A. Exhaled breath leaves the diver’s lungs and is directed into a one-way loop or closed circuit starting at the unit’s exhale counterlung.

B. It then travels, via a water-trap, over the right shoulder into the scrubber unit where it passes upwards through a chemical (Sofnolime) filter stack. This effectively scrubs the breathing gas of carbon dioxide.

C. Inside the mixing chamber, three independent oxygen sensors measure the oxygen pressure, the PO2, of the mix. If the PO2 drops (due to the diver’s metabolism or due to ascending) below a predetermined target level called the PO2 setpoint the oxygen controller opens an oxygen valve or solenoid bringing the PO2 back up to the setpoint – a very simple system.

D. Scrubbed and refreshed the breathing gas returns via another water-trap to the inhale counterlung ready for use in the next cycle.