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As a diver rebreathes through a CCR, during each breathing cycle (that is, one inhale and one exhale) the diver’s body metabolises some of the oxygen as it passes through the body, producing carbon dioxide (CO2). The expired breathing gas thus contains less oxygen than the gas the diver inhaled. In a CCR, that expired gas is cleaned of the dangerous CO2 in a scrubber canister filled with sofnalime and then analysed in a chamber in the rebreather by onboard oxygen sensors. The results trigger a solenoid switch to open and bleed just the right amount of oxygen into the breathing gas to keep the PO2 at the desired level of say 1.3 bar, no matter what depth the diver is at. So, on the ascent, all the way to the surface, the rebreather is trying to inject oxygen to keep the PO2 at say, 1.3 bar. At 20 metres, a CCR diver can be breathing a PO2 of 1.3 bar – but in contrast to breathing EAN50 on open circuit, at the final deco stop at 6 metres a CCR diver is breathing almost pure oxygen. The diver is getting the optimum amount of oxygen, so beneficial to decompression, at any point.