Abstract: An air bag system has an improved temperature compensated, pressure responsive switch of construction adapted to respond to a reduction of vessel gas pressure or to the pressure of gas in the switch itself, to reliably inform the vehicle occupants of an abnormal condition. The pressure responsive switch includes a housing defining a volume that is initially at the same pressure as the pressure of the vessel, and a diaphragm in the switch housing segregating the atmospheres of the housing and vessel from each other. An electrical indication circuit in part composed of a pair of electrodes extending from the switch, helical springs in compression between the electrodes and diaphragm, and a resistor, is maintained normally electrically closed. The diaphragm is so constructed and arranged as to rupture or shear from the switch housing in response to a predetermined pressure differential.