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Chapter 5 — 16
Closed and Semi-closed Circuit
REBREATHING APPARATUS
With this equipment some or all of the diver's exhaled gas is passed through a carbon dioxide
absorber ("scrubber") and then rebreathed from a breathing bag ("counterlung"). This minimises gas
usage, produces fewer bubbles and allows smaller cylinders to be used for an equivalent dive
duration.
Fig. 5.18
Diagram of two types of closed circuit oxygen rebreathing sets.
A military system, using 100% oxygen in a closed circuit, is employed for clandestine operations
(blowing up ships, examining potential landing sites, etc.). Because the diver rebreathes 100%
oxygen, there is a risk of oxygen toxicity, so these sets have a practical depth limit of 9 metres.
Some of these sets have a demand-type system, where gas is supplied automatically when the
volume in the counterlung is reduced. Others have a continual gas supply, with excess being
exhausted.