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 should then be pulled (outwards) to disable the warning and allow access to the remaining pressure in the gas supply.

The reserve gas valve can be pulled during the dive to check that it is not disabled, and then pressed backwards again to enable the reserve gas function. This can be done all the way down to closing pressure (at least 20 bar) without gas being taken from the reserve gas volume.

The reserve gas valve is characterised as an active warning, in other words a signal that the diver cannot help but notice, unlike the pressure gauge which is a passive warning, which gives the diver the same warning information, but that must be read.