Basics of Symbols used in an Instrumentation Diagram
You may be already familiar with many of the symbols used in instrumentation diagrams. Understanding this helps to interpret these diagrams for troubleshooting or diagnostics.
A circle in the square represents shared display or shared control functions.
A diamond in the square represents a programmable logic control or PLC.
A hexagon indicates a computer function.
Function blocks designate computations and conversion within the loop or instrument.You will see function blocks used in several ways,as a standalone symbol that represents only the function aspects of a loop,as a flag designating a function of specific component or on a signal line adjacent to a shared controller.
The letters in the top part of the balloon are the functional identifiers. The first letter identifies the measured or initiating variable. The succeeding letters represent the readout or passive function followed by the output function.
P&ID - Identification Letters