P&ID Instrument Symbols Explained: Bubbles, Tags, and Lines

P&ID drawings are dense with information, but instrument symbols follow a consistent visual language once you know what to look for. This guide covers the shapes, tag formats, line types, and common misreads that trip people up when building I/O lists from drawings.

Instrument bubble shapes

The bubble (or balloon) around a tag number tells you where the instrument is located and how it connects to the control system. ISA 5.1 defines these shapes precisely:

Shape	Meaning	Example
Single circle	Field-mounted, accessible to operator	A local pressure gauge (PI-101)
Circle with horizontal line through center	Field-mounted, behind panel (not operator-accessible)	A transmitter in an enclosure (FIT-201)
Double circle (two concentric circles)	Shared display/control - typically DCS, PLC, or control room panel	A controller on the DCS (FIC-201)
Square with circle	Computer function	Software alarm or calculation block
Dashed circle	Instrument on auxiliary panel or local rack	Local panel-mounted indicator
Diamond	Logic function (interlock, safety)	SIS trip logic (FSL-301)

The distinction between single and double circles matters most in practice. If you see a single circle, that instrument exists as physical hardware in the field. A double circle means the function lives in the control system - no separate physical device, just a software block or shared display point.

Tag number format

Every instrument tag follows this structure:

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

The first letter identifies the measured variable. Subsequent letters describe the function. For example, FIT-101:

• F - Flow (measured variable)
• I - Indicating (readout function)
• T - Transmitting (output function)
• 101 - Loop number
• No suffix

First letter reference

Letter	Measured variable	Notes
F	Flow	Most common on process P&IDs
P	Pressure	Includes vacuum
T	Temperature
L	Level	Tank, vessel, separator
A	Analysis	pH, O2, conductivity, composition
W	Weight / Force	Load cells, weigh scales
Z	Position	Valve position feedback
S	Speed / Frequency	Motor speed, turbine RPM
H	Hand (manual)	Manual loader, hand switch

Letters B, C, D, G, M, N, and O are "user's choice" - each facility assigns them to site-specific variables. Check the drawing legend or project standards before assuming what they mean.

Subsequent letter reference

Letter	Function	Typical signal class
T	Transmitter	AI (4-20mA output)
E	Sensing element	AI (often paired with T)
C	Controller	AO (control output)
V	Valve (control)	AO (modulating) or DO (on/off)
S	Switch	DI (discrete contact)
Y	Relay / Solenoid	DO (energize/de-energize)
I	Indicator	Depends on implementation
R	Recorder	Depends on implementation
G	Gauge / Glass	No signal (local read only)
H	High (modifier)	Alarm or trip qualifier
L	Low (modifier)	Alarm or trip qualifier

Multiple subsequent letters stack. PSHH-301 means Pressure Switch High-High - a discrete input from a safety-rated pressure switch at its high-high setpoint.

Signal lines and connection types

Lines connecting instruments to process piping and control systems carry critical information:

Line style	Meaning
Solid thin line	Process connection (impulse tubing, capillary)
Dashed line	Electrical signal
Dashed line with dots	Pneumatic signal
Double dashed line	Hydraulic signal
Line with lightning bolt	Electromagnetic / RF signal
Line with "S" marks	Software / data link

The signal line type tells you the physical layer. A dashed line from a transmitter to a DCS bubble means it is a wired electrical signal - probably 4-20mA analog or HART. A dashed line with dots means pneumatic, common on older plants with 3-15 psi control valves.

Watch for where signal lines terminate. A line from FIT-101 (field transmitter, single circle) to FIC-101 (DCS controller, double circle) shows the AI signal path. The line from FIC-101 to FCV-101 (control valve) shows the AO path back to the field.

Common symbols engineers misread

Years of building I/O lists from P&IDs surface the same mistakes repeatedly:

Control valves vs. shutoff valves

A control valve (FCV, LCV, PCV) modulates position and is an analog output. A shutoff valve (XV, SDV, BDV) is either open or closed and is a digital output. The tag tells you which, but the valve symbol on the drawing can look identical. Always read the tag, not just the valve body shape.

Position feedback instruments

Tags like ZSO-101 (position switch, open) and ZSC-101 (position switch, closed) provide discrete feedback confirming a valve reached its end position. These are digital inputs. They are easy to miss because they are drawn as small bubbles near the valve, not on a main process line.

Indicators vs. transmitters

PI-101 (pressure indicator) is a local gauge with no signal back to the control system. It does not appear on your I/O list. PT-101 (pressure transmitter) sends a signal and belongs on the list as an AI. The difference is one letter, and it changes whether the instrument needs wiring.

Switches vs. transmitters

PSH-201 (pressure switch high) is a discrete contact - it is either made or broken. That is a DI. PIT-201 (pressure indicating transmitter) sends a continuous analog signal. That is an AI. Both measure pressure, but they produce fundamentally different signal types.

Solenoid valves hidden behind control valves

Many control valve assemblies include a solenoid (FY-101) for emergency shutdown or trip functions. The solenoid is a DO that is separate from the AO to the control valve positioner. Missing these means your I/O count is short by one channel per valve assembly.

Mapping symbols to I/O list entries

Every instrument symbol on a P&ID maps to a row in your I/O list. Here is how a typical control loop translates:

P&ID Symbol	Tag	Signal class	I/O list entry
Single circle, field transmitter	FIT-101	AI	4-20mA flow signal to DCS
Double circle, DCS controller	FIC-101	(software)	No I/O point - internal to DCS
Control valve with positioner	FCV-101	AO	4-20mA to valve positioner
Position switch (open)	ZSO-101	DI	Valve open limit switch
Position switch (closed)	ZSC-101	DI	Valve closed limit switch
Solenoid on valve	FY-101	DO	Trip solenoid, 24VDC

That single flow control loop produces 1 AI, 1 AO, 2 DI, and 1 DO. Multiply across hundreds of loops and you see why accurate reading matters - each missed or misclassified symbol cascades into wrong module counts, wrong wiring, and wrong PLC configuration.

The signal class decision tree

When you are staring at an instrument bubble and need to classify it:

1. Is it a transmitter (T) or sensing element (E)? - AI
2. Is it a controller output (C) or modulating valve (CV)? - AO
3. Is it a switch (S) or position indicator (Z)? - DI
4. Is it a solenoid (Y) or on/off valve (XV)? - DO
5. Is it a local gauge (G), indicator with no signal line, or recorder? - Not on the I/O list (or review case-by-case)

Practical next steps

If you are building an I/O list from P&IDs, start by printing (or viewing) each drawing and systematically scanning left to right, top to bottom. Mark each instrument as you count it. Track your signal class totals per page to catch pages where the count seems suspiciously low - you probably missed something.

For the tag format and letter codes, keep ISA 5.1 Table 1 handy. It is the definitive reference for what each letter combination means. The tables in this guide cover the common cases, but site-specific conventions can override the standard - always check the project legend.

Cross-reference your I/O list against the P&ID instrument index (if one exists) and against the loop diagrams. Discrepancies between these documents are common on brownfield projects and need resolution before you order hardware or start programming.

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Try the interactive ISA 5.1 Tag Decoder to paste any instrument tag and get an instant breakdown of its letters, signal class, and loop number.