What does the ISA 5.1 first letter O stand for?

In ISA 5.1, the first letter O stands for User's choice. It can also stand for No standard assignment.

What second letters commonly follow O in ISA 5.1 tags?

Common combinations include OT (Transmitter, per the project legend).

O - User's choice

Q: What are example ISA 5.1 tags starting with O?

OT-405 (Oxygen analyzer transmitter where the project has assigned O to oxygen service, per the project legend).

Reference for the ISA 5.1 first letter O. Some operating companies park oxygen analyzers under O when they've already burned A on something else and G on gas detection. Other specs leave it unassigned outright. The legend on the drawing set is the only source of truth. Guessing produces wrong tags on the I/O list.

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Also stands for.

No standard assignment

Common second letters.

Transmitter, per the project legend

Example tags.

OT-405

Oxygen analyzer transmitter where the project has assigned O to oxygen service, per the project legend

Typical placement.

Wherever the project legend allows.

Related

GlossaryISA 5.1 ISAISA letter A - Analysis ISAISA letter G - User's choice, commonly Gauging or Gas ISAISA letter N - User's choice