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Rebreather technology is used in the following environments:
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The main advantage of the rebreather over other breathing equipment is the breather's economic use of gas. With the Aqua-Lung, an alternative form of SCUBA, the entire breath is expelled into the surrounding water when the diver exhales. This means that long or deep Aqua-Lung dives require much more gas, which is carried in heavy and bulky cylinders, than rebreathers require. This economy of gas consumption is also useful when then gas being breathed is expensive, such as the helium in trimix. Rebreathers produce many fewer bubbles than Aqua-Lungs, which is useful for photographers and military divers because the rebreather makes those divers less visible.
Rebeathers compared to Aqua-Lungs have some disadvantages including expense, difficulty of operation, unreliablity and complexity of maintenance.
There are several different design variations of diving rebreather. All types have some form of loop that the diver inhales from and exhales into, a counter lung to hold gas when it is not in the diver’s lungs, a carbon dioxide scrubber to remove that gas from the loop and a supply of an oxygen-rich gas to inject into the loop. The gas supplied to rebreathers uses Aqua-Lung technology: the gas is stored in cylinders at high pressure and a regulator delivers the gas at ambient pressure to the loop. Most of the variants of rebreather have some sort of twin hose mouthpiece where the direction of flow of gas through through the loop is controlled by one-way valves.
Rebreather diving equipment is called closed circuit SCUBA as opposed to Aqua-Lung equipment, which is known as open circuit SCUBA.
The main Rebreather design variants are:
In fully closed-circuit systems there is a mechanism that injects oxygen into the loop when it detects that the oxygen supply has fallen below the required level. Often this mechanism is electrical and relies on oxygen sensitive galvanic fuel cells to measure the concentration of oxygen in the loop. The diver adds manually diluent gas to the loop to prevent the loop's gas mixture becoming too oxygen rich: the inert gases, such as nitrogen or helium, in the diluent gas dilute the oxygen in the mixture making it safe to breath.
Diving rebreathers
This is oldest type of rebreather and was commonly used by navies from the early twentieth century. The only gas the rebreather supplies is oxygen. As pure oxygen is toxic when breathed at pressure, oxygen rebreathers are limited to a depth of 6 meters (20 feet).
Military and recreational divers use these because they provide good underwater duration with fairly simple and cheap equipment. Semi-closed circuit equipment generally supplies one gas, which must be a mixture containing oxygen such as air, nitrox or trimix. The gas is injected at a constant rate. As the amount of oxygen required by the diver increases with work rate, the injection rate must be carefully chosen and be controlled to avoid unconsciousness in the diver. Excess gas is constantly vented from the loop in small volumes.
Military, photographic and recreational divers use these because they provide very good underwater duration and produce no bubbles. Closed circuit rebreathers generally supply two gases to the loop: one is oxygen and the other a diluent gas such as air, nitrox or trimix. The major task of the rebreather is to control the oxygen level in the loop and to warn the diver if it's too low or too high. The concentration of oxygen alters with depth as well as the proportion of oxygen in the mix. Too low a concentration results in unconsciousness and death. Too high a concentration results in oxygen toxicity, a condition similar to an epileptic fit.Diving Rebreather Manufacturers
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