The diluent has several functions:

• Automatically injected into the inhale counterlung by the ADV (Auto Diluent addition Valve) on descent in order to maintain counterlung volume (or manually injected by the diver if no ADV is fitted). It is also used for BCD and dry-suit inflation. Only small quantities are used – typically 30-40 bar per dive.

• As a ‘diluent’ or dilutant it dilutes the gas mix so that it is safe to breathe at depths greater than 6m.

• The third and extremely important function is that it provides a reserve gas supply for either a ‘diluent flush’ or open circuit bailout if needed. (See Safety & Bailout)