When breathing, a diver consumes oxygen from gas and exhales carbon dioxide. For a closed circuit to function, the rebreather must do the exact opposite: remove carbon dioxide from gas and replace the consumed oxygen. Immediately after the gas intake of the gas mass treatment unit, there is an electronically controlled valve (solenoid).
Oxygen is added to the loop when this valve opens. After that the gas flows in the space between the canister and the cover of the carbon dioxide scrubber, which is made of finely perforated sheet metal, gas 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 are small granules of slaked lime (calcium hydroxide) with a small amount of added sodium hydroxide. The lime reacts with carbon dioxide and removes it from gas with the concurrent formation of limestone.
After passing through the filter but before exiting the air-mass treatment unit in the direction of the arrows, gas flows around the sensors. 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 gas reaching the filter for purely practical reasons: if oxygen was added after the filter, a small plume of oxygen would immediately come into contact with the sensors, which would thus show incorrect values.