Fig. IV.3: Proportional Band of Feedback Controller

Usually, the workable range of PB is

(IV.5)

The condition of is a bit tricky to visualize. Since the level of the tank can only go up to 100%, a PB of (say) 250% causes the valve to move only through a portion of its available range as the tank goes from empty to 100% full. Let's assume that we set the lower end of the proportional band at -75% and the upper end at 175%. That gives a PB of 250%. Physically it means that the level would have to fall way below the bottom of the tank to fully close the valve and way above to fully open the valve. Since the level of the tank cannot really go that far, it means that the valve will never fully close or fully open. Tuning is the process of finding the gain or PB that provides the optimal response of control valve for controlling the process.