Abstract: A non-intrusive mass flow measuring apparatus includes a sensor unit and a processor unit. The sensor unit includes a sensor conduit, a heating element, and three separate temperature sensor elements. The three temperature sensor elements are spaced out along the length of the sensor conduit from the heating element. The middle, compensating temperature sensor element, enables the coefficient of conductive heat transfer along the conduit to be decoupled from the coefficient of convective heat transfer from the fluid, from which mass flow may be derived. A processor uses the temperatures sensed by the three sensors to determine a normalized temperature parameter related to the coefficient of convective heat transfer, and then utilizes temperature data and the normalized temperature parameter in a preferably empirically determined relationship to determine a mass flow value.