ISA-5.1 · Instrument identification

How to read ISA instrument tags

First letter, succeeding letters, loop numbers, and bubbles, with six common tags decoded. Plus what ISA-5.4 loop diagrams, ISA-20 data sheets, and ISA-84 / IEC 61511 SIL levels mean on the job.

Need the full letter table? It is in the standard — ANSI/ISA-5.1-2024 at isa.org →

How to read a tag, step by step

Step	What you look at	What it tells you
1	The first letter	The measured or initiating variable (flow, pressure, level, temperature…), not what the device manipulates
2	A modifier right after the first letter, if any	Changes the variable, e.g. differential or ratio, so it reads as one unit with the first letter
3	The succeeding letters	What the device does: indicates, records, controls, transmits, switches, alarms, or is a valve / element
4	A trailing modifier like H or L	High or low, usually on a switch or alarm
5	The number	The loop. Every device in one loop shares it (FT-101, FIC-101, FV-101)
6	The bubble around the tag	Where it lives: field-mounted, main panel / control room, or behind the panel; a square around the circle means a shared display (DCS)

Our own explanation, not the ISA-5.1 letter table. Projects often add their own conventions — the P&ID legend sheet and instrument index for your job govern. ISA-5.1 is currently the 2024 edition; older drawings may follow the 2009 or earlier editions.

Six common tags decoded (worked examples)

Tag	Read it as	What it is in the field
FT	Flow + Transmitter	Flowmeter transmitter sending a 4–20 mA or digital signal
PIC	Pressure + Indicating + Controller	A pressure controller that also displays the value, often a DCS or PLC function
LSH	Level + Switch + High	Level switch that trips on high level — often feeds an alarm or shutdown
TE	Temperature + Element	The primary sensor itself: RTD or thermocouple, usually in a thermowell
FCV	Flow + Control + Valve	The valve in a flow loop. Some specs tag this FV and reserve CV for self-actuated regulators, so check the legend
PSV	Pressure + Safety + Valve	Spring-loaded relief device that protects equipment from overpressure. It is not a control valve and must not be adjusted in the field

Worked examples only; see ISA-5.1 for the full letter assignments. Our own explanation, not the ISA-5.1 letter table. Projects often add their own conventions — the P&ID legend sheet and instrument index for your job govern. ISA-5.1 is currently the 2024 edition; older drawings may follow the 2009 or earlier editions.

The ISA documents an I&E crew actually touches

Document	What it is	When you use it
ISA-5.1	Symbols and identification letters used on P&IDs	Reading P&IDs, building the instrument index, tagging devices
ISA-5.4	Instrument loop diagrams: one sheet per loop showing every device, terminal, wire, and cable from field to controller	Termination, loop checks, and troubleshooting
ISA-20 / ISA-TR20.00.01	Instrument specification (data sheet) forms: range, materials, connections, and outputs for each device	Purchasing, receiving inspection, and calibration
ISA-84 (ANSI/ISA-61511)	Functional safety for safety instrumented systems (the US adoption of IEC 61511)	Anything tagged as a SIF / SIS: proof testing, bypass control, and records

Get the documents from ISA. Our own explanation, not the ISA-5.1 letter table. Projects often add their own conventions — the P&ID legend sheet and instrument index for your job govern. ISA-5.1 is currently the 2024 edition; older drawings may follow the 2009 or earlier editions.

Worked example — what a SIL rating means for the crew

Input / step	Value	What it means
Hazard study result	High-pressure trip assigned SIL 2	Set by the process hazard analysis / LOPA, not by the installer
Risk reduction a SIL 2 function must deliver	Between 100 and 1,000 times	Low-demand mode range under IEC 61511
What that drives	Redundant transmitters, voting logic, certified valves, and a proof-test interval	Set in the safety requirements specification (SRS)
Crew impact	No substitutions, documented proof tests, and controlled bypasses	Any change goes through management of change

One band given to illustrate the concept. SIL 1 through 3 are common in process plants; SIL 4 is rare. Not engineering advice — the SRS governs.

