Rebreathers revolutionized the world of both scuba diving and military diving. These units are compact and light so the diver is not weighted down. They are far less costly to operate too. They recycle spent air back into the system so they use less gas. They are much different in use that a normal scuba set so you should seek training before attempting to use one.
The partial rebreather is a breathing set that recycles exhaled air and mixes that with a breathing gas containing oxygen. This reduces the amount of breathing gas needed. It is lighter and much more compact than the standard diving equipment. The exhaled air is filtered through a carbon dioxide scrubber containing a soda lime absorbent. The cleaned air then reenters the line and is mixed with the breathing gas.
The Italians were the first to use rebreathing technology for diving. In the 1930s, the use of rebreathers by spear fisherman came to the attention of the Italian Navy. Italy's Navy developed it into its now famous frogman units. In World War II, the British captured a few Italian frogman and this lead to the development of the British version. By the 1940s, the U.S. Navy was producing them. By the 1950s, divers around the world were using rebreathers.
The main advantage to a rebreather is its ability to use far less gas than standard systems. Because the unit recaptures exhaled oxygen that is lost with normal scuba equipment, the diver only needs to carry a fraction of the amount of gas on board. Except when ascending, the rebreather produces no air bubbles. This allows the diver to be stealthy.
There are several disadvantages to rebreather diving. The most common is sudden blackout caused by too low oxygen pressure in the loop. While ascending there is a drop in ambient pressure that can clamp the closed loop system of the rebreather. Sometimes the oxygen mix is to high in the mask, causing seizures due to oxygen toxicity. If the carbon dioxide gets too high, this can cause disorientation, panic, and headache. These levels can usually be controlled by the diver.
In the 1960s, a cryogenic rebreather was built by Sub-Marine Systems Corp. It used liquid oxygen and no absorbent canister. The carbon dioxide was simply frozen out in a snow box. It had a duration of 6 hours and a dive depth of 200 meters. There are several models of this type of rebreather in production, but the need to insulate the liquid oxygen from the heat from the surrounding water makes it difficult to be put to practical use in diving. If all the kinks are worked out, this could revolutionize the diving world. Sets could be as small as a 2-liter bottle and weigh less than 3 pounds.