How to Read P&ID Instrument Symbols. Bubbles, Tags, Lines.
What the circles, lines, and letter codes on P&ID drawings mean. Covers ISA bubble types, tag number formats, signal line connections, and how to read them.
An instrument symbol on a P&ID encodes three things. Where the instrument sits, read from the bubble shape. What it does, read from the tag letters. And how it signals the control system, read from the line type. Take those three in order and any bubble resolves. The bubble shape gives location, the letters give the measured variable and function, the line style gives the signal path. The rest of this guide takes each in turn, then traces a full loop end to end.
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.
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. The P&ID digitization guide covers the full workflow from drawing preparation through classified I/O list, including how to handle multi-sheet drawing sets and mixed-quality sources.
Related
- P&ID to I/O list. Automated extraction of every bubble, classified per ISA 5.1
- P&ID to equipment list. Pumps, vessels, tanks, exchangers
- P&ID to line list. Pipe numbers with size, spec, service
- Brownfield P&ID digitization. For scanned, faded, or photographed drawings
- ISA 5.1 letter codes cheat sheet
- Instrument index vs I/O list vs line list