There are three different types of rebreathers. These are:
Oxygen Rebreathers: These are rebreathers that are primarily used by the military. Only pure oxygen is injected into the breathing loop, which makes for a fully closed system. The depth to which a diver can dive is limited due to breathing pure oxygen, but there are no bubbles produced and decompression stops are not necessary because there are no inert gases to be absorbed by the diver's system.
Semi-closed Rebreathers (SCR): In the SCR system, the gas used is Nitrox (oxygen enhanced air) which is injected into the system continuously, which means that air must be continuously breathed in and out. The injection rate is matched to the breathing rate of the diver and while there is greater gas economy, decompression stops are still necessary.
Fully Closed Circuit Rebreathers (CCR): In CCR systems there are two gases that are used: pure oxygen and another gas, such as air, tri-mix, or heliox. A dilutant is added to the loop during descent to make up the bulk of the volume in the loop, but it is pure oxygen burned by the diver. The consumption of oxygen is independent of depth and is based on the metabolic rate of the diver. The gas in the breathing loop must be monitored closely in a CCR system. Oxygen can be injected into the loop automatically or manually.