The invention provides an in-line low supply pressure alarm device powered solely by supply flow of pressurised gas from a gas supply for providing an alarm signal when supply gas pressure is below a selected minimum pressure. The alarm device includes a manifold having an input port for communicating with the supply gas supply, an output port for conducting the gas downstream and a manifold chamber disposed therebetween. Gas powered alarms such as an audible reed alarm or a visual pneumatic alarm are connected to the manifold chamber via an alarm supply conduit, and produce an alarm signal when pressurised gas passes to the alarms. A supply gas pressure sensor, in communication with the manifold chamber, produces an actuating flow of pressurised gas by activating a pressure switch, in response to sensing of a supply gas pressure below the selected minimum pressure. A pneumatic alarm output switch, in the alarm supply conduit and in communication with the pressure sensor and pressure switch via an actuation conduit, controls gas flow to the alarms in response to the actuating flow. Preferably, an alarm oscillation system is included for alternating the direction of the actuating flow to and from the alarm output switch, to open and close the alarm output switch thereby turning the alarm on and off in a cyclical fashion.
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