ISA-5.1: how to read the instrument symbols and tags on a P&ID

What ANSI/ISA-5.1-2024 governs, and what it does not

ISA-5.1 establishes a uniform system of graphic symbols and alphanumeric identification for the instruments, devices and functions used in measurement, monitoring and control. That language is meant to be read the same way across every technical document it touches: P&IDs, process flow diagrams, functional diagrams, electrical schematics and instrument loop diagrams. It is ISA’s oldest standard, tracing back to Recommended Practice RP-5.1 in 1949, and 2024 marked its seventy-fifth year.

The scope boundary is the thing most reference pages miss. ISA-5.1 is not a P&ID standard. It governs the instrumentation and control portion of a P&ID or PFD, the bubbles, the tags and the signal lines, but it says nothing about the piping, the mechanical detail, the equipment geometry or the drafting rules for the sheet itself. A useful way to hold the split: ISA-5.1 answers what does this instrument do, and where does it live?, while a diagram standard such as ISO 10628 answers what must this drawing contain, and how is the equipment drawn?

Everything else lives in a sister standard, and knowing which is which stops a reviewer citing the wrong one:

• ISA-5.2 covers binary logic diagrams, the Boolean interlock and sequencing logic used for safety and permissive systems.
• ISA-5.4 covers the instrument loop diagram, the detailed per-loop drawing that uses ISA-5.1 symbols.
• ISA-5.5 covers graphic symbols for process displays, the operator HMI and graphic panels.
• ISO 10628 is the diagram framework itself: how a PFD or P&ID is classified, what it must contain, and how equipment is drawn.

One trap to avoid: ISA-5.3, the old shared-display and DCS symbol standard, was folded into ISA-5.1 years ago and is no longer a current standard. Several diagramming-vendor pages still cite it as if it were live. It is not.

The 2024 edition also moved its non-mandatory guidance out of the standard and into two companion technical reports, ISA-TR5.1.02 for the identification system and ISA-TR5.1.03 for the graphic symbols, leaving the core standard as normative content. The symbols and the tag grammar stayed where they were; the guidance around them moved.

What changed between ISA-5.1-2009 and ISA-5.1-2024

The single most-cited change is the title. The standard was renamed from Instrumentation Symbols and Identification to Instrumentation and Control Symbols and Identification, to signal that control functions, and not just discrete field instruments, belong in the symbol set on equal terms. The 2024 edition was approved by ANSI in July 2024 and published in October 2024.

Getting the lineage right matters, because a good deal of published material gets it wrong. The documented history runs: RP-5.1 in 1949, then ANSI/ISA-5.1-1984 (reaffirmed 1992), a major revision in 2009, a corrective interim revision in 2022, and the substantive revision in 2024. There was no 2012 reaffirmation, and the current edition is not “2009 (R2022)”. If a supplier or a drawing legend cites either of those, it is a signal the reference is out of date. The correct current citation is ANSI/ISA-5.1-2024.

What actually moved in 2024, at the level a reader can rely on:

• Reorganisation for readability. Notes were relocated to sit immediately after each table rather than being grouped at the front, and the tables were renumbered into a simple sequential scheme.
• Guidance spun out. The former annexes became the two technical reports above, so the standard proper is now normative text.
• A dedicated instrument-loop-diagram symbol table was added, reflecting the widened “and control” scope.
• Table-level symbol edits were made across the standard. ISA and the ANSI standards blog publish a change summary at that level of detail; the specifics belong in the standard itself rather than a reference page.

One point of continuity is worth stating plainly, because it is routinely misattributed to 2024: the redefinition of the circle-in-a-square as the basic process control system and the diamond-in-a-square as a safety instrumented system or an alternate control system happened in the 2009 revision, when the old DCS-versus-PLC hardware distinction was dropped. It is not new in 2024.

How to read a tag

An ISA-5.1 tag has two parts. The functional identification is the block of letters that says what the device measures and does. The loop identification is a number, often with an area prefix and sometimes an instance suffix, that ties every device on the same control loop together. Read 12-FC-135A as a flow controller (FC) on loop 135, in area 12, the first of several like devices (A).

