1. A selector valve for controlling the flow of oxygen from a primary, or a secondary, backup source, singly or in combination, either selected manually or automatically to suit environmental condition, said selector valve comprising: a three-position manual switch including a cam surface; a slidable stem controlled by said cam surface; a piston surface mounted on said stem and exposed on one side to a control pressure derived from the primary source; a poppet valve for controlling the flow of oxygen from the secondary source, said poppet valve being coupled to said stem; control pressure vent means for venting said control pressure to atmosphere, whereby, (1) in the first position of the switch the poppet valve is mechanically closed by said stem to prevent the flow of oxygen from the secondary source, (2) in the second position said control pressure maintains the poppet valve mechanically closed, and (3) in the third position said control pressure is vented to atmosphere, allowing the poppet valve to open to allow the flow of oxygen from the secondary source.