Viewed from behind, the air-mass treatment is clearly visible; it is the whole cylinder in the middle. Unit is divided into two parts: the head, in which the electronics are concentrated, and the canister (the lower part) containing the scrubber (chemical filter), which removes carbon dioxide from exhaled air. The control computers are housed in the upper part of the head (circled in red). In the photograph, we can see only their covers located at the top, from which the four connectors of the display units and two breathing hoses protrude (the hoses are far more visible from the front of the apparatus).
The pressure tanks are situated on the sides of the apparatus (diluent at left and oxygen at right). With a rebreather, it generally holds true that the diluent determines the volume of the breathing mixture and oxygen determines its breathability. Air is most commonly used as the diluent for shallower dives and trimix for deeper dives. Unlike open-circuit diving, priority is given to a mixture with a leaner oxygen content, because oxygen is added from a separate tank (other reasons for this are explained in detail in the user and training manuals).
Proper replenishment of oxygen is the main task of the control computers and all electronic and electromagnetic devices in the head. The amount of oxygen – specifically the partial pressure of oxygen in the breathing loop – must be maintained within relatively narrow ranges. This purpose is served by the quartet of ppO2 sensors and two electrically controlled valves (solenoids) for replenishing oxygen. The control computers evaluate data from the sensors and, as needed, open the solenoids, which add oxygen to the loop. Oxygen is transferred to the head via the hose that can be easily seen between the oxygen tank and the canister when the air-mass treatment unit is removed.