U - Multivariable
Reference for the ISA 5.1 first letter U. 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.
Read one of your own drawings.
Drop a P&ID, instrument index, or schedule. Tagsight reads it to the tag and opens a workspace you keep when you sign in.
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Also stands for.
- Calculated or composite measurement
Common second letters.
Transmitter, UT. Multivariable transmitter, e.g. Coriolis with mass plus density plus temperature
Indicator, UI
Example tags.
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.