ISA 5.1 Cheat Sheet
Quick reference for ANSI/ISA-5.1-2009 (R2022) P&ID instrumentation symbology and tag identification. Print this page (Ctrl+P or Cmd+P) and save as PDF for offline reference, or pin to your bookmarks. Built for process, instrumentation, and EPC engineers.
Tag Format
An ISA 5.1 tag is a string of identification letters followed by a loop number, optionally followed by a suffix.
[First letter][Succeeding letters][-][Loop number][Suffix]
Example: FT-101A means Flow (F) Transmitter (T) for loop 101, redundant instance A.
Example: PIC-202 means Pressure (P) Indicating (I) Controller (C) for loop 202.
Example: LIT-303 means Level (L) Indicating (I) Transmitter (T) for loop 303.
First Letter: Measured Variable
The first letter of an ISA 5.1 tag identifies the variable being measured or initiated.
| Letter | Measured Variable |
|---|---|
| A | Analysis |
| B | Burner / Combustion |
| C | Conductivity |
| D | Density |
| E | Voltage (EMF) |
| F | Flow rate |
| G | Gauging / Dimensional |
| H | Hand (manually initiated) |
| I | Current (electrical) |
| J | Power |
| K | Time / Time schedule |
| L | Level |
| M | Moisture / Humidity |
| N | User's choice |
| O | User's choice |
| P | Pressure / Vacuum |
| Q | Quantity |
| R | Radiation |
| S | Speed / Frequency |
| T | Temperature |
| U | Multivariable |
| V | Vibration / Mechanical analysis |
| W | Weight / Force |
| X | Unclassified |
| Y | Event / State / Presence |
| Z | Position / Dimension |
“User's choice” letters (N and O) are deliberately reserved by ISA 5.1 for project-specific assignment. Each project defines their meaning in the instrumentation legend sheet (e.g. N for Nitrogen content, O for Oxygen on one project; different on another). The standard hands these letters back to the project owner.
Succeeding Letters: Function Performed
Letters following the first letter identify the function performed by the instrument.
| Letter | Function |
|---|---|
| A | Alarm |
| B | User's choice |
| C | Control |
| E | Sensor (primary element) |
| F | Ratio (fraction) |
| G | Glass / Viewing device |
| H | High |
| I | Indicate |
| J | Scan |
| K | Time rate of change / Control station |
| L | Light / Low |
| M | Middle / Intermediate |
| N | User's choice |
| O | Orifice / Restriction |
| P | Point (test) connection |
| Q | Integrate / Totalize |
| R | Record |
| S | Safety / Switch |
| T | Transmit |
| U | Multifunction |
| V | Valve / Damper / Louver |
| W | Well / Probe |
| X | Unclassified |
| Y | Relay / Compute / Convert |
| Z | Driver / Actuator / Final element |
“User's choice” letters (B and N) are reserved by ISA 5.1 for project-specific function assignment. Like the first-letter reserved slots, each project documents their meaning in the instrumentation legend sheet.
Common Tag Combinations
The most common ISA 5.1 tag combinations seen on process plant P&IDs, with explanations.
| Tag | Expanded | Description |
|---|---|---|
| PT | Pressure Transmitter | Measures pressure and transmits signal to control system |
| PI | Pressure Indicator | Local pressure gauge or indicator |
| PIC | Pressure Indicating Controller | Indicates pressure and provides automatic control |
| PCV | Pressure Control Valve | Control valve actuated by pressure controller |
| PSV | Pressure Safety Valve | Spring-loaded relief valve for overpressure protection |
| PSE | Pressure Safety Element | Rupture disc or other pressure relief element |
| PAH | Pressure Alarm High | High-pressure alarm setpoint |
| PAL | Pressure Alarm Low | Low-pressure alarm setpoint |
| FT | Flow Transmitter | Measures flow rate and transmits signal |
| FE | Flow Element (primary) | Orifice plate, venturi, vortex, Coriolis sensor |
| FI | Flow Indicator | Local flow indicator |
| FIC | Flow Indicating Controller | Indicates flow and provides automatic control |
| FCV | Flow Control Valve | Control valve actuated by flow controller |
| FQI | Flow Quantity Indicator (Totalizer) | Cumulative flow totalizer / batch counter |
| TT | Temperature Transmitter | Measures temperature and transmits signal |
| TE | Temperature Element | Thermocouple, RTD, or other primary temperature element |
| TI | Temperature Indicator | Local temperature gauge |
| TIC | Temperature Indicating Controller | Indicates and controls temperature |
| TCV | Temperature Control Valve | Control valve actuated by temperature controller |
| LT | Level Transmitter | Measures liquid or solid level and transmits signal |
| LI | Level Indicator | Local level gauge or sight glass |
| LIC | Level Indicating Controller | Indicates and controls level |
| LIT | Level Indicating Transmitter | Indicates level locally and transmits signal |
| LCV | Level Control Valve | Control valve actuated by level controller |
| LG | Level Gauge | Glass sight gauge for visual level reading |
| AT | Analytical Transmitter | Composition analyzer (pH, O2, conductivity, etc.) with transmitter |
| AE | Analytical Element | Primary sensor for analytical measurement |
| AIT | Analytical Indicating Transmitter | Analyzer with local indication and signal transmission |
| HS | Hand Switch | Manual switch (start/stop, open/close) |
| HV | Hand Valve | Manually operated valve |
| SC | Speed Controller | Speed control (e.g. for variable-frequency drive) |
| VT | Vibration Transmitter | Vibration sensor with signal transmission (rotating equipment) |
| WT | Weight Transmitter | Load cell or weigh-bridge with signal transmission |
| ZT | Position Transmitter | Valve position or actuator position transmitter |
| ZSO | Position Switch Open | Limit switch indicating valve is fully open |
| ZSC | Position Switch Closed | Limit switch indicating valve is fully closed |
| YIC | Event/Logic Indicating Controller | Sequencing or interlock controller with indication |
Line Symbols
ISA 5.1 line conventions distinguish process flow from instrument signals.
