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.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!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.The gas is warm rather than cold, as in open circuit. The gas in open circuit is cold due to pressure reduction of substantial amounts of gas, just prior to inhalation. Conversely less gas use in rebreathers means less cool gas is introduced. Our lungs contain a dense vascular system and much heat can be wicked away through exhalation. You will not expel that body heat into the water with CCR, the heat is maintained, in varying degrees, within the loop. Rebreather gases are also warmed by the chemical reaction during the carbon dioxide scrubbing process.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.