The letters are not arbitrary. Each position in the block has a fixed role, and it is the position, not a memorised table, that lets you decode an unfamiliar tag:

1. First letter, the measured or initiating variable. For example F for flow, P for pressure, L for level, T for temperature, A for analysis.
2. Variable modifier, optional, qualifying the variable. D for differential is the common one, as in a differential pressure.
3. Readout or passive function, what the device shows. I to indicate, R to record, G for a gauge or sight glass.
4. Output or active function, what the device does. C to control, T to transmit, S to switch, V for a valve, Y for a relay or compute function.
5. Function modifier, qualifying the function. H for high, L for low, HH and LL for high-high and low-low.

The rule that trips people up is tag by function, not by construction. The identification follows the measured variable of the loop, not the physical device. A differential-pressure transmitter that is measuring the level in a tank is tagged LT, because level is the job, not PDT, even though a pressure cell is what is bolted to the vessel. The same logic makes a valve positioner on a flow loop an FY.

A handful of worked examples covers most of what appears on a real sheet:

• PT is a pressure transmitter.
• FT is a flow transmitter; the primary element it reads is an FE, often drawn as a second bubble.
• PIT is a pressure indicating transmitter, a transmitter with a local display.
• FIC is a flow indicating controller, and TIC a temperature indicating controller. If a device drives a valve, it is a controller.
• LSHH is a level switch, high-high, the discrete trip that feeds safety logic.
• LAHH is a level alarm, high-high, the alarm itself, which is a different device from the switch.
• PSV is a pressure safety valve. ISA-5.1 designates every reclosing overpressure-relief valve PSV regardless of whether it is strictly a safety, relief or safety-relief valve; a rupture disc is a PSE.
• FCV (or FV) is a flow control valve.

The shape says the system; the line says the location

The bubble around the tag carries information of its own, on two independent axes. The outline tells you which system runs the function. The line drawn through the bubble tells you where the function physically lives and whether an operator can reach it.

The mainstream reading of the line is: no line for a field-mounted instrument, a single solid line for the primary location (the main control room, operator-accessible, the front of the panel), a double solid line for an auxiliary or local panel, and a dashed line for a behind-the-panel location that is normally inaccessible. Sources differ at the margins over the exact double-versus-dashed convention, so the project legend always governs; read it before decoding an unfamiliar drawing.

Loop numbering: parallel or serial

The loop number can be assigned two ways, and a set that mixes them collides. Parallel numbering starts a fresh sequence for each first letter, so the same number recurs across variables: a reactor might carry TIC-101, PIC-101 and LIC-101, and it is the letter-plus-number pair that makes each unique. Serial numbering runs one continuous sequence across the area regardless of first letter, as in FRC-101, LR-102, PIC-103. A sequence can begin at any convenient number and can encode area information; ISA recommends keeping it simple. The scheme is informative rather than mandatory, which is exactly why the choice has to be frozen in the legend.

The symbols beyond the bubble

Instruments are only part of the language. Two other families show up on almost every P&ID.

Final control elements, the valves, are drawn as bow-tie bodies oriented along the pipe axis, with the actuation and the valve type carried as detail. The distinction a reviewer looks for is modulating versus on-off: a control valve throttles on an analog output, while a shutoff valve is either open or closed.

Computing and signal-conditioning functions sit in square boxes that carry a symbol for the operation: a square-root extractor to linearise a flow signal, a summer, a high or low signal selector, a ratio station, and the letter-labelled converters such as I/P (current to pneumatic). These are the Y relay and compute functions the tag grammar refers to.

ISA-5.1 and ISO 10628 in European practice

On a European or EPC project the two standards are layered, not chosen between. ISO 10628 governs the diagram: how a PFD or P&ID is classified, what it must contain, and how the equipment is drawn (equipment symbols live in ISO 10628-2). The measuring and control tasks on that diagram are represented per IEC 62424, and the instrument tags are very often written in the ISA-5.1 idiom or a company scheme layered on top.

