Instrument Tag Decoder (ISA 5.1)
Type an instrument tag from a P&ID, loop diagram or datasheet and get its full name, with a letter by letter breakdown based on ISA 5.1 identification letters.
- Read the letters left to right: the first letter is the measured variable, an optional modifier follows, then the function letters
- H, L and M at the end describe high, low and middle settings for alarms and switches
Letter by letter breakdown
| Letter | Position | Meaning |
|---|---|---|
| P | First letter (measured variable) | Pressure |
| D | Variable modifier | Differential |
| I | Function | Indicating |
| T | Function (combined) | Transmitter |
How to Read an Instrument Tag
An ISA 5.1 tag has two parts: the functional identification (letters) and the loop number. The letters follow a fixed order:
- First letter: the measured or initiating variable, for example F for flow, P for pressure, T for temperature, L for level.
- Optional modifier: refines the variable, for example D for differential (PD), Q for totalize (FQ) or S for safety (PSV).
- Succeeding letters: the functions, readout first (I indicate, R record, G gauge, A alarm) then output (C control, T transmit, S switch, V valve, Y relay or converter).
- Function modifiers: H high, L low, M middle, often doubled for high high or low low trips.
Common instrument tags
| Tag | Meaning | Tag | Meaning |
|---|---|---|---|
| FT | Flow Transmitter | PSV | Pressure Safety Valve |
| FIC | Flow Indicating Controller | PCV | Pressure Control Valve |
| FQI | Flow Totalizing Indicator | PDT | Pressure Differential Transmitter |
| FE | Flow Element | TE | Temperature Element |
| FY | Flow Relay / Converter (for example I/P) | TW | Thermowell |
| LT | Level Transmitter | TIC | Temperature Indicating Controller |
| LG | Level Gauge | TSHH | Temperature Switch High High |
| LSLL | Level Switch Low Low | HS | Hand Switch |
| LAH | Level Alarm High | ZSO / ZSC | Position Switch Open / Closed |
| PI | Pressure Indicator (gauge) | XV | On/Off Valve |
Frequently Asked Questions
What does FIC stand for?
Flow Indicating Controller: F is flow, I is indicate and C is control. It is usually a DCS or PLC function block.
What is the difference between PT and PIT?
PT is a pressure transmitter; PIT is a pressure indicating transmitter, one with a local display.
What does the number in an instrument tag mean?
It is the loop number, shared by all instruments in the same control loop. A letter suffix such as A or B distinguishes identical instruments in that loop.
Is PRV an ISA tag?
Not strictly. In ISA 5.1 R means record, so PSV (pressure safety valve) or PCV is preferred, but PRV is widely used for relief or reducing valves.
Learn more on the blog
Instrumentation and control standards
A practical, field focused guide on instrumentationblog.in that explains the theory behind this tool and how engineers apply it on real plants.
Read Full Article