ISA 5.1 Instrument Identification: A Practical Reference

ISA 5.1 defines how instruments are identified on P&ID drawings. Every tag number encodes what an instrument measures, what it does, and how it connects to the control system. This guide covers the letter code system and how to map tags to I/O signal types.

Tag number structure

Every P&ID instrument tag follows this format:

[First Letter][Modifier Letters]-[Loop Number][Suffix]

Example: FIT-101A

• F — measured variable (Flow)
• I — function (Indicating)
• T — function (Transmitter)
• 101 — loop number
• A — redundancy suffix

The letters carry all functional information. The loop number identifies which control loop the instrument belongs to.

First letters — what it measures

Letter	Variable	Common example
F	Flow	FIT-101
P	Pressure / vacuum	PIT-301
T	Temperature	TIT-401
L	Level	LIT-201
A	Analysis (pH, O2, composition)	AIT-301
W	Weight / force	WIT-101
Z	Position / dimension	ZT-101
V	Vibration	VT-101
S	Speed / frequency	ST-101
H	Hand (manual)	HV-101

Letters B, C, D, G, M, N, O are "user's choice" — facilities assign them to site-specific variables like conductivity (C) or density (D).

Modifier letters — what it does

Letter	Function	Typical signal
T	Transmitter	AI
E	Sensing element	AI
C	Controller	AO
V	Valve	DO
S	Switch	DI
Y	Relay / solenoid	DO
I	Indicator	Review
R	Recorder	Review
G	Gauge / glass	Review
H	High (alarm modifier)	—
L	Low (alarm modifier)	—
A	Alarm	—

Signal classification reference

AI — Analog Inputs (transmitters, 4-20mA)

Code	Instrument
PT / PIT	Pressure Transmitter
FT / FIT	Flow Transmitter
LT / LIT	Level Transmitter
TT / TIT	Temperature Transmitter
TE	Temperature Element (RTD/thermocouple)
AT / AIT	Analyzer Transmitter
PDT	Differential Pressure Transmitter
WT / WIT	Weight Transmitter
ZT	Position Transmitter

DI — Digital Inputs (switches, contact closure)

Code	Instrument
PSH / PSL	Pressure Switch High / Low
PSHH	Pressure Switch High-High (safety)
FSH / FSL	Flow Switch High / Low
LSH / LSL	Level Switch High / Low
LSHH / LSLL	Level Switch High-High / Low-Low
TSH / TSL	Temperature Switch High / Low
ZSO / ZSC	Valve Position Open / Closed

AO — Analog Outputs (controller outputs, 4-20mA)

Code	Instrument
FIC	Flow Indicating Controller
PIC	Pressure Indicating Controller
LIC	Level Indicating Controller
TIC	Temperature Indicating Controller
FCV	Flow Control Valve
PCV	Pressure Control Valve
LCV	Level Control Valve
TCV	Temperature Control Valve

DO — Digital Outputs (on/off commands)

Code	Instrument
XV	Block Valve (on/off)
SOV	Solenoid Valve
MOV	Motor Operated Valve
ESDV	Emergency Shutdown Valve
SDV / BDV	Shutdown / Blowdown Valve

Review — may not be wired

Code	Instrument
PI / TI / LI / FI	Indicators (often local gauges)
PG / TG	Pressure / Temperature Gauge
LG	Level Gauge / Glass
TW	Thermowell

Common mistakes

T vs P confusion — These letters look similar at small font sizes on P&IDs. It's the most common transcription error in manual I/O list creation.

Truncated codes — Reading TIC (Temperature Indicating Controller → AO) as TI (Temperature Indicator → Review). If the instrument connects to a control valve, it's a controller.

Split instruments — Some P&IDs show FE-101 (flow element) and FT-101 (flow transmitter) separately. These are one loop and should consolidate as FIT-101 in the I/O list.

FCV classification — Flow Control Valves are AO (analog output), not DO. They modulate position via 4-20mA, unlike XV valves which are strictly on/off.

Quick classification rule

When building an I/O list, look at the last functional letter:

• Ends in T or E → AI
• Ends in S → DI
• Ends in C → AO
• Ends in V or Y → DO
• I, G, R alone → Review (verify if wired)

This is the classification logic Tagsight applies automatically when extracting instruments from P&ID drawings.