The IS-MIX is fitted with a hydrostatic valve. This valve prevents squeezing of the breathing circuit during rapid descents.

The hydrostatic valve is physically installed in the regulator block.

The hydrostatic valve automatically takes the ambient pressure as a reference pressure following each new breath. When a depth change occurs (following a dive/pressure increase) that is greater than one metre without the diver breathing out at the same time, the difference between the reference pressure and the pressure in the breathing circuit is so large that the hydrostatic valve will open and fill the breathing circuit's inhalation side with gas. When the diver then exhales and the bellows are raised, the bellows arm shaft will rotate and pull the hydrostatic valve out via a nipple and reset (pressure equalize) the hydrostatic valve's reference chamber until it has the same pressure as the breathing circuit.

At normal diving speed and breathing frequency, the hydrostatic valve will never open, the bellows volume will be kept constant with gas filling from the gas dosage (influenced by breathing) and by the surplus valve's position-regulated draining.

The volume of gas in the breathing circuit (including the diver) is kept constant and the buoyancy in the IS-MIX is therefore maintained automatically. The diver does not therefore need to compensate the volume in the breathing circuit or buoyancy manually following a change in depth.