The objects and purposes of the invention are met by providing an energy loss detecting system for measuring the rate of flow of the vapor phase of a bi-phase fluid flow, in the presence of an unknown quantity of the liquid phase. Means separate the vapor phase from the liquid phase. Means produce a signal related to the vapor phase flow rate. Energy loss due to a faulty steam trap is measurable by interposing the vapor flow rate measuring apparatus in a steam line between a steam consuming device and the steam trap.