Constant voltage principle electrode is used to measure residual chlorine or hypochlorous acid in water. The constant voltage measurement method is to maintain a stable potential at the electrode measuring end, and different measured components produce different current intensities under this potential. It consists of two platinum electrodes and a reference electrode to form a micro current measurement system. The residual chlorine or hypochlorous acid in the water sample flowing through the measuring electrode will be consumed. Therefore, the water sample must be kept flowing continuously through the measuring electrode during measurement.