When breathing, a diver consumes oxygen from air and exhales carbon dioxide. In order for a closed circuit to function, the rebreather must do the exact opposite: remove carbon dioxide from air and replace the consumed oxygen.
Oxygen is replenished in a relatively simple manner. Immediately after the air intake of the air mass treatment unit, there is an electronically controlled valve (solenoid) marked with the letter S. Oxygen is added to the loop when this valve opens. The means by which it recognizes how much oxygen should be added and the reason why oxygen is added first and carbon dioxide is removed only afterwards will be explained below.
If we continue in the direction of the arrows, we see that air flows in the space between the canister and the cover of the carbon dioxide scrubber, which is made of finely perforated sheet metal. Air flows through the holes in the sheet metal into the scrubber and proceeds to the centre of the scrubber, where it encounters more perforated sheet metal. Between these two sheet-metal components, there is an extremely expensive chemical that was developed for the space program and can be produced by only one factory in the world…actually, no, we’re just kidding. In reality, this component contains small granules of slaked lime (calcium hydroxide) with a small amount of added sodium hydroxide. The actual price of this substance is roughly EUR 2.50 (USD 3.00) per hour of operation. The lime reacts with carbon dioxide and removes it from air with the concurrent formation of limestone (CO₂ + Ca(OH)₂ → CaCO₃ + H₂O).
After passing through the filter but before exiting the air-mass treatment unit in the direction of the arrows, air flows around the sensors (the places where air comes into contact with the sensors are circled). The sensors send information about the amount of oxygen to the control computer, which employs the solenoid to add oxygen so that it is maintained at the required level.
Oxygen is added prior to air reaching the filter for purely practical reasons: if oxygen were added after the filter, a small plume of oxygen would immediately come into contact with the sensors, which would thus show completely incorrect values. Therefore, it is better to allow the breathing mixture with added oxygen to pass through the scrubber, where the oxygen blends sufficiently with the breathing mixture, thus minimizing influence on the sensors.