x the rate of change of this signal x is determined and represented by a signal y the occurrence of surge is sensed and indicated by an output signal z when the signal y exceeds a prescribed threshold value application_number US 1365987 A description the preferred embodiments of the present invention will now be described with reference to fig1 and 2 of the drawing identical elements in the two figures are designated with the same reference numerals as shown in fig1 the flow rate or flow velocity in either the intake or outlet of a compressor is detected and converted to a signal x in a signal converter 7 such a conversion is well known in the art and need not be explained in detail here the measured value x is supplied to a summing point 1 both directly and with a delay produced by a delay element 2 this summing point 1 derives the difference between these delayed and undelayed values expediently a first order delay element is used as the delay element 2 but other types of delay are also possible the term 34 first order delay element 34 should be understood to mean that with a sudden change of the input signal the output signal rises to this input value with a time delay in accordance with an exponential function the time constant t 1 of this rise is variable it constitutes an important settable parameter in this embodiment of the system the system according to the invention may also operate with delay elements of second order or higher order it is operable even with the use of a mere 34 dead time 34 element 3 according to fig2 the dead time element 3 produces.