Let say you want to make a P&ID diagram of a process in which a level sensor transmits its data to a flow controller. The quantity measured is a tank's level, but the quantity controlled is the flow of a medium in a circuit separate from the tank. That is, the flow of the medium does not affect the level of the tank. In such a case, would the measuring instrument be coded as LT (level transmitter) and the controlling instrument as FC (flow controller)?

That is what makes sense to me, but a teacher told me otherwise. The teacher said it is the quantity controlled that matters, and the measuring instrument will be coded thereafter.

Answer in accordance to whatever official standard of P&IDs you wish.

  • $\begingroup$ controlling the level makes it a level controller $\endgroup$
    – Tiger Guy
    Commented Apr 27, 2023 at 13:20

1 Answer 1


The way I understand DIN EN 62424:2016 (german version of IEC62424:2016) the first letter of the oval bubble should always show quantity measured. This is certainly the way I see it on all P&ID. The only exception I'm aware of is that sometimes a pressure difference, if used for flow measurement, is denoted by an "F", not a "P". Similar exceptions are likely possible.

This is an example from the EN62424: enter image description here

Note that the actor is denoted "YS" - Y: "non-electrical actor", S: "binary control (non safety)", I understand this to mean that the valve is controlled, not the flow. This certainly makes more sense. The U in the hexagon means that it is used for a control function.

In you example, I believe the measureing device should be denoted "LC" (level, control).


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