S - Speed, frequency
Reference for the ISA 5.1 first letter S. First slot S means speed. Second slot S means switch. SI is a speed indicator. PS is a pressure switch. The same character, two different jobs, decided entirely by where it sits in the tag. Read the first letter first or you'll wire a tachometer to a discrete input card and wonder why the loop doesn't behave.
Read one of your own drawings.
Drop a P&ID, instrument index, or schedule. Tagsight reads it to the tag and opens a workspace you keep when you sign in.
PDF · DWG · DXF · TIFF · PNG · XLSX
Also stands for.
- Modifier. Switch, when used as a second letter
Common second letters.
Transmitter, ST. Speed transmitter
Indicator, SI. Tachometer
Alarm high, SAH. High-speed alarm
Switch high, SSH. Overspeed switch, often a trip on rotating equipment
Example tags.
Speed transmitter, often on a compressor or turbine shaft
Speed indicator, tachometer reading
Overspeed switch tripping a turbine or compressor on high speed
Typical placement.
On rotating equipment with safety-critical speed monitoring, turbines, compressors, large pumps. Speed measurement uses S as a first letter. The more common second-letter use of S is for switches, PS, FS, LS.