Rebreather diving


Rebreather diving is underwater diving using diving rebreathers, a class of underwater breathing apparatus which recirculates 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.
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.

Concept

Rebreather diving is a mode of underwater diving which uses a diving rebreather for breathing gas supply at ambient pressure in an underwater environment.

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.
A rebreather retains most of the exhaled gas for re-use and does not discharge it immediately to the surroundings. 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. 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.

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% 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 use at about 25%; the remaining 75% is lost. As the remaining 79% of the breathing gas is inert, the diver on open-circuit scuba only uses about 5% of the cylinders' contents.
At depth, this advantage of a rebreather is even more marked. The diver's metabolic rate is independent of ambient pressure, 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.

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 diluent cylinder to last for eight dives.

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; 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.
  • 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 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.
  • 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.
  • 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.
  • Oxygen rebreathers 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. 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.