Magnetic Flowmeter in Demin Water


We are currently facing the problem of swinging/erratic reading of our electromagnetic flowmeter even though we already did the empty pipe calibration. Is this problem cause by improper installation? What are possible solutions for this one?

Possible reasons for magmeter fluctuations

- conductivity lower than the minimum required by the flow meter to get a reliable reading (depends on how de-mineralized the water is)

- vertical down flow (attempted) measurement

- entrained air

- remote transmitter mounted remotely and wired to the tube without the mfg's recommended shielded (twisted pair) low-noise cable, (typically with THHN).

- remote transmitter-tube cabling routed next to power cabling

- I'm not sure what an empty pipe calibration is. Mags should fault on empty pipe. Most people want them to flag a fault for a partial full situation. Most modern mags do not even have a zero adjustment.

- lack of grounding rings in plastic pipe

- stray currents through medium

- pulsating flow
Besides the fact that you did not provide much information, the first thing that comes to mind that Demin water's conductivity is very low, and a mag meter may not be right flow meter to measure it. The second thing that comes to mind is improper grounding.

Here is an Emerson technical note on Mag Meter grounding:

William (Bill) L. Mostia, Jr. PE
ISA Fellow, FS Engineer (TUV Rheinland)
Winner of the 2018 ISA Raymond D. Molloy Award
Sr. Safety Consultant
SIS SILverstone, LLC
A magmeter will not work with demineralized water. It's not conductive enough, use a vortex shedding meter or a mass flow meter.