Less common, but becoming increasingly available, are closed and semi-closed rebreathers. Open-circuit sets vent off all exhaled gases, but rebreathers reprocess each exhaled breath for re-use by removing the carbon dioxide buildup and replacing the oxygen used by the diver. Rebreathers release few or no gas bubbles into the water, and use much less oxygen per hour because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications
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