Abstract: A gas mass flow sensor or probe (35, 37, 39) of an air flow sensing and control system (17, 19, 21) based upon hot-wire type devices where flow sensing is achieved by controlling the difference between two temperature sensing elements (51 and 53) suspended in the flow stream (41, 43, 45). The first element (53) is used to measure the ambient temperature of the fluid flow and the second element (51) is maintained at a programmed temperature (121) above ambient by a current-fed heater (49). The mass flow rate density is determined from heater current (117) required to maintain the temperature difference and the total flow in the duct is inferred. To measure the flow in which the elements (51, 53) are submerged, electronic circuitry monitors the two temperature elements (51, 53) and controls the amount of current through the heater (49) such that there will always be a predetermined difference between the ambient and heated temperatures. The sensor provides an output signal (101) for use by other devices (23).