How to Read Instrument Identification Tags on a P&id

Walk onto any process plant floor, open a P&ID, and you will see letters and numbers stamped next to every valve, transmitter, controller, and sensor. These are instrument identification tags the shared language engineers, operators, and technicians use to talk about specific equipment without confusion. If you cannot read these tags, the drawing is just shapes and lines. If you can, every circle, square, and line tells you exactly what the instrument does, where it sits in the process, and how it connects to the control system. Learning how to read instrument identification tags on a P&ID is one of the most practical skills a process engineer, instrumentation technician, or piping designer can pick up.

What Are Instrument Identification Tags on a P&ID?

An instrument identification tag sometimes called an instrument tag number, loop identifier, or instrument code is the alphanumeric label assigned to every measurement and control device on a piping and instrumentation diagram. Each tag follows a structured format so that anyone familiar with the standard can decode what the instrument measures, whether it is local or connected to a control room, and what type of device it is.

The most widely adopted standard for this system is ISA 5.1 – Instrumentation Symbols and Identification. If you want to understand the full symbol set these tags sit inside, our breakdown of ISA standard P&ID symbols covers that in detail.

How Does the ISA Tag Number Format Work?

The standard format follows a defined sequence. While companies sometimes add extra fields for unit numbers or project codes, the core structure looks like this:

[First Letter(s)] – [Succeeding Letter(s)] – [Loop Number]

For example, in the tag LT-2051:

L Measured variable (Level)
T Function (Transmitter)
2051 Unique loop number

That single tag tells you the device is a level transmitter in loop 2051. No guessing, no looking up a separate legend the information is right there in the label.

What Does the First Letter in an Instrument Tag Mean?

The first letter identifies the measured or initiating variable. This is the physical quantity the instrument deals with. The ISA standard defines these commonly used first letters:

A Analysis (pH, conductivity, oxygen content)
B Burner or combustion
C User's choice (often conductivity or commissioning)
D User's choice (often density or differential)
E Voltage
F Flow
G Gaging or position
H Hand (manual)
I Current (electrical)
J Power
K Time or schedule
L Level
M User's choice (often moisture)
N User's choice
O User's choice
P Pressure
Q Quantity or totalizer
R Radiation
S Speed or frequency
T Temperature
U Multivariable
V Vibration
W Weight or force
X Unclassified
Y Event, state, or presence
Z Position or dimension

A tag starting with P deals with pressure. A tag starting with F measures flow. This is the foundation get the first letter right and you already know what the instrument is watching.

What Do the Succeeding Letters Tell You?

Letters after the first one describe the function of the instrument what it does with the measurement. Here are the most common succeeding letters:

A Alarm
C Controller
D Differential
E Element or sensor (primary element)
G Glass or local viewing (sight glass)
I Indicate
K Control station
L Light or lamp (low alarm context)
R Record
S Switch
T Transmit
V Valve or damper
Y Converter, relay, or computing

So when you see FIC, you read it as: F (flow) I (indicate) C (controller). It is a flow indicating controller. The tag PDT means: P (pressure) D (differential) T (transmitter) a differential pressure transmitter.

How Do You Read Combined Letters Step by Step?

Let's work through real examples you will find on most plant P&IDs.

Example 1: TE-101

T = Temperature (measured variable)
E = Element or sensor
101 = Loop number

Meaning: A temperature sensing element typically a thermocouple or RTD in loop 101. You will often see this connected to a TIC (temperature indicating controller) elsewhere on the same loop.

Example 2: LIC-3025

L = Level
I = Indicate
C = Controller
3025 = Loop number

Meaning: A level indicating controller. This instrument receives a level signal, displays it, and sends a control output often to a level control valve.

Example 3: PSV-4010A

P = Pressure
S = Switch
V = Valve
4010 = Loop number
A = First of a set (often indicates parallel/redundant devices)

Meaning: A pressure switch valve commonly used to identify a pressure safety valve. The suffix A tells you this is the first in a pair or group.

Example 4: FCV-2001

F = Flow
C = Controller (function modifier here meaning "control")
V = Valve
2001 = Loop number

Meaning: A flow control valve. This is the final control element that a flow controller (FIC) modulates. The letters CV together indicate a control valve whose purpose is defined by the preceding variable letter. If you need a refresher on valve symbols, our article on API and ASME valve symbols used in instrumentation diagrams walks through those.

