Rebreather diving is underwater diving using diving rebreathers, a class of underwater breathing apparatus which recirculate the breathing gas exhaled by the diver after replacing the oxygen used and removing the carbon dioxide metabolic product. Rebreather diving is practiced by recreational, military and scientific divers in applications where it has advantages over open circuit scuba, and surface supply of breathing gas is impracticable. The main advantages of rebreather diving are extended gas endurance, low noise levels, and lack of bubbles.^[1]

Rebreathers are generally used for scuba applications, but are also occasionally used for bailout systems for surface-supplied diving. Gas reclaim systems used for deep heliox diving use similar technology to rebreathers, as do saturation diving life-support systems, but in these applications the gas recycling equipment is not carried by the diver. Atmospheric diving suits also carry rebreather technology to recycle breathing gas as part of the life-support system, but this article covers the procedures of ambient pressure diving using rebreathers carried by the diver.

Rebreathers are generally more complex to use than open circuit scuba, and have more potential points of failure, so acceptably safe use requires a greater level of skill, attention and situational awareness, which is usually derived from understanding the systems, diligent maintenance and overlearning the practical skills of operation and fault recovery. Fault tolerant design can make a rebreather less likely to fail in a way that immediately endangers the user, and reduces the task loading on the diver which in turn may lower the risk of operator error.

Comparison with open circuit

Basic principle

At shallow depths, a diver using open-circuit breathing apparatus typically only uses about a quarter of the oxygen in the air that is breathed in, which is about 4 to 5% of the inspired volume. The remaining oxygen is exhaled along with nitrogen and carbon dioxide – about 95% of the volume. As the diver goes deeper, much the same mass of oxygen is used, which represents an increasingly smaller fraction of the inhaled gas. Since only a small part of the oxygen, and virtually none of the inert gas is consumed, 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 breathing gas supply.^[2][1]

A rebreather retains most of the exhaled gas for re-use and does not discharge it immediately to the surroundings.^[3][4] The inert gas and unused oxygen is kept for reuse, and the rebreather adds gas to replace the oxygen that was consumed, and removes the carbon dioxide.^[3] Thus, the gas recirculated in the rebreather remains breathable and supports life and the diver needs only to carry a fraction of the gas that would be needed for an open-circuit system. The saving is proportional to the ambient pressure, so is greater for deeper dives, and is particularly significant when expensive mixtures containing helium are used as the inert gas diluent. The rebreather also adds gas to compensate for compression when dive depth increases, and vents gas to prevent overexpansion when depth decreases.^[2][5][1]

Advantages

Efficiency advantages

The main advantage of the rebreather over open circuit breathing equipment is 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 with cylinders filled with compressed air is about 21%^[6] 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.^[6] This leaves the available oxygen use 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 the cylinders' contents.^[1]

At depth, this advantage of a rebreather is even more marked. The diver's metabolic rate is independent of ambient pressure (i.e. depth), so the oxygen consumption rate does not change with depth. The production of carbon dioxide does not change either since it also depends on the metabolic rate. This is a marked difference from open circuit where the amount of gas consumed increases as depth increases since the density of the inhaled gas increases with pressure, and the volume of a breath remains almost unchanged.^[1]

Feasibility advantages

Very long or deep dives using open circuit scuba 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 of a rebreather is also useful when the gas mix being breathed contains expensive gases, such as helium. In normal use at constant depth, only oxygen is consumed: small volumes of inert gases are lost during any one dive, due mainly to venting of the gas as it expands on ascent. For example, a closed circuit rebreather diver theoretically need not use up any more diluent gas after reaching the full depth of the dive. On ascent, no diluent is added, but most of the gas in the loop is lost as it expands and is vented. A very small amount of trimix could therefore last for many dives. It is possible for a 3-litre (19 cubic foot nominal capacity) diluent cylinder to last for eight 40 m (130 ft) dives.^[1]

