Valve for breathing apparatus

A positive pressure demand valve for breathing apparatus incorporates a tubular "balanced piston" valve member (6) for regulating the flow of breathing gas from an inlet (2) to an outlet (15). A coaxial tension spring (18) acts to bias the valve member open and a lever (25) coupled to a pressure-responsive diaphragm (20) acts to push the valve member closed when a predetermined pressure is attained within the outlet. A combined pressure-relief and bypass valve member (13) enables the operation of the main valve member (6) to be bypassed by porting gas through the centre (9) of the main valve seat (8). A "first breath" mechanism is provided comprising a hooked link (17) which can be pressed in by a button (36) to maintain the main valve member closed and can be released by contact by the diaphragm (20) when a negative pressure is applied to the outlet (15). The outlet is in the form of a bayonet connector which is locked into a corresponding inlet fitting on the user's mask by means of the same button (36).