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Field vs Control Room Instruments on a P&ID

The line inside an instrument bubble tells the reader where the function is accessible.

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Comparison: a field-mounted pressure indicator drawn as a plain circle, and a pressure indicator available to the operator in the control room drawn as a circle with a horizontal barPIField-mountedread at the pipePTPIControl roomshown to the operator
Same measurement, different accessibility. The bar tells the reader the function sits with the operator, not out on the pipe.

Same circle, different meaning

A plain circle is a field-mounted instrument: you read or adjust it at the equipment. A circle with a single horizontal bar is a function in the primary location, normally the control room, available to the operator.

InduDraft also draws a double bar for an auxiliary location such as a local panel, and a dashed bar for functions behind a panel. You set this as the instrument's location in the right panel; the tag stays the same.

Why it matters

Location tells operators and maintenance where to find a reading and tells the control engineer which signals must reach the control system. A PT in the field may feed a PI on the operator screen: same loop number, two bubbles.

Instrument bubble

PI-101 · control room

Try this symbol

See also: ISA guide · instrument tags explained.

Try instrument locations in InduDraft

Place an instrument and switch between field, control room and local panel.

Start ISA P&ID

Official references

Illustrations are drawn with InduDraft's own symbols. No content is copied from the standards.