Other advantages

• Except on ascent, closed circuit rebreathers produce no bubbles during normal operation, and make no bubble noise and much less gas hissing, compared to open-circuit scuba;^[6] 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.^[7]
• This lack of bubbles allows wreck divers to enter enclosed areas on sunken ships without slowly filling them with air, which can accelerate rusting, and is also an advantage in cave diving if there is loose material on the ceiling which can be dislodged by bubbles, reducing visibility.
• The fully closed circuit rebreather can be used to optimise the proportion of inert gases in the breathing mix, and therefore minimise the decompression requirements of the diver, by maintaining a specific and nearly constant relatively high oxygen partial pressure (${\displaystyle P_{O_{2}}}$) at all depths.
• The breathing gas in a rebreather loop is warmer and more humid than the dry and cold gas from open circuit equipment, making it more comfortable to breathe on long dives and causing less dehydration and chilling of the diver.^[8]
• Many rebreathers have a system of oxygen sensors, which allow the diver or a control circuit 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 during decompression, permitting shorter decompression times. Care must be taken that the partial pressure of oxygen is not set to a level where it can become toxic. Research has shown that a partial pressure of oxygen of 1.6 bar can produce acute toxicity symptoms with extended exposure.^[9]
• Mass loss over the dive is reduced as a much smaller amount of gas is used, so buoyancy at constant depth does not vary much as the dive progresses, and less ballast weight is needed to compensate for gas consumption.

Disadvantages

When compared with open circuit scuba, rebreathers have some disadvantages, including expense, complexity of operation and maintenance, and more critical paths to failure. A malfunctioning rebreather can supply a gas mixture which contains too little oxygen to sustain life, too much oxygen which may cause convulsions, or it may allow carbon dioxide to build up to dangerous levels. Some rebreather designers 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.^[10]

• Oxygen rebreathers (simple closed circuit) are limited to a shallow depth range of approximately 6 m, beyond which the risk of acute oxygen toxicity rises to unacceptable levels very rapidly.
• Semi-closed circuit rebreathers are less efficient than closed circuit, and are more mechanically complex than open circuit scuba or oxygen rebreathers.
• Closed circuit rebreathers are yet more mechanically complex, and generally rely on electronic instruments and control systems to monitor and maintain a safe breathing gas mixture. This makes them more expensive to produce, more complex to maintain and test, and sensitive to getting their circuitry wet.
• Depending on the complexity of the rebreather, there are more failure modes than for open circuit scuba, and several of these failure modes are safety-critical and not easily recognized by the diver without technological intervention. A major disadvantage of a rebreather is that, due to a failure, gas may continue to be available for breathing, but the mixture provided may not support consciousness, and this may not be apparent to the user. With open circuit, this type of failure can only occur if the diver selects an unsuitable gas, and the most common type of open circuit failure, the lack of gas supply, is immediately obvious, and corrective steps like changing to an alternative supply would be taken immediately.

The bailout requirement of rebreather diving can sometimes 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.^[11] Some rebreather divers choose 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 deprecated due to the perceived extremely high risk of death if the rebreather fails.^[12]

Other differences

A major difference between rebreather diving and open-circuit scuba diving is in fine control of neutral buoyancy. When an open-circuit scuba diver inhales, a quantity of highly compressed gas from their 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 sink slightly with each exhalation. This does not happen to a rebreather diver, because the diver is circulating a roughly constant volume of gas between their lungs and the counterlung. This is not specifically an advantage or disadvantage, but it requires some practice to adjust to the difference.^[5][1]

Function

A rebreather functions by removing carbon dioxide from the exhaled gas, replenishing oxygen used, and providing the recycled gas at ambient pressure for the diver to inhale.^[1]

Scrubber endurance

In rebreather diving, the typical effective endurance of the scrubber will be half an hour to several hours of breathing, depending on the type and size of the scrubber, the absorbent characteristics, the ambient temperature and pressure, the operational mechanics of the rebreather, and the amount of carbon dioxide produced by the diver, which mainly depends on their metabolic work rate.^[8]

Controlling the mix

A basic need with a rebreather is to keep the partial pressure of oxygen (${\displaystyle P_{O_{2}}}$) in the mix from getting too low (causing hypoxia) or too high (causing oxygen toxicity). In humans, the urge to breathe is normally caused by a high level of carbon dioxide in the blood, rather than lack of oxygen. If not enough new oxygen is being added, the proportion of oxygen in the loop may become too low to support consciousness, and eventually too low to support life. The resulting serious hypoxia causes sudden blackout with little or no warning. This makes hypoxia a deadly hazard for rebreather divers.^[1]

The method used for controlling the range of oxygen partial pressure in the breathing loop depends on the type of rebreather.

In an oxygen rebreather, once the loop has been thoroughly flushed, the mixture is effectively static at 100% oxygen, and the partial pressure is a function only of depth. In some early oxygen rebreathers the diver had to manually open and close the valve to the oxygen cylinder to refill the counter-lung each time the volume got low. In others a small continuous oxygen flow is provided by a flow restricting valve, but the set also has a manual bypass valve for descent and when consumption exceeds supply. In more advanced oxygen rebreathers, the pressure in the counter-lung controls the oxygen flow with a demand valve which will add gas when the bag is empty and internal pressure drops below ambient.

