A positive pressure demand valve for breathing apparatus incorporates a tubular "balanced piston" valve member for regulating the flow of breathing gas from an inlet to an outlet. A coaxial tension spring acts to bias the valve member open and a lever coupled to a pressure-responsive diaphragm acts to...http://www.google.com/patents/US5501213?utm_source=gb-gplus-sharePatent US5501213 - Gas flow control valves
