Rebreather
Inspiration Closed Circuit Diving Rebreather
Rebreather technology is used in many environments:
- underwater - where it is known as "closed circuit SCUBA" as opposed to Aqua-Lung equipment, which is known as "open circuit SCUBA"
- mine rescue - where poisonous gases may be present or oxygen may be absent
- space suits - space is a near vacuum where there is not enough oxygen to support life
- hospital anesthesia breathing systems
- submarines and hyperbaric oxygen therapy chambers where the atmosphere for the habitat must remain safe
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.
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2 Parts of a rebreather 3 Main Rebreather design variants 4 Rebeather risks 5 Diving Rebreather Manufacturers 6 Sources |
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 or heliox. Rebreathers produce many fewer bubbles than Aqua-Lungs, which is useful for photographers and military divers because the rebreather makes those divers less visible both underwater and from the surface.
Back of an Inspiration Diving Rebreather
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 to have carbon dioxide removed. At present, there is not 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 note 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 because rebreathers allow very long dives where long decompression stops may be needed: knowing that the rebreather will 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.
The mouthpiece often has a valve allowing the diver to remove the mouthpiece 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 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 of 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. 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. Excess gas is constantly vented from the loop in small volumes.
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 oxygen and the other is a diluent gas such as air, nitrox or trimix. The major task of the rebreather is to control the oxygen concentration, or partial pressure, in the loop and to warn the diver if it's becoming dangerously low or high. The concentration of oxygen depends on two factors: depth and the proportion of oxygen in the mix. Too low a concentration results in hypoxia leading to unconsciousness and death. Too high a concentration results in oxygen toxicity, a condition similar to an epileptic fit.
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, from the diluent diving cylinder are used to dilute the oxygen in the loop making it safe to breathe.
In addition to the other diving disorders suffered by divers, rebreather divers are also more susceptible to :
Diving rebreathers
Parts of a rebreather
Main Rebreather design variants
Oxygen rebreather
Semi-closed circuit rebreather
Fully closed circuit rebreather
Rebeather risks
Rebeathers compared to Aqua-Lungs have some disadvantages including expense, difficulty of operation, unreliability and complexity of maintenance.Diving Rebreather Manufacturers
Sources