The SFS Ferro Sensor records the number of ferromagnetic particles accumulating at the permanent magnet at the sensor head. In this regard, the sensor can distinguish between fine particles in the micrometer range and coarse ferromagnetic fragments in the millimeter range. According to the output signal of 0 … 100% the distribution of ferromagnetic particles at the sensor surface can be read off. Furthermore, the sensor may compensate the magnetic field of the permanent magnet, whereupon the particles are released from the sensor head (automatic cleaning process). With the time intervals between two cleaning processes, a change in wear can be assumed.