A device is provided for measuring the velocity of flow of a fluid in a respiration system and includes a first thermal sensor element (5) provided with a controllable heating element (50) and a second thermal sensor element (6). The thermal sensor elements (5, 6) are arranged at spaced locations from one another at a path of flow, so that a thermal signal generated by the first sensor element (5) with the heating element (50) is transmitted to the second sensor element (6), and the second sensor element (6) is designed to detect the thermal signal from the fluid flow. The second sensor element (6) is connected to the first sensor element (5) via feedback (12) which triggers another thermal signal. A controlling and analyzing device (13, 15) is connected to the sensor elements (5, 6) to start the generation of a first thermal signal and to read and analyze the signal frequency as an indicator of the velocity of flow.