Abstract: A flowmeter with a rotor (21) is provided with blades (22) which are rotatably and coaxially supported in an essentially cylindrical bore (15). The rotor (21) is preceded by a screw-shaped and coaxial distributor (16) imparting a spin to the incident medium before this medium loads the blades (22). The rotation of the rotor (21) is detected in a contactless manner by a light barrier (29, 30, 31). In order to linearize as much as possible the flowmeter characteristics also in the case of impulsive flow, the rotor (21) is provided with blades of which the surfaces (22') loaded with the spinning flow are concave and are curved solely about axes of curvature which are parallel to the rotor axis (15'). Each blade (22) is provided with a slot (28) starting at one end of its side edges. The light beam (29) of the light barrier (29, 30, 31) orthogonally crosses the rotor axis (15') at a distance whereby one count signal is generated for each blade.