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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 the 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, 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|archive-date=20 July 2020|archive-url=https://web.archive.org/web/20200720181608/http://processflowsystems.com/piping-instrumentation-diagram-pid/|url-status=dead}}</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.
==Loop and control equipment tagging==
To aid unique identification of equipment, each loop and its elements are identified by a "tagging" system and each element has a unique tag identification.<ref>{{cite web|title=Components of control loops and ISA symbology|url=http://www.pacontrol.com/download/Process%20Control%20Fundamentals.pdf|website=Instrumentation and control - process control fundamentals.|accessdate=16 September 2017|archive-date=23 November 2018|archive-url=https://web.archive.org/web/20181123082627/http://www.pacontrol.com/download/Process%20Control%20Fundamentals.pdf|url-status=dead}}</ref>
Based on the standards [[ANSI]]/[[Instrument Society of America|ISA]] S5.1 and [[ISO 14617]]-6, the identifications consist of up to 5 letters.
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