JPH05233074

PURPOSE:To speedily reach a target pH value without generating overshoot by successively and stepwisely switching targets until the final set value in respect to the pH value of objective liquid corresponding to a change caused by dropping acid or alkaline liquid, and controlling the quantity of liquid to be dropped. CONSTITUTION:At the time of starting pH control, a target switcher 9 sets the target value at S1 and outputs the corrected target value from a temperature corrector 6 corresponding to the measured temperatures of a temperature gauge 4. The temperature corrector 6 corrects the target value corresponding to measured temperature difference from the specified temperature. A fuzzy controller 10 controls the aperture of a control valve 5 corresponding to the deviation value of the corrected target value and the pH value from a pH gauge 3 and a differentiated value from a differentiator 7. When the pH of objective liquid 2 is close to the target value S1 and the deviation value and the differentiated value are reduced and turned to be smaller than a set threshold value, a fuzzy target value switching judger 8 switches the target value by using the target switcher 9. Near the next target value S2, the quantity of liquid to be dropped is controlled so as not to exceed the final target value S3, and the objective liquid 2 is controlled at the set pH value.