubber, suppressing this natural warning. If not enough new oxygen is being added, the proportion of oxygen in the loop may be too low to support life even though there is plenty of gas to breathe. The resulting serious hypoxia causes sudden blackout with little or no warning. This makes hypoxia a deadly problem for rebreather divers.
In many rebreathers the diver can control the gas mix and volume in the loop manually by injecting each of the different available gases to the loop and by venting the loop. The loop often has a pressure relief valve to prevent over-pressure injuries caused by over-pressure of the loop.
In some early rebreathers the diver had to manually open and close the valve to the oxygen cylinder to refill the counter-lung each time. In others the oxygen flow is kept constant by a pressure-reducing flow valve like the valves on blowtorch cylinders; the set also has a manual on/off valve called a bypass. In some modern rebreathers, the pressure in the breathing bag controls the oxygen flow like the demand valve in open-circuit scuba; for example, trying to breathe in from an empty bag makes the cylinder release more gas. Most modern closed-circuit rebreathers have electro-galvanic fuel cell sensors and onboard electronics, which monitor the ppO2, injecting more oxygen if necessary or issuing an audible warning to the diver if the ppO2 reaches dangerously high or low levels.
Counterlung
The counterlung is a flexible part of the loop, which is designed to change in size by the same volume as the diver's lungs are able to when breathing. Its purpose is to let the loop expand to hold the gas exhaled by the diver and to contract when the diver inhales letting the total volume of gas in the lungs and the loop remain constant throughout the diver's breathing cycle.
Underwater, the position of the breathing bag, on the chest, over the shoulders, or on the back, has an effect on the ease of breathing. This is due to the pressure difference between the counterlung and the diver's lung caused by the vertical distance between the two. It is easier to inhale from a front mounted counterlung and exhale to a back mounted counterlung for diver swimming facedown and horizontally.
The design of the rebreathers' counterlungs can also affect the swimming diver's streamlining due to location of the counterlungs themselves. Some are designed as over-the-shoulder lungs (e.g. InterSpace Systems Megalodon), while others incorporate the counter lungs into a solid case (e.g. The KISS Classic).
For use out of water, this does not matter so much: for example, in an industrial version of the Siebe Gorman Salvus the breathing bag hangs down by the left hip.
A rebreather whose counterlung is rubber and not in an enclosed casing, should be sheltered from sunlight when not in use, to prevent the rubber from perishing due to UV light.
Bailout
While the diver is underwater the rebreather may fail and be unable to provide a safe breathing mix for the duration of the ascent back to the surface. In this case the diver needs an alternative breathing source: the bailout.
Although some rebreather divers, "Alpinists", do not carry bailouts, bailout strategy becomes a crucial part of dive planning particularly for long and deeper dives in technical diving. Often the planned dive is limited to the capacity of the bailout and not the capacity of the rebreather.
Several types of bailout are possible.
- An open-circuit demand valve connected to the rebreather's diluent cylinder. While this option has the advantages of being permanently mounted on the rebreather and not heavy, the quantity of gas held by the rebreather is small so the protection offered is low.
- An open-circuit demand valve connected to the rebreather's oxygen cylinder. This is similar to the diluent option except it can only safely be used in depths of 6 metres or less.
- An independent open-circuit system. The extra cylinders are heavy and cumbersome but larger cylinders let the diver carry more gas providing protection for the ascent from deeper and long dives. The breathing gas mix must be carefully chosen to be safe at all depths of the ascent.
- An independent closed-circuit system.
Casing
Many rebreathers have their main parts in a hard backpack casing. This casing needs venting to let surrounding water or air in and out to allow for volume changes as the breathing bag inflates and deflates. In a diving rebreather this needs fairly large holes, including a hole at the bottom to drain the water out when the diver comes out of water. The SEFA, which is used for mine rescue, to keep grit and stones out of its working, is completely sealed, except for a large vent panel covered with metal mesh, and holes for the oxygen cylinder's on/off valve and the cylinder pressure gauge. Underwater the casing also serves for streamlining, e.g. in the IDA71 and Cis-Lunar.
Disadvantages of rebreather diving
Risks
The percentage of deaths that involve the use of a rebreather among United States and Canadian residents increased from approximately 1 to 5% of the total diving fatalities collected by the Divers Alert Network from 1998 through 2004. Investigations into rebreather deaths focus on three main areas: medical, equipment, and procedural.
Many very competent divers have died using rebreathers in accidents, which are often put down to operator error. Rebreathers are generally considered safer in extreme conditions such as deep dives (75m = 246 feet or more) or overhead environments, as they reduce the risk of running out of breathable gas.
Closed circuit disorders
In addition to the other diving disorders suffered by divers, rebreather divers are also more susceptible to:
- Sudden blackout due to hypoxia caused by too low a partial pressure of oxygen in the loop. A particular problem when using a closed circuit rebreather is the drop in ambient pressure caused by the ascent phase of the dive, which reduces the partial pressure of oxygen to hypoxic levels leading to what is sometimes called deep water blackout.
