The body of the unit is divided into two parts: the head, in which the electronics are housed, and the canister with the chemical filter. The control computers are housed in the upper part of the head. 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 or heliox for deeper ones. Unlike open-circuit diving, priority is given to a mixture with a leaner oxygen content, because oxygen is added from a separate tank. 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.