^ 
^ In instrumentation and process diagrams the origin and quantity of a measurement or a function is described with one to four letters. The measurement or the function is futher identified by a unique number.
Example: PIC-107 - P Pressure, I Indication, C Control, loop number 107.
| LETTER | MEANING | |
|---|---|---|
| as the 1st | as any other | |
| letter | ||
| A | Alarm | |
| B | Audiovisual function | |
| C | Control | |
| D | Density | Difference |
| E | Electrical | Sensory function |
| F | Flow | Ratio |
| G | Length, position | |
| H | Hand | Manual |
| I | Indication | |
| J | Intermittant function | |
| K | Time, timer, scheduler | |
| L | Level | |
| M | Moisture | Signal transformer |
| N | User defined, e.g. amount | |
| O | User defined | |
| P | Pressure | Testing, sampling function |
| Q | Quality, e.g. analysis result, concentration, conductivity, consistency | combination or addition, calculation |
| R | Radio activity | Recording, plotting |
| S | Speed, velocity, frequency | Switching function |
| T | Temperature | Transmitter function |
| U | Several measurands, multivariable | Multi-function |
| V | Viscosity | Valve, functional unit |
| W | Weight, force | |
| X | Undefined quantity | Undefined function |
| Y | User defined | Counter function |
| Z | Emergency, security, interlock | |