Consider the level control problem first described in Chapter 1 and shown in Figure 5-1. A level measurement device (usually a differential pressure cell) senses the level and sends a signal to the controller. Notice that the controller is comparing the tank height with the desired setpoint and sending a controller output (pressure signal) to the valve, which changes the valve position and therefore the volumetric flow rate of stream 1, F1. These signals are shown as dashed lines on the figure.
Figure 5-1. Level control, inlet flow rate manipulated. The instrumentation signals are shown as dashed lines. The level transmitter sends a signal to the level control, which sends a signal to the control valve.
The question is, what is ...