What U means on a P&ID.
Smart instruments are dragging U into common use. A Coriolis flow meter delivers mass flow, density, and temperature on one wired tag. That's a U-class instrument. Some operating companies stick with FT and flag the secondaries in the I/O list. Others adopt U-prefix to make multivariable scope explicit. House standard decides. The 2022 ISA 5.1 revision is more permissive than older editions on this front.
Also stands for.
- Calculated or composite measurement
Common second letters.
| Tag | Meaning |
|---|---|
| UT | Transmitter, UT. Multivariable transmitter, e.g. Coriolis with mass plus density plus temperature |
| UI | Indicator, UI |
Example tags.
| Tag | Meaning |
|---|---|
| UT-301 | Multivariable transmitter, often a Coriolis flow meter reporting mass flow plus density plus temperature on one tag |
Typical placement.
On smart instruments that produce multiple measurements over a single signal, HART or fieldbus. Coriolis meters and multivariable DP transmitters are the most common U-prefix services.
Common questions
What does the ISA 5.1 first letter U stand for?
In ISA 5.1, the first letter U stands for Multivariable. It can also stand for Calculated or composite measurement.
How is letter U typically used on a P&ID?
On smart instruments that produce multiple measurements over a single signal, HART or fieldbus. Coriolis meters and multivariable DP transmitters are the most common U-prefix services.
What second letters commonly follow U in ISA 5.1 tags?
Common combinations include UT (Transmitter, UT. Multivariable transmitter, e.g. Coriolis with mass plus density plus temperature), UI (Indicator, UI).
What are example ISA 5.1 tags starting with U?
UT-301 (Multivariable transmitter, often a Coriolis flow meter reporting mass flow plus density plus temperature on one tag).
Run it on your drawings.
Upload a P&ID set and see every tag read in the convention the drawing already uses.