- W.G. — Water Gauge (plus a height dimension).
- W.C. — Water Column or Column of Water.
- Inches H20 — Inches of Water (Column).
- S.P. — Static Pressure (usually in inches).
- W.L. — Water Lift
These are all the same thing. They can reference pressure or vacuum, but they are normally associated with dust collector fan performance. In vacuum systems, the terms are most often used when talking about pressure differential across a filter media.