The single hose regulator separates the demand valve from the cylinder, giving the diver air at the ambient pressure at the mouth, rather than ambient pressure at the cylinder valve.
42.
The second stage demand valve regulator, supplied by a low-pressure hose from the first stage, delivers the breathing gas at ambient pressure to the diver's mouth.
43.
In demand valves where the cracking pressure is adjustable by the diver, it may also occur as a result of maladjustment of the cracking pressure ( dial a breath ) knob.
44.
A final alternative sometimes used by recreational divers, is to bail out to a standard demand valve by removing the full face mask completely, and then fitting a backup half mask.
45.
The alternative is to donate the primary demand valve that the donor is currently breathing from, on the principle that it is known to be working and is immediately recognizable and accessible.
46.
A helmet or full face mask will automatically equalise as any pressure differential will either vent through the exhaust valve or open the demand valve and release air into the low-pressure space.
47.
Two general systems have evolved for carrying and deploying the backup demand valve one more prevalent in recreational diving and the other commonly found in technical diving ( although some crossover exists ).
48.
In single hose regulators, the demand valve is the second stage, which is either held in the diver's mouth by a mouthpiece or attached to the full-face mask or helmet.
49.
The cracking pressure of a sensitive demand valve is often less than the hydrostatic pressure difference between the inside of an air-filled housing and the water below the diaphragm when the mouthpiece is pointed upwards.
50.
Towards the end of inhalation the bellows bottoms out and activates an addition valve, in much the way that a regulator diaphragm activates the demand valve, to make up the gas discharged by the inner bellows.
