Combined Controllers and Final Control Element:

A proportional controller is reverse-acting so in which the control valve throttles down to decrease steam flow as the hot water outlet temperature increases; the control valve will open additional to increase steam flow as the water temperature decreases.

A combined action of the control valve and controller for different modifications in the measured variable is displays in given figure.

Figure: Combined Controllers and Final Control Element Action

Initially, the measured variable value is equal to 100^oF.  The controller has been set so in which this value of measured variable corresponds to a 100% output, or 15 psi, that in turn, corresponds to a "full open" control valve position.

At time t₁, the measured variable increases through 100^oF, or 50%, of the measured variable span. That 50% controller input change causes a 100% controller output change because of the controller's proportional band of 50%.  A direction of the controller output change is decreasing since the controller is reverse-acting.  The 100% decrease corresponds to a decrease in output for 15 psi to 3 psi that causes the control valve to go from fully open to fully shut.

At time t₁, the measured variable decreases through 50^oF, or 25%, of the measured variable span. The 25% controller input decrease causes a 50% controller output increase. This conclude in a controller output increase from 3 psi to 9 psi, and the control valve goes from fully shut to 50% open.