- Seizures due to oxygen toxicity caused by too high a partial pressure of oxygen in the loop. This can be caused by the rise in ambient pressure caused by the descent phase of the dive, which raises the partial pressure of oxygen to hyperoxic levels. In fully closed circuit equipment, aging oxygen sensors may become "current limited" and fail to measure high partial pressures of oxygen resulting in dangerously high oxygen levels.
- Disorientation, panic, headache, and hyperventilation due to excess of carbon dioxide caused by incorrect configuration, failure or inefficiency of the scrubber. The scrubber must be configured so that no exhaled gas can bypass it; it must be packed and sealed correctly. Another problem is the diver producing carbon dioxide faster than the absorbent can handle, for example, during hard work or fast swimming. The solution to this is to slow down and let the absorbent catch up. The scrubber efficiency may be reduced at depth where the increased concentration of other gas molecules, due to pressure, stops all the carbon dioxide molecules reaching the active ingredient of the scrubber.
- The rebreather diver must keep breathing in and out all the time, to keep the exhaled gas flowing over the carbon dioxide absorbent, so the absorbent can work all the time. Divers need to lose any air conservation habits that may have been developed while diving with open-circuit scuba. In closed circuit rebreathers, this also has the advantage of mixing the gases preventing oxygen-rich and oxygen-lean spaces developing within the loop, which may give inaccurate readings to the oxygen control system.
- "Caustic cocktail" in the loop if water comes into contact with the soda lime used in the carbon dioxide scrubber. The diver is normally alerted to this by a chalky taste in the mouth. A safe response is to bail out to "open circuit" and rinse the mouth out.
Restoring the oxygen content of the loop
Many diver training organizations teach the "diluent flush" technique as a safe way to restore the mix in the loop to a level of oxygen that is neither too high nor too low. It only works when partial pressure of oxygen in the diluent alone would not cause hypoxia or hyperoxia, such as when using a normoxic diluent and observing the diluent's maximum operating depth. The technique involves simultaneously venting the loop and injecting diluent. This flushes out the old mix and replaces it with a known proportion of oxygen from the diluent.
Divers using oxygen rebreathers are advised to flush the system when they start the dive, to get surplus nitrogen out of the system.
Compared with open circuit
When compared with Aqua-Lungs, rebreathers have some disadvantages including expense, complexity of operation and maintenance and fewer failsafes. A malfunctioning rebreather can supply a gas mixture which contains too little oxygen to sustain life. Typically rebreathers try to solve these problems by monitoring the system with electronics, sensors and alarm systems. These are expensive and susceptible to failure, improper configuration and misuse.
The bailout requirement of rebreather diving can sometimes also require a rebreather diver to carry almost as much bulk of cylinders as an open-circuit diver so the diver can complete the necessary decompression stops if the rebreather fails completely. Some rebreather divers prefer not to carry enough bailout for a safe ascent breathing open circuit, but instead rely on the rebreather, believing that an irrecoverable rebreather failure is very unlikely. This practice is known as alpinism or alpinist diving and is generally maligned due to the perceived extremely high risk of death if the rebreather fails.
Sport diving rebreather technology innovations
Over the past ten or fifteen years rebreather technology has advanced considerably, often driven by the growing market in recreational diving equipment. Innovations include:
- The electronic, fully closed circuit rebreather itself - use of electronics and electro-galvanic fuel cells to monitor oxygen concentration within the loop and maintain a certain partial pressure of oxygen
- Automatic diluent valves - these inject diluent gas into the loop when the loop pressure falls below the limit at which the diver can comfortably breathe.
- Dive/surface valves or bailout valves - a device in the mouthpiece on the loop which connects to a bailout demand valve and can be switched to provide gas from either the loop or the demand valve without the diver taking the mouthpiece from his or her mouth. An important safety device when carbon dioxide poisoning occurs.
- Integrated decompression computers - these allow divers to take advantage of the decompression benefits provided by the ideal mix in the loop of a fully closed circuit rebreather. By monitoring the oxygen content of the mix they can work out the inert gas content and generate a schedule of decompression stops.
- Carbon dioxide scrubber life monitoring systems - temperature sensors monitor the progress of the reaction of the soda lime and provide an indication of when the scrubber will be exhausted.
See also
- Escape set
- SCBA (surface-only (industrial) rebreathers)
- Portable Life Support System
- NIOSH Docket # 123, titled "Reevaluation of NIOSH limitations on and precaution for safe use of positive-pressure closed-circuit SCBA" is available at the link http://www.cdc.gov/niosh/review/public/123/default.html Public comment is sought from responder and manufacturer stakeholders NLT September 30, 2008.
External links
Information sources
- and many useful references in its "Further Reading" section
- , published in 1970, plenty of images, including mountaineering rebreathers, may be slow to download
- : English and French articles about rebreathers (Cedric Verdier)
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