In a semi-closed rebreather the loop mix depends on a combination of factors:

• the type of gas addition system and its setting, combined with the gas mixture in use, which control the rate of oxygen added. Oxygen fraction is limited by the gas mix. It can be lower but not higher.
• work rate, and therefore the oxygen consumption rate, which controls the rate of oxygen depletion, and therefore the resulting oxygen fraction.
• depth, which has the usual effect of increasing partial pressure in proportion to ambient pressure and oxygen fraction.

In manually controlled closed circuit rebreathers the diver controls the gas mix and volume in the loop by manually injecting oxygen and diluent gases to the loop and by venting the loop. The loop has a pressure relief valve to prevent damage caused by over-pressure of the loop when the mouthpiece valve is closed.

Electronically controlled closed-circuit rebreathers have electro-galvanic oxygen sensors which monitor the partial pressure of oxygen, and electronic control systems, which inject more oxygen to maintain the set point, and issuing an audible, visual and/or vibratory warning to the diver if the partial pressure of oxygen reaches dangerously high or low levels.^[1]

The volume in the loop of both SCRs and CCRs is usually maintained by a pressure controlled automatic diluent valve, which works on the same principle as a demand valve, to add diluent when inhalation lowers the pressure in the loop during descent or if the diver removes gas from the loop by exhaling through the nose.^[1]

Set-points

A set-point (or set point) is a factory set or user programmable limit value for the desired partial pressure of oxygen in a rebreather loop. The feedback of actual oxygen partial pressure measured by the oxygen sensors is compared with the set-points, and if it deviates outside of the limits of upper and lower set-points, the control system will activate a solenoid valve to add oxygen or diluent gas to the loop to correct the oxygen content until it is back within the set-point limits. Usually the user can override the gas addition by manual activation of injection valves.^[5][1]

Some control systems allow depth activated switching of set-points, so that one pair of set-points can be selected for the main part of the dive, and another pair, usually richer, for accelerated decompression above the limiting depth. The changeover is automatic during ascent, but the high set-points are not activated before ascent as they are generally undesirable during descent and the main part of the dive.^[5][1]

The deep sector set-point is chosen to minimise decompression obligation while also maintaining a low risk of oxygen toxicity over the expected dive duration. Values ranging from around 1.4 bar for a short dive to 1.0 bar for a very long dive can be used, with 1.2 to 1.3 bar being a frequent general purpose compromise. (see US Navy rebreather tables). The decompression set-point tends to be a bit higher to accelerate elimination of inert gases, while retaining a low risk of oxygen toxicity. Values between 1.4 and 1.6 bar are generally chosen, depending on the expected duration of decompression.^[5][1]

Gas endurance

Gas endurance depends on the amount of gas available and the control system for injection to maintain the oxygen partial pressure set points. These include constant mass flow, manual control, and automated control by injecting gas via a solenoid valve. The injection may follow the "bang-bang", "on-off", or "hysteresis" model, where the valve is activated and gas is injected until it reaches the upper set point limit, deactivated until the partial pressure reduces to the lower set point limit, and injection is started again, or more complex models such as proportional-integral-derivative (PID) control, in which the injection rate is controlled taking into account current rate of use, and changes to the rate of use. The gas endurance can be affected by the control model used.^[8]

Calculating the loop mix

In closed circuit rebreathers the breathing loop gas mixture is either known (100% oxygen) or monitored and controlled within set limits, by either the diver or the control circuitry, but in the case of semi-closed rebreathers, where the gas mixture depends on the predive settings and diver exertion, it is necessary to calculate the possible range of gas composition during a dive. The calculation depends on the mode of gas addition.

Oxygen partial pressure in a semi-closed rebreather

A diver with a constant workload during aerobic working conditions will use an approximately constant amount of oxygen ${\displaystyle V_{O_{2}}}$ as a fraction of the respiratory minute volume (RMV, or ${\displaystyle V_{E}}$). This ratio of minute ventilation and oxygen uptake is the extraction ratio ${\displaystyle K_{E}}$, and usually falls in the range of 17 to 25 with a normal value of about 20 for healthy humans. Values as low as 10 and as high as 30 have been measured.^[13] Variations may be caused by the diet of the diver and the dead space of the diver and equipment, raised levels of carbon dioxide, or raised work of breathing and tolerance to carbon dioxide.

${\displaystyle K_E=\frac{V_E}{V_{}}}$Zdroj:https://en.wikipedia.org?pojem=Rebreather_diving
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