Troubleshooting
Why an electromagnetic flow meter reads zero on a plastic pipe
A magnet inserted into a plastic pipe will still generate a voltage. The instrument does not care about the pipe; it cares about the reference. When the sensor has no conductive path back to the liquid, the amplifier sees a floating signal and reports zero, or a number that jumps whenever a pump starts.
Check the ground before the meter
The fix is a grounding ring on each flange, or a grounding electrode in a non-conductive pipe, bonded to the meter’s earth terminal with a cable at least as heavy as the signal cable. On PVC or lined pipe with no metal in contact with the liquid, both rings are needed; one ring is not enough.
Two other causes look identical. A partially full pipe, common on horizontal runs with no back pressure, gives an unstable reading that rises and falls with flow. And a lining that has lifted away from the pipe wall lets liquid creep behind it, which shorts the electrodes. Neither is fixed by changing the zero.