A Proportional+Integral+Derivative (PID) controller is to be used to control the level of liquid in a tank, measured as the Process Variable (PV) by a level transducer. The setpoint (SP) for the controller is obtained from a faceplate control, which also displays the PV as well as the output value (XOUT). The output value is used to set the position of a valve which determines the rate of flow of liquid into the tank. This level is to be maintained in the presence of exogenous changes in the rate of liquid removal by a variable-speed pump.