Rebreather: Difference between revisions

A Rebreather is a type of breathing equipment that provides an oxygen-based breathing gas and recycles exhaled gases, which econmises on breathing gas consumption making rebreathers suitable for long duration life support in environments where humans cannot breathe.

Rebreather technology is used in the following environments:

• underwater - for commercial, scientific and recreational SCUBA diving
• 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

An inhalation typically contains twenty times more gas than the oxygen that is actually used by the body's metabolism. 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 are designed to capture the unmetabolised exhaled gas, remove carbon dioxide from it and inject sufficient oxygen back into the gas mixture so that the mixture is able to support life, consciousness and possibly high work rates of the user of the rebreather.

Diving rebreathers

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 booth underwater and from the surface.

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 diving cylinders at high pressure and a diving 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:

• Oxygen rebreather

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).

• Semi-closed circuit rebreather

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.

• Fully closed circuit rebreather

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 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 breath.

Diving Rebreather Manufacturers

• Ambient Pressure Diving (http://www.ambientpressurediving.com/) - closed circuit rebreathers
• Drager (http://www.draeger.com ) – semi-closed circuit rebreathers
• Halcyon (http://www.halcyon.net/index.shtml) – semi-closed circuit rebreather
• Kiss (http://www.jetsam.ca/)) – closed circuit rebreather
• Steam Machines (http://www.steammachines.com/) - rebreather

Sources

• Richard Pyle’s rebreather page (http://www.bishopmuseum.org/research/treks/palautz97/rb.html )
• Engineering aspects of Apollo (http://www.hq.nasa.gov/alsj/apollo.engin.html)
• The Rebreather Site (http://www.therebreathersite.nl)