Local pressure gauge.
PI in an instrument circle on a pipe tap marks a local mechanical pressure gauge. Mechanical-only like the level sight glass, intended for operator local indication only.
How it's drawn.
An instrument circle carrying PI on a pipe or vessel tap, marking a local mechanical pressure gauge. Like the sight glass it is read at the equipment, with no signal behind it. It is common to see a PI gauge beside a pressure transmitter on the same tap.
Typical usage.
Pump discharge, vessel overhead, instrument-air supply. Common to find a PI gauge alongside a PT transmitter on the same tap. The gauge survives a power loss, the transmitter doesn't.
Telling it apart.
- A PI gauge is local indication read by eye. A PT transmitter on the same tap is the remote, signalled measurement.
- The gauge keeps reading without power, which is why it often sits next to a transmitter as a backup.
- PI in the bubble is pressure indication. Adding a transmitter function would change the tag.
Related.
Get the P&ID symbol cheat sheet.
One page of instrument bubble and primary-element geometry, including the pressure gauge and its mounting conventions.