A mag meter measures velocity and multiplies by the pipe area. Too slow and accuracy suffers and coatings build up; too fast and you get head loss and wear. Check the velocity at your normal and minimum flows, or find the size that gives a target velocity.
Mag Meter Velocity Check
Formula
Velocity, ft/s = Flow, gpm x 0.002228 / Area, ft² Area, ft² = 3.1416 x (d, in / 12)² / 4 Shortcut: Velocity, ft/s = 0.4085 x gpm / d² (d in inches)
Inputs used
- Flow (normal or maximum) (gpm)
- Meter (or pipe) inside diameter (in)
- Minimum flow (optional) (gpm)
Worked example (fictional plant data)
- 10 in mag meter on a RAS line, 1,200 gpm normal, 300 gpm minimum
- Area = 3.1416 x (10 / 12)² / 4 = 0.5454 ft²
- Velocity = 0.4085 x 1,200 / 10² = 4.90 ft/s
- At 300 gpm: 0.4085 x 300 / 100 = 1.23 ft/s (low for sludge)
Meter Size for a Target Velocity
Formula
d, in = square root of (0.4085 x gpm / Velocity, ft/s)
Inputs used
- Design flow (gpm)
- Target velocity (ft/s)
Worked example (fictional plant data)
- 1,200 gpm at a target of 5 ft/s
- d = square root of (0.4085 x 1,200 / 5) = 9.90 in, so a 10 in meter
Cautions
- Use the meter bore (liner inside diameter). It can be smaller than the pipe.
- Keep the meter full: install it in a rising line or a low point, never at a high point.
- Give it the straight run the manufacturer asks for (often 5 diameters upstream and 2 or 3 downstream).
- Ground the meter as the manual shows, especially in plastic or lined pipe.
- Velocity ranges here are general guides. The manufacturer sizing guide governs.
References
- AWWA Manual M33, Flowmeters in Water Supply.
- Manufacturer sizing guides for electromagnetic flowmeters (velocity limits vary by make and liner).
Results are estimates for operator and technician use. Equations follow the references above; always check against the instrument manual, your calibration procedure and plant data.