Rebreathers

With rebreathers, the gas the diver exhales is stored between breaths in a "counterlung". In some rebreathers, one-way valves direct the gas through a "loop". In other rebreathers, the inhaled and exhaled gas goes back and forth along a single tube: this is called the pendulum system. The oxygen consumed by the diver is replaced, nearly always from a cylinder. The exhaled carbon dioxide generated by the diver is removed by passing the gas through a "scrubber": a canister full of soda lime. Then the gas is fit to be re-inhaled. This type of scuba equipment is known as 'closed circuit'.

Since 80% or more of the oxygen remains in normal exhaled gas, and is thus wasted, rebreathers use gas very economically, making longer dives possible and special mixes cheaper to use at the expense of more complicated technology and more experience and longer training. There are three variants of rebreather: oxygen rebreathers, semi-closed circuit rebreathers, and fully closed circuit rebreathers.

The rebreather's economic use of gas, typically 1.6 litres of oxygen per minute, allows dives of much longer duration than is possible with open circuit equipment where gas consumption is typically 10 times higher. Oxygen rebreathers have a maximum operating depth of around 6 meters / 18 feet, but several types of fully closed circuit rebreathers, when using a helium-based diluent, can dive deeper than 100 meters / 330 feet. The main limiting factors on rebreathers are the duration of the carbon dioxide scrubber, which is generally at least 3 hours, and that the scrubber gets less efficient at depth because the scrubber's inside is more crowded with diluent molecules hindering the carbon dioxide molecules from reaching the absorbent as quickly.