The surplus valve has an additional integrated function as the breathing circuit's overpressure safety valve.

The safety valve comes into operation in cases of fault which leads to increased pressure in the breathing circuit

If, for example, the surplus valve fails (cord snaps), the overpressure valve will act as a safety valve for the breathing circuit, and thereby the diver will continue to be able to use the breathing circuit. In this case, the dive must be aborted and the IS-MIX submitted for maintenance.