[red]What are the advantages of Rebreathers?[/red]
Rebreathers in general, and closed-circuit rebreathers in particular, provide three fundamental advantages over open-circuit scuba systems: more efficient use of gas, optimized decompression characteristics, and near-silent operation.
Gas Efficiency
Perhaps the most significant advantage that closed-circuit rebreathers (and to a lesser extent, semi-closed rebreathers) offer is greatly increased gas efficiency. Under normal circumstances, a diver only uses a small fraction of the oxygen of each inhaled breath; most of the oxygen leaves the lungs unused when the diver exhales. When using open-circuit scuba, the oxygen and other gases in the exhaled gas are wasted in the form of bubbles. As the depth of the dive increases, this inefficiency of open-circuit systems is compounded: because of the increased pressure at greater depths, more gas molecules are lost with each exhaled breath. A rebreather, on the other hand, retains most or all of the exhaled breath, processes it, and returns it to the diver. In the case of closed-circuit rebreathers, because there are almost no exhaled bubbles at all, there is no change in gas usage efficiency at greater depths. Thus, the deeper the dive, the more advantageous (from a gas efficiency perspective) rebreathers become. For example, a standard scuba cylinder contains enough gas to sustain an average resting person for about an hour and a half at the surface. The same cylinder will last only 45 minutes 30 feet / 10 meters underwater, and less than 10 minutes at a depth of 300 feet / 90 meters. But if that same cylinder were filled with oxygen and used to supply a closed-circuit rebreather, the diver could theoretically stay underwater for two days -- regardless of the depth!
Decompression Efficiency
The second advantage has to do with decompression optimization. This advantage only applies to closed-circuit rebreathers, not oxygen or semi-closed rebreathers. Oxygen rebreathers are limited to depths where decompression is not an issue. The reason it applies only to closed-circuit rebreathers and not semi-closed rebreathers has to do with differences in the breathing gas dynamics of these two types of rebreathers. As mentioned above, semi-closed rebreathers maintain a more-or-less constant fraction of oxygen in the breathing gas, whereas closed-circuit rebreathers maintain a constant partial pressure of oxygen in the breathing gas. A closed-circuit rebreather maintains the concentration of oxygen in the breathing gas at it maximum safe value throughout the dive. This means that the non-oxygen portion of the breathing gas (the part that determines decompression obligations), is kept at a minimum. This allows the diver to stay longer at depth without incurring a decompression obligation, and also to speed up the decompression process whenever an obligation is incurred.
Silence
With each exhaled breath, a diver using conventional scuba releases a large burst of noisy bubbles. The effect of this on the behavior of marine-life varies, but in most cases, fishes behave warily and are reluctant to allow a diver to approach closely. Semi-closed rebreathers reduce the volume of exhaled bubbles, and closed-circuit rebreathers essentially eliminate bubbles entirely. With rebreathers, divers are able to approach marine life much more closely while disturbing behavioral patterns much less severely. This is especially important for specimen collection and photographic activities.
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[red]What are the disadvantages of Rebreathers?[/red]
Discipline and Training
All kinds of rebreathers have certain specific complexities which introduce forms of risk not experienced by scuba divers. The fundamental difference between open-circuit scuba and rebreather systems is that on scuba, if a diver can breathe and is not outside well-established depth limits, the breathing gas is going to be life-sustaining (assuming the cylinder was filled properly). If there is a problem with an open-circuit system, the problem is usually very self-evident to the diver, so the diver at least is aware of the problem and can takes steps toward a solution.
With rebreathers, however, the breathing gas may be dynamic, and thus the oxygen concentration may drift out of life-sustaining range within the course of a single dive. In the case of oxygen rebreathers, if the breathing loop is not adequately flushed prior to commencing the dive, the fraction of nitrogen in the breathing gas may be high. For oxygen rebreathers with passive-addition oxygen control systems, it is possible that the diver may breathe-up all of the oxygen in the breathing loop before the oxygen addition valve is triggered, thus leaving only nitrogen. In the case of semi-closed rebreathers, oxygen concentration in the breathing loop depends on diver workload. Under certain circumstances, especially during high exertion and/or during an ascent, the oxygen concentration in a semi-closed rebreather could drop to dangerously low levels. The inherent weakness of closed-circuit rebreathers is the reliance on electronics to control the oxygen concentration in the breathing loop. As any underwater photographer knows, electronics and water (particularly salt water) do not mix. Indeed, closed-circuit rebreathers have earned a somewhat notorious reputation as being "unreliable", largely due to failures of the electronic O2 control system (leading to either too much, or too little oxygen in the breathing loop).
These problems can be largely avoided if oxygen rebreathers are adequately flushed with pure oxygen prior to a dive, if the gas supply rate of semi-closed rebreathers is adjusted carefully and the breathing loop is flushed with fresh gas prior to an ascent, and if multiple redundant oxygen sensors and oxygen control systems are incorporated into closed-circuit rebreathers. Unfortunately, symptoms associated with hypoxia and oxygen toxicity cannot be regarded as reliable precursors to black-out. Therefore it is ultimately up to the diver to take steps to ensure a continuous life-sustaining gas mixture in the breathing loop at all times. This level of discipline requires a great deal of discipline and training. Thus rebreather divers must have a higher dedication to equipment maintenance and operation than is generally required for open-circuit divers. Furthermore, rebreathers are generally more complex devices than open-circuit scuba gear, which also accounts for why they require more training time.
Expense
Another disadvantage of rebreathers is monetary expense. Even low-end rebreather designs can cost a bit, and sophisticated closed-circuit rebreathers can cost more than your lawn mowing savings. After the initial purchase price, however, operational expenses are not significantly greater than they are for conventional scuba. As the technology increases and these become more common the price will drop.
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