DE3937133

A system having a control variable, such as a proportion of a certain substance in a solution, maintains a set value of that control variable by obtaining a measuring signal as a function of the control value. That function is characterized by a curve having an extreme value dividing the curve into monotonically ascending and descending sections. The measuring signal is compared with a reference value corresponding to the set value of the control variable to obtain a different value. When the difference value lies within a predetermined range, a first signal generator is actuated to correct the deviation of the control variable. When the difference value lies outside of that range, a second signal generator is actuated to correct the deviation of the control value within a section of the curve which would be unstable if correction were instead to be made by the first signal generator.