invention first the position sensors are calibrated at 501 which position sensors are of the type described in connection with fig1 by pushing the position sensors out from the refining surface of the stator and moving them into contact with the refining surface of the rotor thereafter a non contact measurement of the distance between the refining surfaces at each measurement position according to the reluctance method is performed at 502 at three different measurement positions in the region of the refining gap and the refining surfaces by means of three position sensors positioned at different measurement positions where each position sensor measures the reluctance in a circuit in which at least the refining surface of the rotor and the refining gap are included the measurement of the distance is performed substantially along one and the same diameter of the refining surface of one of the refining discs after this the signals measurement results from the position sensors are processed at 503 the measurement results are analysed at 504 which analysis comprises comparison of the measurement results with each other the refining surface of the stator is adjusted based on this comparison in step 504 until substantially the same measurement results are obtained at all measurement positions whereby correct alignment between the refining surfaces is attained thereafter this process is preformed recurrently during the operation of the refining apparatus fig6 shows a flow diagram.