a gas flow detection device having an inlet for connection to a source of gas and an outlet for connection to a gas - using appliance is disclosed . gas for feeding a pilot flame of the appliance flows through a permanently open orifice . when the appliance is switched on the pressure at the devices outlet drops and a piston which is in a tube is lifted by gas pressure at the device &# 39 ; s inlet . gas then flows from the inlet to the outlet . a light source and a light sensitive cell provide a signal that indicates that the piston has lifted . a monitoring system which makes use of the signal provided by the cell is also disclosed .