Common field mistakes

Mistake	What to do instead
Naming the valve after what it moves instead of the loop variable	A valve driven by a level controller is LV, not FV. The first letter follows the measured variable
Treating a PSV like a PCV or PRV	A PSV is a safety device, while PCV and PRV control or regulate pressure. Never field-adjust a PSV
Assuming every project uses textbook ISA letters	Read the P&ID legend first. Owners add their own letters and conventions
Terminating from the P&ID instead of the loop diagram	P&IDs show function. Loop diagrams (ISA-5.4) show terminals and wires
Swapping a SIS transmitter with a "same model" spare	SIS devices are tied to the SRS and may need specific certification. Follow MOC

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Reading a P&ID like the I&E foreman does

This page teaches the logic; it does not reproduce the ISA letter table. The full identification letters and symbols are in ANSI/ISA-5.1-2024, available from ISA. Your project’s P&ID legend sheet always governs.

The one rule that matters most: the first letter is the variable being measured, not the thing being moved. A control valve on a tank outlet that holds tank level is tagged for level (LV), even though it throttles flow. Once that clicks, most tags decode themselves. The letters after the first describe the job the device does, and the number ties together everything in the same loop.

Bubbles tell you where to look. A plain circle is a field device. A line through the circle means it is on the main control panel or in the control room, and a dashed line means behind the panel. A circle inside a square is a shared display or control function, typically the DCS. Other shapes cover computer and PLC functions. Edition differences exist, so check the legend.

From P&ID to termination. The P&ID shows function. The instrument index lists every tag. The ISA-20 style data sheet defines what to buy and how to calibrate it. The ISA-5.4 loop diagram shows every wire and terminal. If a loop is part of a safety instrumented system under ISA-84 / IEC 61511, its SIL rating drives redundancy, proof testing, and change control. Treat those tags differently from ordinary control loops.

Related guides: hazardous area classification (API RP 500/505), API codes including RP 552 signal transmission and RP 14C SAFE charts, and insulation resistance testing. Track instrument punch items with our free punch list template and vendor data with the submittal log. Field PM keeps loop-check records and photos with the job.

FAQ

How do you read an ISA instrument tag?+

Read the first letter as the measured or initiating variable (F for flow, P for pressure, L for level, T for temperature, and so on). Read the letters after it as what the device does, such as indicate, control, transmit, switch, or act as an element or valve. The number is the loop number, and every device in the loop shares it. The full letter assignments are in ANSI/ISA-5.1, available from ISA.

What does PIC mean on a P&ID?+

Pressure Indicating Controller: a controller that acts on a pressure measurement and also displays it. On modern jobs it is usually a function in the DCS or PLC, drawn as a circle inside a square, rather than a panel-mounted instrument.

What is the difference between a PSV and a PRV?+

A PSV (pressure safety valve) is a relief device that opens to protect equipment from overpressure. It is a safety device sized by the engineer and should never be adjusted in the field. A PRV is usually a pressure-regulating (or pressure-reducing) valve that holds a downstream set pressure during normal operation. Some owners use PRV to mean pressure relief valve, so check the P&ID legend.

What is an ISA-5.4 loop diagram?+

A loop diagram is one drawing per instrument loop showing every device, junction box, terminal, cable, wire number, and I/O point from the field instrument to the controller. Technicians use it for termination and loop checks. The P&ID shows what the loop does, and the loop diagram shows how it is wired.

What do SIL levels mean?+

SIL (safety integrity level) rates how much risk reduction a safety instrumented function must provide, under IEC 61511 (adopted in the US as ANSI/ISA-61511, known as ISA-84). Higher SIL means more risk reduction and stricter design, testing, and change control. For example, a SIL 2 function in low-demand mode must reduce risk by a factor between 100 and 1,000.

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