| Line Type | Representation | Purpose |
|---|---|---|
| Process Line | Solid line | Main process flow (gas, liquid, solid) |
| Pneumatic Signal | Solid line with double slashes (//) | Compressed air signal between instruments |
| Electric Signal | Dashed line | Electric signal (analog or digital) between instruments |
| Hydraulic Signal | Solid line with single slashes (/) | Hydraulic signal between instruments |
| Capillary Tubing | Solid line with X markers | Filled thermal system (diaphragm seal, capillary connection) |
| Software / Data Link | Line with circles or hexagons at intervals | Data link between DCS, PLC, or computer systems |
| Electromagnetic / Sonic Signal | Wavy line | Wireless or sonic signal (guided wave radar, ultrasonic) |
| Mechanical Link | Solid line with circles at intervals | Direct mechanical linkage between elements |
Function Symbol Location
The shape of the function symbol bubble indicates where the instrument lives in the plant.
| Location | Representation | Description |
|---|---|---|
| Field-mounted | Plain circle (no line) | Instrument physically mounted at the process equipment |
| Discrete control room | Circle with horizontal line through middle | Panel-mounted discrete instrument accessible to operator |
| Behind control panel (inaccessible) | Circle with dashed horizontal line | Auxiliary instrument behind a control room panel |
| Shared display / shared control (DCS) | Diamond inside circle (or hexagon inside circle) | DCS function block, shared display with shared control |
| Computer function | Hexagon | Function implemented in a computer or business system |
| PLC function | Diamond inside square | Function implemented in a programmable logic controller |
Fail-Safe Position Codes
Control valves and final control elements carry a fail-safe code indicating their position on signal loss.
| Code | Meaning | Description |
|---|---|---|
| FO | Fail Open | Valve goes to fully open position on loss of signal or power |
| FC | Fail Closed | Valve goes to fully closed position on loss of signal or power |
| FL | Fail in Last position | Valve stays where it is on loss of signal or power |
| FI | Fail Indeterminate | Failure position not predictable (avoided in safety design) |
Quick Notes
Loop numbers group instruments that work together. Every instrument in loop 101 (FT-101, FIC-101, FCV-101) is part of the same control loop.
Redundant instruments on the same loop carry a suffix letter (A, B, C). FT-101A and FT-101B are two transmitters on the same loop, typically for safety-critical measurement.
ISA 5.2 covers binary logic diagrams for Safety Instrumented Systems (SIS) and interlock logic. ISA 5.1 governs the P&ID symbology; ISA 5.2 sits alongside for logic-side content.
ISA 5.4 covers detailed instrument loop diagrams (ILDs). ISA 5.1 defines the high-level symbology you see on the P&ID; ISA 5.4 defines the loop-detail drawings beneath.
IEC 60617-12 is the international equivalent. ISA 5.1 dominates in North America, India, Middle East, and Southeast Asia. IEC 60617 dominates in some European markets. Most symbols overlap, but a handful of function-block representations differ.
Project-specific deviations from ISA 5.1 are documented in the project instrumentation symbol legend issued at the start of every EPC project. Always check the project legend before assuming a symbol matches ISA 5.1 strictly.
Quick FAQs
Common ISA 5.1 tag lookups engineers reach for.
What does PT mean on a P&ID?
PT = Pressure Transmitter. Per ISA 5.1: P (first letter) = Pressure or Vacuum measured variable; T (succeeding letter) = Transmit function. PT-101 means the pressure transmitter for loop number 101. Add a suffix letter (PT-101A, PT-101B) for redundant instruments on the same loop, common on safety-critical measurement.
What does FIC mean on a P&ID?
FIC = Flow Indicating Controller. F (first letter) = Flow rate; I (succeeding letter) = Indicate; C (succeeding letter) = Control. FIC-202 means the flow indicating controller for loop 202: it both shows the flow value locally and provides automatic control output (typically to a control valve like FCV-202 on the same loop).
What does PSV mean on a P&ID?
PSV = Pressure Safety Valve. P (first letter) = Pressure; S (succeeding letter) = Safety; V (succeeding letter) = Valve. A spring-loaded relief valve that opens automatically when pressure exceeds the set point, protecting equipment from overpressure. PSVs are sized per API 520 / 521 and ASME Section VIII UG-125 through UG-137.
What is the difference between FO and FC on a control valve?
FO = Fail Open: the valve goes fully open on loss of signal, instrument air, or power. FC = Fail Closed: the valve goes fully closed under the same failure conditions. The choice depends on process safety analysis: a cooling water valve typically fails open (keep cooling on), a fuel gas valve typically fails closed (cut fuel on failure). The fail-safe code is shown next to the valve symbol on the P&ID.
How are redundant instruments named in ISA 5.1?
Redundant instruments on the same loop carry a suffix letter (A, B, C) after the loop number. FT-101A and FT-101B are two flow transmitters on loop 101, typically for safety-critical measurement where one transmitter alone is insufficient. The control system votes on the readings (1oo2, 2oo3) per the safety integrity level (SIL) requirement defined in the SIS specification per IEC 61511.
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