mmWC and inWC
Millimetres and inches of water column for low-pressure and draft measurement.
Learning objectives
- Explain mmWC and inWC in technician-friendly language.
- Identify where where mmWC and inWC appear in plant instrumentation appears in live plant work.
- Connect the topic to pressure measurement, calibration, or loop checking.
- Avoid the main field mistake: using a bar-range transmitter for millimetre-water-column service.
mmWC and inWC is part of the Pressure Measurement master module. Millimetres and inches of water column for low-pressure and draft measurement.
The main focus is where mmWC and inWC appear in plant instrumentation. In the field, the technician must read the tag, confirm the pressure reference and unit, check the process connection, and compare the instrument reading with the expected plant condition. The topic also links to calibration records, DCS scaling, alarm settings, and safe isolation before any instrument work.
A practical way to study this topic is to ask four questions: what pressure is being measured, what reference is used, how the signal reaches the DCS, and what failure would make the reading unsafe or misleading. This keeps the lesson tied to real maintenance and commissioning decisions rather than only theory.
Field examples
- Furnace draft gauges and duct pressure instruments are typically specified in mmWC or inWC.
- Compare the local gauge, transmitter HART value, and DCS indication before deciding which value is credible.
- Record the unit, range, LRV, URV, and tag number before calibration or loop checking.
Common mistakes
- using a bar-range transmitter for millimetre-water-column service
- Trusting one reading without checking the pressure reference, unit, range, and process condition.
- Opening tubing, manifolds, or vents without proving isolation and zero stored pressure.
Safety
Low-range DP instruments can be damaged by accidental overpressure from standard supply lines.
Key takeaway
mmWC and inWC helps a technician understand pressure readings, field installation, and safe troubleshooting. Always verify the unit, reference, range, and isolation state before acting on the measurement.
Imported study note from pressure_lesson_063. The app generates this text from a lesson seed. It is not a long textbook page.