Breathing apparatus

Claim 1

In a single stage breathing apparatus, a demand valve comprising a valve body having a diaphragm chamber, an outlet means from said chamber for connection to a face mask, a diaphragm clamped within said chamber and being displaceable by pressure changes therein, and an inlet portion connected to said chamber comprising a plurality of body parts providing an inlet for admission of compressed breathable gas, a valve seating, a valve member urged into engagement with the valve seating, means for moving the valve member away from the seating when suction is applied to the outlet means to enable the breathable gas to pass from the inlet to the outlet means, and by-pass means enabling the breathable gas to by-pass the valve member and seating and thereby reach the outlet means to provide an emergency supply of breathable gas in the event of the valve member becoming lodged in the closed position, the by-pass means including a flow passage sized to allow a predetermined volumetric flow rate of breathable gas at a pressure within a predetermined range, a first chamber communicating with the sized flow passage and an orifice leading from the inlet to said first chamber, and a constant pressure reduction valve comprising a piston, one end of which is adapted to close said orifice leading from the inlet to said first chamber, biasing means urging the piston in a direction to open said orifice, means normally restraining the piston from movement by the biasing means but operable in an emergency to permit movement of the piston to open said orifice, another end of the piston forming a wall of a second chamber having a greater cross-sectional area than the cross-sectional area of the first chamber, and the piston having a bore therethrough communicating said first and second chambers enabling breathable gas under pressure to pass between said first and second chambers to control the pressure of breathable gas in the first chamber within the predetermined range when the constant pressure reduction valve is open.