CCUBA

Rebreather

A fully closed circuit electronic rebreather (Ambient Pressure Diving Inspiration)
Acronym	CCUBA (closed circuit underwater breathing apparatus); CCR (closed circuit rebreather), SCR (semi-closed rebreather)
Uses	Breathing set
Related items	Davis apparatus

A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the user. This differs from an open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment.

Rebreather technology may be used where breathing gas supply is limited, such as underwater or in space, where the environment is toxic or hypoxic, as in firefighting, mine rescue and high-altitude operations, or where the breathing gas is specially enriched or contains expensive components, such as helium diluent or anaesthetic gases.

Rebreather technology is used in many environments:

This may be compared with some applications of open-circuit breathing apparatus:

The recycling of breathing gas comes at the cost of mass, bulk, technological complexity and specific hazards, which depend on the specific application and type of rebreather used.

As a person breathes, the body consumes oxygen and produces carbon dioxide. Base metabolism requires about 0.25 L/min of oxygen from a breathing rate of about 6 L/min, and a fit person working hard may ventilate at a rate of 95 L/min but will only metabolise about 4 L/min of oxygen The oxygen metabolised is generally about 4% to 5% of the inspired volume at normal atmospheric pressure, or about 20% of the available oxygen in sea level air. Exhaled air at sea level still contains roughly 16% oxygen.

The situation is even more wasteful of oxygen when the oxygen fraction of the breathing gas is higher, and in underwater diving, the compression of breathing gas due to depth makes the recirculation of exhaled gas even more desirable, as an even larger proportion of open circuit gas is wasted. Continued rebreathing of the same gas will deplete the oxygen to a level which will no longer support consciousness, and eventually life, so gas containing oxygen must be added to the breathing gas to maintain the required concentration of oxygen.