Drawings / Instrumentation legend
Instrumentation symbol legend
Bubble shapes and tag letters in the style commonly taught from ISA 5.1 ideas, written here in our own words for planners. Confirm every convention against your site legend.
Bubble shapes
Tag letter table (teaching)
First letter is the measured variable. Succeeding letters are the function. Loop number ties devices together.
| Letters | Meaning (teaching) | Example |
|---|---|---|
F | Flow | FT, FIC, FV |
L | Level | LT, LIC, LSHH |
P | Pressure | PT, PIC, PSV |
T | Temperature | TE, TT, TIC |
I (succeeding) | Indicate | FI, LI, PI |
C (succeeding) | Control | FIC, LIC |
T (succeeding) | Transmit | FT, LT, PT |
S / H / L | Switch / high / low | LSHH, PSL |
Y | Relay / compute / convert | FY I/P, UY logic |
V | Valve / damper / final element | FV, LV, UV |
E | Sensor / primary element | TE thermocouple |
HS | Hand switch | HS-1401 |
This table is a planner cheat sheet, not a full standard reprint. Site legend and ISA publications govern formal work.