Wiktionary
n. A mechanism in a diving regulator which controls the supply of gas by opening to provide flow when the user inhales, and shuts off supply when inhalation stops.
Usage examples of "demand valve".
Air sighed softly into the helmet, its flow through the demand valve regulated by the rise and fall of my chest-I could reset it to speed up or slow down by the chin control.
Because the demand valve is higher in the water than the lungs there is a difference of pressure to be overcome which is awkward at first.
In fact I suggested as much to him and ordered him to check the demand valve and the rest of his breather gear before he went outside again.
Hawks motioned impatiently as the demand valve in his helmet began to draw air from his tanks again, its sigh filling his helmet.
They tested the twin cylinders each containing a thousand litres of free air compressed to two hundred atmospheres and Bond found the manipulation of the demand valve and the reserve mechanism simple and fool-proof.