What is the difference between open and closed circuit systems?
In an open circuit system you exhale into the water, and the bubbles float away. This means a few things, one any oxygen left over in the air you exhale is lost, and two you create a sizable disturbance in the water around you. In a closed circuit system your exhaled air is sent to “scrubbers” to remove the carbon dioxide from it and then put back in the reservoir to be breathed again. This leads to almost no bubbles being released by a closed system. It also means you can stay down much longer with the same air supply, because humans only use about 20% of the oxygen in any breath.
