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.
At 215 miles (346 km), it is the longest river entirely in England and the second longest in the United Kingdom, after the River Severn.
Rebreather technology is used in many environments: 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.
The volume of gas used is reduced compared to that of open circuit; therefore, a smaller cylinder or cylinders, may be used for an equivalent dive duration.
Rebreathers extend the time spent underwater compared to open circuit for the same gas consumption, they produce fewer bubbles and less noise than scuba which makes them attractive to covert military divers to avoid detection, scientific divers to avoid disturbing marine animals, and media divers to avoid bubble interference.