Abstract: To detect the volume of gas remaining in a gas bottle or a fixed tank of a system for supplying liquefied gas as fuel for a combustion engine, a positive temperature coefficient (or PTC) probe (16) is placed in the gas feed line (12) between the gas source and the combustion engine. The PTC probe (16) detects the change of phase, from the liquid phase of the liquefied gas to the gaseous phase, in the feed line (12) and delivers an electrical signal to an electronic circuit that triggers a visual or audible signal in response to the detection of the phase change.