The principle of relay action when pH controller controls alkali addition is as follows:

Add alkali, set the low point, and connect the output switch signal to the low point (LOW) relay. When the pH value of the incoming water is less than the LOAL value, the state of the low point relay starts the alkali pump, and the pH value changes positively. When the alkali is added to LOAL, in order to prevent the dosing pump from frequently operating, a value of Lb is set. When the pH continues to rise to (LOAL+Lb), the low point relay will activate and the alkali addition will stop. Subsequently, the incoming water was continuously replenished, and the pH value decreased in reverse. When the pH dropped to (LOAL+Lb), the low point relay did not activate, but only activated when the pH dropped to LOAL, continuing to add alkali.