In order to prevent the diver unwittingly using up his gas supply, the reserve gas valve will close the gas supply to the IS-MIX. This occurs when there is still at least 20 bar of pressure in the gas supply which is approximately 10 minutes of remaining diving time, although this time is dependent on the diver's activity.

When the IS-MIX is then depressurised, the IS-MIX's dosage warning is activated and the diver experiences breathing resistance to attract his attention.

The reserve gas valve handle should then be pulled (outwards) to disable the warning and allow access to the remaining pressure in the gas cylinder.