 |
|
| ||||||||||||||||||||||||||
|
|
| ||||||||||||||||||||||||||
|
|
|
|
|
| ||||||||
A Rebreather is a type of breathing equipment that provides an oxygen-based breathing gas and recycles exhaled gases. This recycling reduces the volume of breathing gas used making a rebreather a lightweight and compact machine for supplying breathing gas for long durations in environments where humans cannot safely breathe from the atmosphere.
Rebreather technology is used in many environments:
The rebreather takes advantage of the fact that an inhalation typically contains twenty times more gas than the body’s metabolism actually consumes. The rebreather is designed to recycle the useful portion of the exhaled gas for further inhalation. The body also produces, as part of the metabolic process, carbon dioxide, which is poisonous and therefore must be removed from the breathing gas before it is re-inhaled.
Rebreathers capture the exhaled gas, remove carbon dioxide from it and inject supplemental oxygen back into the gas mixture so that the mixture, which is about to be returned to the diver to breathe, is able to support life and the physical activity of the diver.
The first closed circuit breathing device using stored oxygen and adsorption of carbon dioxide by a caustic soda was invented by Henry Fluess in 1879 for the rescue of mineworkers who were trapped by water.
The main advantage of the rebreather over other breathing equipment is the breather's economic use of gas. With the "open circuit" Aqua-Lung, an alternative form of SCUBA, the entire breath is expelled into the surrounding water when the diver exhales. So, long or deep dives using open circuit equipment require much more gas than when using a rebreather. This open circuit gas must be carried by the diver in heavy and bulky diving cylinders.
The economy of gas consumption is also useful when the gas being breathed is expensive, such as the helium in trimix or heliox gas mixes used in technical diving. Also, rebreathers produce many fewer bubbles than Aqua-Lungs, making photographers and military divers much less visible.
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 diving cylinders at high pressure and a diving regulator delivers the gas at ambient pressure to the loop.
The active ingredient of the scrubber is often soda lime. It is important that all gas moving through the loop passes through the soda lime so that its carbon dioxide is removed. At present, there is no effective technology for detecting the end of the life of the scrubber or a dangerous increase in the concentration of carbon dioxide. The diver must monitor the exposure of the scrubber and replace it when necessary. Carbon dioxide gas sensors do exist, but they are not sensitive enough to be used in a rebreather - the scrubber "break through" occurs quite suddenly and the diver shows symptoms before the sensor indicates a dangerous build-up of carbon dioxide.
Even if a sensitive gas sensor is developed, it may not be useful as the primary tool for monitoring scrubber life when underwater because rebreathers allow very long dives where long decompression stops may be needed: knowing that the rebreather will begin to deliver a poisonous breathing gas in five minutes may not be useful to a diver needing to carry out an hour or more of decompression stops.
Most of the variants of rebreather have some sort of "twin hose" mouthpiece where the direction of flow of gas through the loop is controlled by one-way valves. Some have a "pendulum" hose, where the inhaled and exhaled passes through the same tube but in different directions.
The mouthpiece often has a valve allowing the diver to take the mouthpiece from the mouth underwater without water entering the loop. Many rebreathers have "water traps", which prevent large volumes of water entering the loop if the diver removes the mouthpiece underwater without closing the valve.
In many rebreathers the diver is able to control the gas mix in the loop manually by injecting each of the different available gases to the loop and by venting the loop. The loop often has a pressure relief valve preventing the "hamster cheek" effect on the diver by over-pressure of the loop.
The position of the "counter lungs", on the chest, over the shoulders or on the back, has an effect on the ease of breathing.
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 inhaled 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 breathing gas such as air, nitrox or trimix. The gas is injected at a constant rate. Excess gas is constantly vented from the loop in small volumes.
The diver must fill the cylinders with gas mix that has a maximum operating depth that is safe for the depth of the dive being planned. As the amount of oxygen required by the diver increases with work rate, the injection rate must be carefully chosen and controlled to prevent unconsciousness in the diver due to hypoxia.
Military, photographic and recreational divers use these because they allow long dives and produce no bubbles. Closed circuit rebreathers generally supply two breathing gases to the loop: one is pure oxygen and the other is a diluent or diluting gas such as air, nitrox or trimix.
The major task of the fully closed circuit rebreather is to control the oxygen concentration, known as the oxygen partial pressure, in the loop and to warn the diver if it is becoming dangerously low or high. The concentration of oxygen in the loop depends on two factors: depth and the proportion of oxygen in the mix. Too low a concentration of oxygen results in hypoxia leading to sudden unconsciousness and ultimately death when the oxygen is exhausted. Too high a concentration of oxygen results in oxygen toxicity, a condition causing convulsions, which when they occur underwater can lead to drowning.
In fully closed-circuit systems there is a mechanism that injects oxygen into the loop when it detects that the partial pressure of oxygen in the loop has fallen below the required level. Often this mechanism is electrical and relies on oxygen sensitive electro-galvanic fuel cells to measure the concentration of oxygen in the loop.
The diver may be able to manually control the mixture by adding diluent gas or oxygen. Adding diluent can prevent the loop's gas mixture becoming too oxygen rich. Manually adding oxygen is risky as additional small volumes of oxygen in the loop can easily raise the partial pressure of oxygen to dangerous levels.
Many diver training organizations teach the "diluent flush" technique as a safe way to restore the mix in the loop to a level of oxygen that is neither hypoxic nor hyperoxic. It only works when partial pressure of oxygen in the diluent alone would not cause hypoxia or hyperoxia, such as when using a normoxic diluent and observing the diluent's maximum operating depth. The technique involves simultaneously venting the loop and injecting diluent. This has the effect of flushing out the old mix and replacing it with a known proportion of oxygen from the diluent.
In addition to the other diving disorders suffered by divers, rebreather divers are also more susceptible to :
When compared with Aqua-Lungs, Rebeathers have some disadvantages including expense, difficulty of operation, unreliability and complexity of maintenance.
|
||||
| View Live Article | This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License | |
||
