The bellows have a surplus valve, which empties the excess gas to the surroundings and also acts as an overpressure safety valve. The adjuster knob is used by the technical diving staff to adjust the opening position of the surplus valve.

The breathing circuit is supplied with breathing gas from the gas supply and carbon dioxide from the diver. Oxygen is taken from the breathing circuit that the diver uses.

The excess gas in the breathing circuit is expelled through a valve controlled via end position. When the breathing bellows are in their upper position, the cord is stretched whereupon the surplus valve opens. The gas that then leaves the breathing circuit is used breathing gas with a high C02 content and low oxygen content.

The canister is placed in the breathing circuit so that it only cleans the gas that is to be breathed again.

As the surplus valve has a volume controlled opening, this means that:

no work is needed to drain excess gas,

there is never any overpressure in the bellows,

the buoyancy of the IS-MIX remains constant regardless of depth,

there is always a full breath for the diver to take.