ISA-5.1: A Practical Reference for HMI Engineers

ISA-5.1 — “Instrumentation Symbols and Identification” is the standard that lets a process engineer in Stuttgart and a panel operator in São Paulo read the same P&ID and arrive at the same mental model of the loop. It defines the bubble-letter shorthand you see on every drawing — FIC-101, PSV-204, LSHH-301 — and the geometric conventions that distinguish a field instrument from one wired into the DCS.

This is a practitioner's reference, not a verbatim restatement. It focuses on the parts that matter when you are drawing HMI symbols — or picking one that another engineer has published — tag format, letter codes, and mounting symbols.

1. Tag format

Every ISA-5.1 tag is a two-part identifier:

• Function letters (1–4 letters) describe what the instrument measures and what it does.
• Loop number (typically 3 digits) identifies the specific loop. Conventionally grouped by area: 1xx for unit 1, 2xx for unit 2, etc.

Joined by a hyphen: FCV-104. Optional suffix letters (A/B/C, S, X) handle redundancy, switches, and accessories: PT-101A, HS-204, FCV-104S.

2. First letters — the measured variable

3. Modifier and succeeding letters

The letters after the first describe the function of the instrument:

Putting it together:

• PIT-101 — pressure indicating transmitter (a transmitter with a local display).
• FIC-301 — flow indicating controller (a controller with a built-in display).
• LSHH-401 — level switch high-high (a discrete trip device for safety logic).
• FQI-501 — flow totalizing indicator (a flow totalizer with display).
• FCV-104 — flow control valve (a valve actuated by the flow controller).
• PSV-201 — pressure safety valve (a relief valve).

4. Mounting symbols

The bubble around the tag tells you where the instrument lives, which is critical because it determines what the operator can see and touch:

Modern HMIs blur the line between DCS and PLC, but the convention persists on P&IDs because it reflects where the logic lives, not where the data is shown. A PLC-controlled loop with a SCADA faceplate is still drawn with the PLC square — the SCADA is just a window into it.

5. Common loop examples

Pressure control loop

PT-101 → PIC-101 → PCV-101. A transmitter senses pressure, the controller (DCS or PLC, hence the bubble convention matters) calculates output, and the control valve modulates. The loop number 101 is shared.

Level interlock

LIT-401 is the analog indicating transmitter for normal control. LSHH-401 is a separate discrete switch (often a vibrating-fork or capacitance probe) wired into the safety system independent of the DCS. ISA-84 (functional safety) usually mandates this independence.

Motor start/stop

HS-201 (hand switch — start), YS-201A (state running), YS-201B (state alarm). Some sites use M-201 as the “motor object” umbrella tag and break out individual signals as suffixes.

6. Drawing them in HMI software

Most HMI platforms ship with a generic instrument library that technically follows ISA-5.1, but:

• The shapes are often dated (3D bevels, gradient fills) and clash with ISA-101.
• Tag visibility is hard-coded — you cannot easily turn it off when the HMI handles tagging through Text Objects on top of the bitmap.
• Mounting symbol variants (field/DCS/PLC/shared) are usually missing or duplicated as separate library entries with no naming convention.

How HMI Library handles this, in three layers:

• Generic icon library — the official set ships the four mounting shells (field, DCS, PLC, shared) and the logic diamond as empty bubbles, plus the ISA-5.1 signal-line types (process pipe, electrical, pneumatic, hydraulic, capillary, data link) and the alarm/state badges you combine with them. Tag text is optional on every icon, so you can leave it off when the HMI handles tagging with a Text Object on top.
• Symbol Builder — in the free in-browser editor you drop a bubble stamp, type the function letters and loop number, add a P1/P2 badge or a “no signal” state if you need it, and export clean SVG or transparent PNG. One tagged instrument takes about a minute; a full P/T/F/L family with SH/SL/SHH/SLL variants is a template you save once and reuse per project.
• Community library — engineers publish finished instruments (transmitters, indicating controllers, switches, discrete sensors such as ZS/LS/FS) under CC BY 4.0 at Explore → Instrumentation. Symbols that pass the automatic ISA-101 check (palette, stroke, structure) carry an “ISA-101 compliant” mark. If one is close but not quite your tag convention, open it in the Builder, edit, and republish with credit to the original.

7. ISA-5.1 vs ISA-5.5 vs ISA-101

These three are often confused:

• ISA-5.1 — instrumentation identification: tags, letters, mounting bubbles. P&ID-centric.
• ISA-5.5 — graphic symbols for process equipment (vessels, columns, pumps in P&ID style). On HMI Library those equipment shapes are what the community draws in the Builder and publishes on Explore; the official library deliberately stays on generic HMI iconography.
• ISA-101 — HMI design: how to compose a screen, what colors mean, alarm priorities. See our ISA-101 guide.

A well-designed HMI uses ISA-101 for layout and color, embeds ISA-5.1 tags exactly as the P&ID drew them so operators can cross-reference, and represents equipment with shapes that are recognizable to anyone who has read the P&ID.

8. References

• ISA-5.1-2009 — “Instrumentation Symbols and Identification”
• ISA-5.5 — “Graphic Symbols for Process Displays”
• ISA-101.01-2015 — “Human Machine Interfaces for Process Automation Systems”
• ISA-18.2-2016 — “Management of Alarm Systems”