ISA 5.1 Instrument Identification Letters
The complete table of instrument identification letters used on P&IDs, with a quick lookup for any letter.
ISA 5.1 identification letters
| Letter | First letter: variable | Variable modifier | Readout / passive | Output | Modifier |
|---|---|---|---|---|---|
| A | Analysis | Alarm | |||
| B | Burner, combustion | User’s choice | User’s choice | User’s choice | |
| C | User’s choice (often conductivity) | Control | |||
| D | User’s choice (often density) | Differential | |||
| E | Voltage | Sensor, primary element | |||
| F | Flow, flow rate | Ratio | |||
| G | User’s choice | Glass, gauge, viewing device | |||
| H | Hand | High | |||
| I | Current (electrical) | Indicate | |||
| J | Power | Scan | |||
| K | Time, schedule | Time rate of change | Control station | ||
| L | Level | Light | Low | ||
| M | User’s choice (often moisture) | Middle, intermediate | |||
| N | User’s choice | User’s choice | User’s choice | User’s choice | |
| O | User’s choice | Orifice, restriction | |||
| P | Pressure | Point (test) connection | |||
| Q | Quantity | Integrate, totalize | |||
| R | Radiation | Record | |||
| S | Speed, frequency | Safety | Switch | ||
| T | Temperature | Transmit | |||
| U | Multivariable | Multifunction | Multifunction | ||
| V | Vibration, mechanical analysis | Valve, damper, louver | |||
| W | Weight, force | Well | |||
| X | Unclassified | X axis | Unclassified | Unclassified | Unclassified |
| Y | Event, state, presence | Y axis | Relay, compute, convert | ||
| Z | Position, dimension | Z axis | Driver, actuator, final element |
Using the Letter Table
The same letter can mean different things depending on its position. L is level as a first letter, light as a readout and low as a modifier. S is speed first, safety as a modifier and switch as an output.
PSV = Pressure (first) + Safety (modifier) + Valve (output)
Letters marked user’s choice have no fixed meaning; the project must define them on its legend sheet. This table summarises ISA 5.1 identification letters; refer to the current edition of the standard for the full notes.
Frequently Asked Questions
What is ISA 5.1?
The ANSI/ISA standard for instrumentation symbols and identification, used worldwide on P&IDs and loop diagrams.
Why does the same letter have different meanings?
Letter meaning depends on position: first letter, modifier, readout, output or function modifier.
What are user’s choice letters?
Letters left free by ISA (such as C, D, G, M, N, O as first letters) so each project can assign them; they must be defined on the P&ID legend.
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