Its not for no reason that many manufacturers of electromagnetic flow meters suggest grounding rings as required accessory. In non-conductive pipes or when measuring abrasive products, electromagnetic flow meters require these rings to reduce electromagnetic interference, which would compromise measurement accuracy. Even without expected electromagnetic interference, grounding rings can improve flow measurement so they are usually worth the investment.
When installing grounding rings on an electromagnetic flow meter, it is important that the rings match the meter size rather than the pipe size.
For most applications, accurate measurement with a flow meter requires an inlet run of straight pipe five times as long as the pipe’s diameter and an outlet run of straight pipe three times as long as the pipe’s diameter after the flow meter.