Instrumentation basics · 1.3

Reading P&ID symbols and ISA 5.1 tag numbers

25 October 2026 · 10 min read · ISA 5.1

A P&ID is the map every instrument engineer works from. The instrument bubbles on it follow ISA 5.1, the standard for instrument symbols and identification. Learn its few rules, and a tag such as PDIT-1203 or LSHH-4101 tells you what the instrument is before you open its datasheet.

Where this matters

The tag is the key that links every instrument document: the instrument index, the datasheet, the hook-up, the loop diagram, the I/O list, the cable schedule, the purchase order and the maintenance system. A tag misread on the P&ID becomes the wrong instrument bought, the wrong range configured or the wrong cable pulled. Getting the identification right at the start saves rework in every later deliverable.

How it works

The tag: letters plus loop number

For example, 10-PDIT-1203A is plant area 10, P (pressure) with modifier D (differential), I (indicate), T (transmit), loop 1203, and suffix A, the first of two similar instruments. Area prefixes and suffixes are project conventions. The letters follow ISA 5.1.

First letters: the measured variable

C, D, G, M, N and O are "user's choice" letters that a project defines on its legend. First-letter modifiers change the variable: D differential (PD, TD), F ratio (FF), Q totalise (FQ), K rate of change, J scan and S safety (PSV, PSE).

Following letters: what it does

Alarms and switches take H (high), L (low) and M (middle), doubled for the second level: LAH, LAHH, PSLL.

Bubbles: where the function lives

The SIS is often shown with the PLC symbol or a project-specific symbol, so check the legend.

Signal lines

Thin solid lines are process connections and instrument supply. Dashed lines are electrical signals. Lines with double cross-hatches are pneumatic signals. Lines with small circles are data or software links. Lines with crosses are capillaries.

What the standards and practice say

• ISA 5.1 (the 2009 edition is the one most projects still cite) defines the letters, bubbles and lines above. A letter's meaning depends on its position: first letter means the variable, later letters mean the function.
• IEC 62424 covers how process control functions are shown on P&IDs and exchanged between tools. Many European projects mix it with ISA letters.
• One loop, one number. Every device in a loop shares the loop number, so PT-1203, PIC-1203 and PV-1203 belong together.
• Tag by function, not by hardware. A transmitter with a local display is PIT. Its DCS indication is PI shown in a shared-display bubble.
• Company rules override. Many operators add area codes, separate SIS loop series or extra letters. The project's legend sheet and numbering procedure are the authority.

Worked example: decoding a P&ID snippet

Here are the instruments around a compressor suction drum:

What this example shows

• Count instruments by function. The drum has 13 tags, but the SIS level loop 4102 is independent of control loop 4101 because it has its own number.
• One valve can have a whole family. XV-4106 brings position switches, a hand switch and usually a solenoid (XY-4106). Each needs I/O and cables.
• The bubble tells you the I/O. Field transmitters become analogue inputs, the DCS bubble is configuration, not hardware, and switches become digital inputs.

When it fails: the fixes in order

1. Check the legend sheet. Most "wrong" tags are project conventions that differ from the textbook.
2. Check the loop. If devices that work together carry different numbers, raise it with the process engineer before the instrument index is built.
3. Check the function letters against the hardware. A "PI" that actually needs a 4–20 mA signal is a PIT, and that changes the datasheet and the I/O count.
4. Fix it at the source. Correct the P&ID and the instrument index together. Don't fix it quietly in one downstream document.

Common mistakes

• Reading a letter by its common meaning instead of its position. The S in PSV is the safety modifier, but the S in PSL means switch.
• Giving the SIS transmitter the same loop number as the control transmitter.
• Missing the accessories, such as the thermowell TW, the solenoid XY and the limit switches ZS.
• Assuming every bubble is a physical instrument. DCS and PLC bubbles are functions.
• Re-using a loop number from a deleted instrument, which confuses document history.
• Ignoring the user's-choice letters and the X definitions on the legend.

Checklist

• The legend sheet has been read and its letter and symbol definitions noted.
• Each tag's first letter matches the measured variable.
• The function letters match the hardware (local only, transmitter, switch or valve).
• All devices in a loop share the loop number.
• SIS loops are numbered separately from control loops.
• Accessories (TW, XY, ZS, HS) are tagged and listed.
• The bubble location (field, panel, DCS or PLC) is consistent with the I/O list.
• The suffixes (A/B) for duplicated instruments are applied consistently.
• The tags on the P&ID and the instrument index match exactly.

Quick reference

Further reading

• ISA 5.1, Instrumentation Symbols and Identification
• IEC 62424, Representation of process control engineering: requests in P&ID diagrams and data exchange
• In this series: 1.2 The instrument loop · next, 1.4 Range, span, accuracy and turndown

Letter meanings follow ISA 5.1. Projects adapt it, so the project's legend sheet and numbering procedure take precedence. The example tags are illustrative. ISA and IEC are named for identification only.