Piping and instrumentation diagrams (PIDs) are schematics used in instrumentation and control to understand process and instrumentation interconnection. Most industries standardize symbols according to ISA-S5.1. PIDs use symbols and circles to represent instruments and how they are interconnected. Tag numbers identify device type and function. The first tag letter designates the measured variable, succeeding letters designate function or modify the first letter. Instrument location is shown by presence or absence of a line from the symbol. PIDs are important tools for understanding process instrumentation.

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