Abstract: A device for monitoring and/or measuring the flow velocity of a flowing fluid comprises temperature sensing means adapted for location in said flowing fluid. The sensing means comprises two identical monolithic semiconductive crystals (integrated circuits) each arranged in a heat conducting case. Each crystal contains a number of series connected diode paths serving as temperature sensors. One of the crystals in addition includes a heating member, such as a semiconductor device having a diode, transistor or Zener diode characteristic with a low temperature coefficient. A constant voltage is supplied to said heating member thereby developing constant power, and thus a constant temperature rise, in said one crystal over a large temperature range. Means are arranged for supplying current in the forward direction in the two diode paths, and a comparison circuit is connected to the diode paths for comparing the currents therein, whereby the flow velocity may be determined.