Example 5: LSHH-1050

L = Level
S = Switch
HH = High-high (two letters meaning it is a safety-level alarm)
1050 = Loop number

Meaning: A level switch that activates on a high-high condition. The double letter is an important detail HH or LL signals a safety or trip set point, not just a regular alarm.

What Do the Numbers in an Instrument Tag Mean?

The number after the letters is the loop number or instrument number. This number groups related instruments together. For instance, instruments tagged 2051 FT-2051, FIC-2051, FCV-2051 all belong to the same control loop. They are physically and logically connected.

Companies use different numbering conventions:

Three-digit numbers (101, 205) common in smaller systems
Four-digit numbers (1051, 4025) common in larger plants
Numbers with unit prefixes sometimes a leading digit identifies the process unit (e.g., 2051 = unit 2, loop 051)

When you see the same number across multiple tags, you know they belong together. FT-105, FRC-105, and FCV-105 form a flow control loop a transmitter sends data to a recording controller, which operates a control valve.

What Do the Letter Boxes on a P&ID Symbol Mean?

On a P&ID, instrument tags appear inside a circle (for field-mounted instruments), a circle inside a square (for instruments mounted in a primary location like a local panel), or a diamond shape (for shared displays or DCS-located instruments). The location of the tag symbol tells you where the instrument physically exists:

Circle alone Field mounted, accessible at the equipment
Circle inside a square Located in a local panel or auxiliary panel
Hexagon Located in a DCS or computer-based control system

This distinction matters during commissioning and maintenance. A field-mounted pressure transmitter needs a technician at the pipe. A DCS-indicated controller is accessed from the control room. For a full review of symbol placement and the broader set of standard notations, see our reference on ISA P&ID symbols.

What Are Common Mistakes When Reading Instrument Tags?

Even experienced engineers make these errors and they can lead to wrong assumptions during HAZOP reviews, maintenance planning, or instrument sizing.

Confusing the first letter with the function. A tag starting with P is about pressure. A tag starting with F is about flow. Mixing these up for example, assuming a PIC is a flow controller leads to wrong interpretations.
Ignoring the double letter convention. LSHH is not the same as LSH. The HH means high-high a safety trip point. Missing this during a safety review is a serious oversight.
Assuming all companies follow ISA 5.1 identically. Many organizations customize the system. Some use additional letters, project-specific prefixes, or non-standard suffixes. Always check the project's instrument designation key, usually found on the first sheet of the P&ID set.
Forgetting that "V" in a tag means valve, not voltage. Voltage is the first-letter variable E. Seeing V as a succeeding letter almost always means a valve or actuator.
Skipping the line class context. An instrument tag sits on a process line. If you are unsure about the piping specifications tied to that line, reading our guide on common piping line class codes can help you understand what the line is rated for, which affects instrument selection.

How Can You Read Instrument Tags Faster?

With practice, reading tags becomes automatic. Here are tips that speed up the learning curve:

Memorize the first six measured variables first. P (pressure), T (temperature), F (flow), L (level), A (analysis), and S (speed) cover the vast majority of instruments you will encounter.
Learn the ten most common succeeding letters. I (indicate), C (controller), T (transmitter), A (alarm), S (switch), V (valve), R (record), G (glass), E (element), and D (differential) appear on nearly every P&ID.
Read tags in pairs. If you see FT-105, look for FCV-105 and FIC-105 on the same drawing. Understanding the loop connection makes each individual tag clearer.
Use the ISA 5.1 standard as a reference. Keep a copy at your desk or bookmark it. A quick lookup eliminates guesswork. The standard is available through ISA's publication page.
Start with simple P&IDs. Single-loop drawings or utility systems (steam, cooling water, instrument air) use basic tags and are good practice material before moving to complex process units.

What Should You Do After Learning to Read Tags?

Reading the tag is the starting point. From there, you can use that knowledge to trace control loops, prepare for HAZOP or LOPA studies, write instrument data sheets, and troubleshoot process upsets. The next step is connecting tag knowledge with understanding the physical symbols the balloons, squares, and lines that surround them on the drawing.

Here is a practical checklist to build this skill step by step:

Pull a real P&ID from your project or a public reference drawing.
Identify 10 instrument tags and decode each letter using the format described above.
Group tags into loops find the transmitter, controller, and final element that share the same number.
Note the symbol shape (circle, circle-in-square, hexagon) to determine where each instrument is physically located.
Check the line number each instrument connects to and verify the piping class using a line class reference.
Write out your decoded tags in a table format: Tag | Variable | Function | Location | Loop Group.
Review ISA 5.1 Section 5 for any letters you are unsure about.