Abstract: A fluid flow sensing device comprises first and second p-n junctions, one of which is arranged to be disposed in the fluid flow to be measured and the other of which is arranged to be disposed at a position such that its temperature depends only on that of the fluid. An amplifier monitors the voltage drop across the first p-n junction. A comparator compares the measured voltage drop across the first p-n junction with a reference level to provide a feedback voltage which is used to control the heating of the first p-n junction such as to maintain the temperature of that junction constant irrespective of variations in the fluid flow or the fluid temperature. A current is passed through the second p-n junction and the voltage drop across it is monitored and compared with the feedback voltage to provide a signal which is a function of fluid flow alone.