Brush Up on Mag-Meter Basics
Jane Alexander | April 23, 2019
Mag meters (electromagnetic flow meters) offer a number of benefits in fluid-handling applications.
Among other things, they’re obstruction-less, cost-effective for aggressive chemicals and slurries, and highly accurate in measuring volumetric flow.
Mag meters work by way of an inline sensor that measures induced voltage generated by fluid as it flows through a pipe. The transmitter takes voltage generated by the sensor, converts it into a flow measurement, and transmits that measurement to a control system.
Experts with Emerson Flow Solutions, St. Louis, compiled the following list of some best practices for keeping your mag meters functioning within their original parameters.
• Select magnetic flow-sensor materials that are chemically compatible with all expected process fluids and cleaning/flushing agents. Beware of steam-out conditions that require a meter rated for higher temperatures and full vacuum.
• Orient the mag meter properly to keep it full and avoid two-phase conditions. Direct flow upward through the meter, if possible.
• Use recommended upstream and downstream pipe lengths to ensure proper fluid conditioning for best flow-measurement results. Consult the manufacturer if space requirements won’t allow the minimum diameters.
• If insulation is used, don’t cover the coil housing or electronics as these components can overheat and fail.
• Size mag meters properly to achieve best flow-measurement performance across the full process flow range. Most will perform well when selected to match the process-pipe size. But, for processes that operate at low flowing velocities, adding a reducer to accommodate a smaller magnetic sensor will improve measurement accuracy by increasing the fluid velocity through the sensor.
• Isolate wiring from the transmitter to the magnetic sensors in its own conduit. Do not run mag-meter wiring in a conduit shared with any other equipment wiring. Any electrical interference can cause measurement offsets.
• Avoid stripping too much insulation from mag-meter wiring. Any exposed wires that aren’t covered by a braided shield can allow extraneous electronic interference into the measurement circuits.
• Ensure that mag-meter electronics are grounded to the process fluid (in addition to earth ground) by using ground straps, ground rings, lining protectors, or a dedicated grounding electrode.
• Verify that calibration constants from the flow sensor are reflected properly in the electronics. For sensors capable of driving the magnetic coils at multiple frequencies, there should be a unique calibration number for each coil drive frequency.
• Perform a zero calibration if using an AC-powered mag meter, or when operating at high-coil-drive frequencies on pulsed-DC meters.
• Determine a field baseline for the circuit-health-monitoring-diagnostic test values (meter verification) if required by the manufacturer. EP
For more information on fluid-handling/flow-control topics, visit emersonflowsolutions.com.