It is helpful to look at the differences between the two types of control looked at so far (P and PI). The proportional control was not oscillatory, but did not bring the response back to the set-point. The PI control was oscillatory, but did bring the level in the tank back to the set-point. Which type of control is better? The answer will depend on the process that we are controlling: normally it is best if the response of the process is returned to the set-point. However in the case of liquid levels in ta there is no reason to insist that the level return to the set-point, as long as the liquid level does not reach the top or bottom of the tank. In fact tight process control of liquid level can be bad control, as we can see if we look at the manipulated variable (the flow out of the t in Example 5.2(the response of the PI process to a load disturbance). This variable is plotted in Figure 5.9