The principle of measurement is based on the fact that a vane wheel rotates at a speed proportional to the flow velocity of a fluid into which it is immersed. The rotational speed is almost independent of density, pressure and temperature of the medium. The sensing of the vane wheel rotation is by means of a proximity switch without the slightest braking effect on the vane wheel. By fitting a further proximity switch, sensing of the direction of the vane wheel rotation and the indication of the +/-direction of flow is made possible. Determined by its light weight, the vane wheel rotational speed adapts in milliseconds to velocity increases, even when being used in gases. All vane wheel flow sensors are calibrated on the same frequency, so that sensors of the same type are interchangeable.