from because the procedure has been described sufficiently clearly by this cross reference an unfiltered value is established in each case and a value which underwent low pass filtering it is favorable that the filter is a low pass filter of first order further it is checked in step 103 whether the currently established filtered reference value is greater than the previously established maximum value of the filtered reference values if this is the case the currently established filtered reference value will be memorized as the maximum value of the filtered reference values likewise it is checked in step 103 whether the currently established filtered reference value is smaller than the previously established minimum value of the filtered reference value if this is the case the currently established filtered reference value is memorized as the minimum value of the filtered reference values the mean value of the unfiltered reference values and the pertinent quality gauge is determined continuously from the unfiltered reference value in step 103 thereafter there is a transition to step 104 checking whether a defined number of reference values has already been established in the negative the method is terminated for the time being until a number of reference values is established in further cycles of the method which corresponds to the defined number when it is found out in step 104 that a defined number of reference values has already been established there will be a transition to step 105 step 105 checks whether the difference between the maximum values and minimum.