Field-mounted instrument (discrete)

ISA 5.1-2009/2022 (general instrument and function symbols)

Drawn as

A plain circle with no horizontal line, containing the tag letters and usually the loop number.

Meaning

A discrete instrument mounted in the field, on or next to the pipe or vessel: transmitters, gauges, switches and local controllers. These are the bubbles that become physical instruments; read the letters to choose the component (PT → pressure-transmitter, PI → pressure-gauge, etc.).

Variants

Bubble size varies; ISA suggests about 11 mm (7/16")
Wireless field instruments may carry a small antenna mark
Two touching circles = one device with two functions