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Rebreathers
Autor: Tom Stolarski, 17-07-2010

 

A rebreather is a self-contained breathing set that supplies the diver with the breathing gas and partially or completely reuses the exhaled air (absorbs the carbon dioxide and adds the oxygen or gas mixture from the cylinder). It enables a more efficient use of gas. Another advantage of using rebreathers is their silent operation due to reduced volume of exhaled bubbles (semi-closed rebreathers) no bubbles at all (closed-circuit rebreathers).

 Rebreathers typically consists of a breathing loop with a mouthpiece, tubes that transport the inhaled and the exhaled air, and a breathing bag. A breathing bag (often called a counterlung) serves to store the gas - it inflates when exhaling, and deflates when inhaling. However, a rebreather would not work without a CO2 absorbent canister. This piece removes carbon dioxide from the breathing gas. The set would not be complete without an oxygen source that injects the gas to the circulation. Proper functioning of a rebreather is possible thanks to the valves (an upstream check-valve and a downstream check-valve). There are three main types of rebreathers: oxygen rebreathers, semi-closed rebreathers, and closed-circuit rebreathers.

  • Oxygen rebreathers

 

An oxygen rebreather is the oldest and the simplest type. It consists of two stages: constant flow and a bypass. This type of rebreather supplies only pure oxygen either at a constant rate or at a rate that matches the metabolic consumption rate with a use of mechanical valve. The oxygen rebreathers are limited in function to a depth of about 6 meters due to the toxic properties of pure oxygen inhaled at pressure.

 

  • Semi-closed rebreathers

 

In a semi-closed rebreather the gas mixture (nitrox or trimix) is injected into the breathing loop at a constant rate. The excess gas that is not consumed by the diver (usually nitrogen or helium) is vented out of the breathing loop (very small bubbles). The gas mixture in the cylinder has to match the diving depth (maximum operating depth). Semi-closed rebreathers as well as closed-circuit rebreathers are not limited in function in terms of depth.

 

  • Closed-circuit rebreathers

 

Closed-circuit rebreathers also supply the gas mixture but in a different way than semi-closed type. All the gas is recycled (no bubbles emitted). A closed-circuit rebreather has two independent gas supply cylinders. One of them injects pure oxygen into the breathing loop. The other cylinder (called diluent) contains compressed air or gas mixture (nitrox, heliox, neox, or trimix). Thanks to electronic oxygen sensors, closed-circuit rebreathers maintain a comparatively constant partial pressure of oxygen in the breathing gas and automatically adds oxygen to the breathing loop when the partial pressure of oxygen drops below the set point.