The gap that makes this necessary is deliberate: ISO 10628-2 specifies no tagging methodology at all. It defers identification to a project or national standard, which in European practice is IEC 62424. An ISO-framework project therefore has to define its instrument-identification convention separately, whereas a project working in the ISA idiom gets functional tagging built in. That is a scope boundary, not a defect in either standard.

What engineers actually do about it on real projects comes down to a few habits:

• A layered legend. An ISO 10628-structured drawing that uses ISA-5.1 conventions for the instrument bubbles and tags is common. ISA is the instrument language; ISO is the drawing framework.
• A company standard on top. Owner and operator specifications select and constrain the base standards, and most plants adapt them into an internal standard.
• The legend sheet as the contract. Because ISA-5.1 is flexible by design (user’s-choice letters, optional numbering schemes), the legend is where the numbering philosophy, the symbol set and any deviations are frozen. It is the single most important harmonisation artefact on the drawing.

For a pharma or biotech facility the practical picture is ISA-5.1-based symbology, supplemented by a company standard, with extra attention to clean-in-place and steam-in-place connections and to keeping the drawing current as an as-built record. That last point is where the standard meets the quality system, which is the subject of the next section.

The tagging and symbol mistakes reviewers catch

These are the errors that recur in P&ID reviews and HAZOP walkdowns, drawn from engineering forums, review checklists and training material rather than from the standard alone. They are worth a checklist of your own.

• Tagging by construction instead of function. The differential-pressure cell measuring level tagged PDT when it should be LT; an analyser’s target gas encoded into the first letters instead of annotated outside the bubble.
• Switch confused with alarm. A device is a switch (LSH) when switching is what it does, and an alarm (LAH) only when it contains the alarm function. Teams invent a non-standard third H (LSHHH) or argue over whether a two-H switch can drive a one-H alarm. It cannot; split the switching from the alarm and never invent a third H.
• The wrong line for the signal type. Confusing a pneumatic signal (a line with cross-hatches) with an electrical one (a dashed line) or a software link. Misread during a HAZOP, it produces a wrong failure-mode assumption.

• Controller read as indicator. Reading TIC (a controller, with an analog output) as TI (an indicator, monitor-only), or transcribing the visually similar T and P wrongly at small font. If the instrument drives a valve, it is a controller.
• Relief-valve designation errors. Under ISA-5.1 every reclosing overpressure device is a PSV, whatever its construction; a rupture disc is a PSE. Confusion comes from the mechanical codes, where PRV is used and is itself overloaded (pressure relief, reducing or regulating).
• Missing fail positions. A control valve with no fail-safe annotation (FC for fail-closed, FO for fail-open) is a recurring HAZOP finding, especially on utility systems. Label the fail position rather than relying on the symbol fill.
• Inconsistent formatting and colliding numbers. TIC-101 in one place and TIC101 in another breaks software integration and loop checks; duplicate loop numbers, or a set that mixes parallel and serial numbering, causes collisions. Duplicate tags are a named finding on nearly every review checklist.
• Mixed standards or mixed editions with no legend. Brownfield and post-acquisition plants accumulate ISA, ISO and DIN forms of the same valve across one drawing set, and a package drawn to 2024 can drop silently into a plant frozen on an older edition. The legend is what keeps the set readable.
• As-built drift. The safety-critical one: instruments replaced or reconfigured while the drawing was not updated. For a regulated facility this maps directly onto management-of-change and data-integrity expectations, which is why an out-of-date P&ID is a finding, not a nuisance.

Where these symbols come from

The instrument bubbles, valves and computing-function symbols shown on this page were rendered from the curated ISA-5.1 library in PharmaDiagrams, a browser-based PFD and P&ID editor. The library aims at partial conformance over a curated set, the ISA-5.1 symbols pharma teams actually use rather than every symbol in the standard, with tags admitted freeform so the grammar above stays in the engineer's hands. Diagrams are shared by link, commented on the tag, versioned, and backed by an audit trail, which is what keeps a P&ID current instead of drifting out of date in the way the last section warns about.

See also: ISO 10628 for PFDs and P&IDs · GAMP Category 1: how much you have to validate a P&ID tool