device for underwater
use that reuses at least part of each breath
, in contrast, expells the entire breath
into the surrounding water
Rebreathers are able to reuse the oxygen not used by your body during each breath and remove CO2 with a chemical that acts as a "trap" agent. This results in extended dives with a small tank. Also, a rebreather produces no bubbles and thus a much quieter dive.
There are basically two classes of rebreathers: The semi-closed and the fully-closed. Semi-closed rebreathers expel a part of the breathing gases, whereas fully-closed rebreathers are completely self-contained. Both models, however, need to expel gases during the ascent, to maintain proper gas pressure in the system.
Some rebreathers allow a diver to stay underwater with very little gas for over eight hours.