ISA Tag Decoder
Type a tag from a P&ID or instrument index to read it letter by letter. Modifier letters after the first letter (D differential, F ratio, J scan, K rate of change, Q totalize, S safety) are recognised, as are loop numbers and suffixes.
Instrument Bubble Shapes (Location/Accessibility)
| Symbol | Meaning |
|---|---|
| Circle, plain | Field-mounted, locally mounted instrument — no shared display |
| Circle, single horizontal line | Primary location, normally accessible to the operator (e.g. control room panel-front) |
| Circle, double horizontal line | Auxiliary location, accessible to the operator (panel-back or secondary panel) |
| Square with inscribed circle | Shared display/control, computer (DCS) function — normally accessible |
| Hexagon | Programmable logic controller (PLC) function |
| Diamond in square | Safety instrumented function (SIS) logic |
Instrument Line Types
| Line | Meaning |
|---|---|
| Solid thin line | Process piping (main process flow) |
| Long-dash line | Pneumatic signal |
| Solid line with arrowheads | Electrical signal |
| Dash-dot line | Capillary tubing (filled thermal system) |
| Line with small loops | Electromagnetic, sonic, or radio (wireless) signal |
| Line with double-slash ticks | Software or internal data link within a computer/shared system |
ISA Tag Letters — First Letter (Measured Variable)
| Letter | Variable |
|---|---|
| A | Analysis |
| B | Burner, combustion |
| C | User's choice |
| D | Density / specific gravity |
| E | Voltage |
| F | Flow rate |
| G | User's choice |
| H | Hand (manually initiated) |
| I | Current (electrical) |
| J | Power |
| K | Time, time schedule |
| L | Level |
| M | Moisture / humidity |
| N | User's choice |
| O | User's choice |
| P | Pressure, vacuum |
| Q | Quantity / totalized |
| R | Radiation |
| S | Speed / frequency |
| T | Temperature |
| U | Multivariable |
| V | Vibration, mechanical analysis |
| W | Weight / force |
| X | Unclassified |
| Y | Event, state, presence |
| Z | Position, dimension |
ISA Tag Letters — Succeeding Letters (Function)
| Letter | Function |
|---|---|
| A | Alarm |
| C | Control |
| E | Sensor (primary element) |
| G | Glass / gauge (local viewing device) |
| H | High (modifier) |
| I | Indicate |
| K | Control station |
| L | Light (pilot light) or Low (modifier) |
| M | Middle / intermediate (modifier) |
| O | Orifice, restriction |
| P | Point (test connection) |
| R | Record |
| S | Switch |
| T | Transmit |
| U | Multifunction |
| V | Valve, damper, louver |
| W | Well |
| X | Unclassified |
| Y | Relay / compute / convert |
| Z | Driver, actuator, final control element |