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In this example the value of the PV is always the same as the MV, as they are in series in the pipeline. However if the feed from the valve was to a tank, and the controller function was to control level using the fill valve, the PV would be the tank level, and the MV would be the flow to the tank.
The controller function can be a discrete controller, or a function block in a computerised control system such as a [[distributed control system]] or a [[programmable logic controller]]. In all cases a control loop diagram is a very convenient and useful way of representing the control function and its interaction with plant. In practice at a process control level the control loops are normally abbreviated using standard symbols in a [[Piping and instrumentation diagram]], which shows all elements of the process measurement and control based on a [[process flow diagram]].<ref>{{cite web|title=Piping and instrumentation diagram P&ID|url=http://processflowsystems.com/piping-instrumentation-diagram-pid/|website=Process Flow Systems|accessdate=16 September 2017}}</ref>
At a detailed level the control loop connection diagram is created to show the electrical and pneumatic connections. This greatly aids diagnostics and repair, as all the connections for a single control function are on one diagram.
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