"When we exhale, a large portion of the oxygen we inhaled, around 80 percent, is exhaled..."
Adam Altman, Long Island Divers Association
Scuba diving usually involves filling a tank with air, strapping it to your back, and breathing from it underwater. This simple system is called open circuit scuba (or self-contained underwater-breathing apparatus). Before this method of scuba diving caught on, however, people were using rebreathers. In 1878, Henry Fleuss built a diving system that allowed the user to breathe the same air over and over again. Using a rubber mask, a breathing bag, a copper tank, and a bit of string, he constructed the first scuba rebreather.
A rebreather works by removing carbon dioxide from the diver's exhaled gas and recycling its usable components. The contraption uses an expandable breathing bag to hold the exhaled gas and a system of valves to keep the gas flowing in only one direction. A carbon dioxide scrubber then filters the exhaled gas into a breathable form.
In the carbon dioxide scrubber, the exhaled air moves past an absorbent mixture often containing soda lime. The carbon dioxide reacts with the soda lime, and the other components of air pass through to be inhaled again. Air lasts longer with a rebreather, so users can use smaller tanks.
The rebreather is more efficient than normal scuba systems, whose users lose 75 percent of the available oxygen when exhaling. Rebreathers also produce dramatically fewer bubbles than their counterparts, thus helping naval divers to remain inconspicuous. Similar systems are used in space suits and by firefighters. Rebreathers are also now affordable enough for recreational divers to experiment without paying exorbitant costs.