A tag is a compact engineering sentence
An instrument tag normally combines a measured or initiating variable with one or more functional meanings and a loop or equipment identifier. The exact interpretation belongs to the project's approved legend.
Do not infer safety, control authority, or physical location from artwork alone. Symbols and tags work together, and project conventions take precedence.
Read the symbol in layers
| Layer | Question | Project check |
|---|---|---|
| Letters | What variable and functions are being identified? | Confirm the project's letter-code legend. |
| Identifier | Which loop or equipment item does it belong to? | Confirm plant and unit numbering rules. |
| Bubble treatment | Where is the function accessible or implemented? | Confirm mounting and system conventions. |
| Connections | What kind of signal or relationship is shown? | Confirm the line legend and drawing type. |
Frequently asked questions
- Can this page replace an ISA-5.1 legend?
- No. Always use the current standard, the project's approved legend, and the owner's conventions.
Sources and boundaries
KonnectSymbols is independently developed and is not endorsed by ISA, IEC, or ISO. These educational notes do not reproduce or replace current standards, project specifications, regulations, or an approved project legend.
- ANSI/ISA-5.1-2024 Instrumentation Symbols and IdentificationInternational Society of Automation. Official publisher page for the current ISA-5.1 edition and related technical reports.