Abstract: Mass flow of a fluid flowing in a conduit is detected by mounting an active device in thermal contact with an external wall of the conduit, the active device being characterized by a positive temperature coefficient of resistance of at least 10%/.degree.C. for a range of temperatures, driving the active device to operate within the temperature range, and detecting mass flow based on the resistance of the active device. In another aspect, a passive temperature sensor is arranged to sense the temperature of the fluid at one position along the conduit, the passive temperature sensor being substantially non-self-heating, an active device is positioned to exchange heat at another position along the conduit, the conduit having a resistance dependent on temperature, and mass flow is detected based on signals from the active device and from the sensor.