How an ISA tag is built
ISA 5.1 gives every instrument on a P&ID a compact name with a strict grammar. The first letter is the measured variable — what the instrument senses. An optional modifier may follow it. The remaining letters are succeeding letters describing what the device does: indicate, transmit, control, alarm, record.
Read left to right, the tag becomes a sentence. FIC-101 is "flow, indicating, controller, loop 101" — a flow controller with a local display. FT-101 is the transmitter feeding it, and FCV-101 the control valve it drives: three devices, one loop number, one control loop.
First letters — the measured variable
| Letter | Variable | Letter | Variable |
|---|---|---|---|
| A | Analysis | L | Level |
| B | Burner, combustion | M | Moisture, humidity |
| C | Conductivity | P | Pressure |
| D | Density | Q | Quantity, totalised |
| E | Voltage | R | Radiation |
| F | Flow | S | Speed, frequency |
| H | Hand (manual) | T | Temperature |
| I | Current | V | Vibration |
| J | Power | W | Weight, force |
| K | Time, schedule | Z | Position |
Succeeding letters — the function
| Letter | Function | Letter | Function |
|---|---|---|---|
| A | Alarm | R | Record |
| C | Control | S | Switch |
| E | Element (primary sensor) | T | Transmit |
| G | Glass, sight gauge | V | Valve, damper |
| I | Indicate | Y | Relay, compute, convert |
| K | Control station | Z | Driver, actuator |
Note that several letters mean different things depending on position: A is analysis as a first letter but alarm as a succeeding one; I is current first, indicate after. Context is everything, which is exactly what makes these tags hard to look up letter by letter.
Alarm and trip suffixes
High and low alarms append H and L, doubled for the second, more urgent level. LAHH-310 is a level alarm high-high — typically a shutdown trip rather than an operator warning. LSLL is the corresponding low-low switch. On safety instrumented functions these tags matter: HH and LL generally indicate the trip layer rather than the control layer.
Field notes
- The loop number ties devices together, not the letters. FT-101, FIC-101 and FCV-101 are one loop. When troubleshooting, collect the whole number family before assuming which device is at fault.
- Numbering schemes are project conventions. ISA 5.1 standardises the letters, not the digits — many projects encode plant area in the first digits, so 101 and 201 sit in different units.
- Y means "compute something" — a square root extractor, an I/P converter, a signal selector. Seeing FY next to FT on a DP flow loop usually means the square root is being taken there.
- PDT is the tag to recognise on sight. Differential pressure transmitters carry most DP level and DP flow measurement in a plant.
Frequently asked questions
What does FIC mean on a P&ID?
Flow Indicating Controller. F is the measured variable flow, I adds indication, and C adds control — so the device displays the flow and controls it.
What is the difference between PT and PDT?
PT is a pressure transmitter measuring pressure at one point. PDT is a pressure differential transmitter, where D is the modifier meaning differential — it measures the difference between two pressures, and is the workhorse of DP flow and DP level measurement.
What does LAHH mean?
Level Alarm High High. L is level, A is alarm, and the doubled H indicates the second, more urgent high trip level — usually a shutdown rather than a warning.
What do the numbers in an instrument tag mean?
They identify the loop, and their structure is a project convention rather than an ISA rule. A common scheme encodes the plant area in the leading digits and the sequential loop number after — so FIC-101 and FT-101 belong to the same loop.
Based on the ISA 5.1 letter conventions, for reference and education. Project drawings may use house conventions that differ — the drawing legend governs. See our disclaimer.