A device for measuring a parameter such as the mass concentration or the viscosity of a fluid medium such as a suspension or a liquid, including a rotor for rotation in the medium, a drive motor, a drive transmission with a drive shaft for the rotor, and a sensor device for sensing the torque which is transmitted to the rotor and which represents the parameter. The device is distinguished by that the sensor device includes an excitable frequency gauge which is deformed by the torque, and the prevailing natural frequency of which his dependent on the deformation and directly detectable by a stationary detector, which is placed outside the transmission.