Application Corner: The wrong coil in a magnetic flowmeter?

Magnetic flowmeters are used ubiquitously to measure liquid flows, many of which are water-based. The effluent flow from a paper mill was measured with an existing magnetic flowmeter that was used by the treatment plant for billing purposes. The flowmeter was regularly calibrated over the years by inputting simulated electrode voltages corresponding to different flow rates. The flowmeter failed calibration after approximately 40 years of service.

The plant treating the effluent generated a graph with annual treatment flows that varied significantly from year to year. The paper mill explained that multiple changes were made over the years to reduce water consumption, and hence, reduce effluent flows. Despite project descriptions detailing the changes, the treatment facility alleged that the effluent flowmeter measured approximately half of the actual flow, and pursued arbitration to collect payment for the unbilled half.

Interestingly, the treatment company’s expert suggested that the flowmeter measured the alleged flow because that the wrong size coil was installed in the flowmeter when it was manufactured. This begs the question of explaining how a (say) 16-inch diameter coil will fit around a (say) 24-inch diameter magnetic flowmeter tube. In addition, the flowmeter was almost certainly wet-calibrated prior to shipment, even though the manufacturer could only locate its specification.

It was impractical to locate magnetic flowmeter coils and bodies of this size and bring them to the arbitration proceeding, so photographs were taken that included an 8-inch coil sitting on a 12-inch magnetic flowmeter tube showing that the small coil would not fit around the larger tube. The other expert’s explanation of how this works would have been interesting, but the photographs were not presented at arbitration because the case settled before the experts testified.