from fourier analysis device 32 a evaluation unit 32 has a computing unit 32 b for the detection of a characteristic change in the phase angle of individual harmonics only the analysis of one harmonic i e the 8 th harmonic will be described below the evaluation can however also take place on the basis of several harmonics computing unit 32 b has a differentiator which differentiates the phase angle signal the differential of the phase signal as a function of time is a measure of the decrease in the phase angle the computing unit compares the differential of the phase angle signal with a preset limiting value if the differential exceeds the limiting value a disruption is assumed an acoustic and or optical alarm can be emitted since the detection of the disruption takes place before the membrane of the dialyser is blocked up countermeasures can be taken in good time in the case where a disruption is detected evaluation unit 32 sends a signal via data line 34 to central control unit 25 of the dialysis apparatus in order to initiate an intervention in the machine control central control unit 25 controls first substituate pump 22 in such a way that thickening of.