A fully closed circuit electronic rebreather (Ambient Pressure Diving Inspiration) |
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| Acronym | CCUBA (Closed Circuit Underwater Breathing Apparatus); CCR (Closed circuit rebreather), SCR (Semi closed rebreather) |
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| Uses | Breathing set |
| Related items | Davis apparatus |
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).
Rebreather technology is used in many environments:
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As a person breathes, the body consumes oxygen and makes carbon dioxide. At shallow depths, a person with an open-circuit breathing set typically only uses about a quarter of the oxygen in the air that is breathed in (4%–5% of the inspired volume). The remaining oxygen is exhaled along with nitrogen and carbon dioxide. As the diver goes deeper, roughly the same quantity of oxygen is used, which represents an increasingly smaller fraction of the compressed air breathed in. Because exhaled air can contain as much as 79% nitrogen (which is not utilized in the body) and 16% (or more) unused oxygen, every exhaled breath from an open-circuit scuba set represents at least 95% wasted, potentially useful gas volume, which has to be replaced from the air supply.
The rebreather recirculates the exhaled gas for re-use and does not discharge it to the atmosphere or water.[1][3] It absorbs the carbon dioxide, which otherwise would accumulate and cause carbon dioxide poisoning. It removes the carbon dioxide by a process called scrubbing.[1] The rebreather adds oxygen, to replace the oxygen that was consumed.[1] Thus, the gas in the rebreather's circuit remains breathable and supports life and the diver needs only a fraction of the gas that would be required for an open-circuit system.
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%[15] 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.[15] This leaves the available oxygen utilization at about 25%; the remaining 75% is lost. As the remaining 79% of the breathing gas (mostly nitrogen) is inert, the diver on open-circuit scuba only uses about 5% of his cylinders' contents.
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 consumed increases as depth increases.
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 are reused during (only) one dive, due to venting of the gas on ascent. 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 added, however most of that in circuit is lost. 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 40 m (130 ft) dives.
Except on ascent, closed circuit rebreathers produce no bubbles and make no bubble noise and much less gas hissing, unlike open-circuit scuba;[15] 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.[16] This lack of exhale also allows shipwreck divers to enter enclosed areas on sunken ships and avoid slowly filling them with air, which then supports the growth of rust.
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[17]
One major difference between rebreather diving and open-circuit scuba diving is in keeping neutral buoyancy. When an open-circuit scuba diver inhales, a quantity of highly compressed gas from his cylinder is reduced in pressure by a regulator, and enters the lungs at a much higher volume than it occupied in the cylinder. This means that the diver has a tendency to rise slightly with each inhalation, and lower slightly with each exhalation. This does not happen to a rebreather diver, because the diver is circulating a roughly constant volume of gas between his lungs and the breathing bag.
This is the oldest type of rebreather and was commonly used by navies from the early twentieth century. Oxygen rebreathers can be remarkably simple designs, and their invention predates that of open-circuit scuba. The only gas that it supplies is oxygen.[18] As pure oxygen is toxic when inhaled at pressure, oxygen rebreathers are currently limited to a depth of 6 meters (20 ft); some say 9 meters (30 ft). In the past they have been used deeper (up to 20 meters) but such dives were more risky than what is now considered acceptable. 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.
The diving pioneer Hans Hass used Dräger oxygen rebreathers in the early 1940s.
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.[9] 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.
Oxygen rebreathers are no longer commonly used in diving because of the depth limit imposed by oxygen toxicity. However, they are still the most commonly used for industrial applications on the surface, (SCBA) such as in mines, due to their simplicity and compact size.
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.[19]
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.[20] A higher gas injection rate reduces the likelihood of hypoxia but consumes more gas and wastes more oxygen.
Military, photographic, and recreational divers use these because they allow long dives and produce no bubbles.[21] 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 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.
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.[22] 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.[22]
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. The set's liquid oxygen tank must be filled immediately before use. They include these types:
A cryogenic rebreather 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.
A cryogenic 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 sea 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.[23]
Cryogenic rebreathers were widely used in Soviet oceanography in the period 1980 to 1990.[24][25]
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.
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.
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.[15]
Some absorbent chemical designed for diving applications are Sofnolime, Dragersorb, or Sodasorb. Some systems use a prepackaged Reactive Plastic Curtain (RPC)[26] based cartridge: Reactive Plastic Curtain (RPC) was first used between Micropore Inc. and the US Navy to describe Micropore's absorbent curtains for emergency submarine use, and then more recently RPC has been used on the web to describe their Reactive Plastic Cartridges – ExtendAir.
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.
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:
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.
A basic need with a rebreather is to keep the partial pressure of oxygen (ppO2) in the mix from getting too low (causing hypoxia) or too high (causing oxygen toxicity). If not enough new oxygen is being added, the proportion of oxygen in the loop may be too low to support life. In humans, the urge to breathe is normally caused by a build-up of carbon dioxide in the blood, rather than lack of oxygen. 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.
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 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. Innerspace 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.
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—referred to as "alpinists"—do not carry bailouts, bailout strategy becomes a crucial part of dive planning, particularly for long dives and deeper dives in technical diving. Often the planned dive is limited by the capacity of the bailout and not the capacity of the rebreather.
Several types of bailout are possible:
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.
Some military rebreathers have a diffuser over the blowoff valve, which helps to conceal the diver's presence by masking the release of bubbles.[30]
The parts of a rebreather can be arranged on the wearer's body in many ways. For example:
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.[31] Investigations into rebreather deaths focus on three main areas: medical, equipment, and procedural.[31]
In mountaineering, closed-circuit rebreathers are ideal to treat various altitude related illnesses as the user is brought back to sea level in terms of oxygen partial pressure (pp). The danger is that a sick climber using a rebreather might become unconscious. Because an absolute atmospheric seal is required for rebreathers to work correctly, such a seal could conceivably cause an unconscious user to suffocate when the oxygen ran out or the scrubber became exhausted. (Because there has been very little use of mountaineering rebreathers, this danger is still only theoretical.)
In addition to the other diving disorders suffered by divers, rebreather divers are also more susceptible to the following disorders (all of which are directly connected with the effectiveness of actual rebreather designs and construction, not with the theory of rebreathing):
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
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, or it may allow carbon dioxide to build up to dangerous levels. 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.[32] 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.[33]
Over the past ten or fifteen years rebreather technology has advanced considerably, often driven by the growing market in recreational diving equipment. Innovations include:
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