A piping and instrumentation diagram (P&ID) is a detailed diagram in the process industry which shows the piping and process equipment together with the instrumentation and control devices
Superordinate to the piping and instrumentation diagram is the process flow diagram (PFD) which indicates the more general flow of plantprocesses and the relationship between major equipment of a plant facility
| Letter | Column 1 (Measured value) | Column 2(Modifier) | Column 3 (Readout/passive function) | Column 4 (Output/active function) | Column 5 (Function modifier) |
| A | Analysis | Alarm | |||
| B | Burner, combustion | User choice | User choice | User choice | |
| C | User's choice (usually conductivity) | Control | Close | ||
| D | User's choice (usually density) | Difference | Deviation | ||
| E | Voltage | Sensor | |||
| F | Flow rate | Ratio | |||
| G | User's choice (usually gaging/gauging) | Gas | Glass/gauge/viewing | ||
| H | Hand | High | |||
| I | Current | Indicate | |||
| J | Power | Scan | |||
| K | Time, time schedule | Time rate of change | Control station | ||
| L | Level | Light | Low | ||
| M | User's choice | Middle / intermediate | |||
| N | User's choice (usually torque) | User choice | User choice | User choice | |
| O | User's choice | Orifice | Open | ||
| P | Pressure | Point/test connection | |||
| Q | Quantity | Totalize/integrate | Totalize/integrate | ||
| R | Radiation | Record | Run | ||
| S | Speed, frequency | Safety | Switch | Stop | |
| T | Temperature | Transmit | |||
| U | Multivariable | Multifunction | Multifunction | ||
| V | Vibration, mechanical analysis | Valve or damper | |||
| W | Weight, force | Well or probe | |||
| X | User's choice (usually on-off valve as XV) | X-axis | Accessory devices, unclassified | Unclassified | Unclassified |
| Y | Event, state, presence | Y-axis | Auxiliary devices | ||
| Z | Position, dimension | Z-axis or Safety | Actuator, driver or unclassified final control element |
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