Magnetic Flowmeter Calibration Verification and Diagnostics

Magnetic flowmeters (magmeters) are often used in water and wastewater applications due to the conductivity properties of the water. In a Water & Wastewater Asia magazine article, Better flowmeter management with EDDL, Emerson’s Jonas Berge describes how intelligent magmeters provide:

…accurate and reliable measurement, more reliable empty pipe detection, and low maintenance costs for calibration…

For those not familiar with how a magmeter measures flow, Jonas describes it as being [hyperlink added]:

…based on the principle of Faraday’s Law: A conductor moving through a magnetic field induces a voltage proportional to the velocity of the conductor. In the case of a magnetic flowmeter, two coils located at the top and bottom of the flowtube are driven by the transmitter to generate a pulsed magnetic field of consistent strength. The induced voltage is proportional to the velocity of the conductive liquid passing through the flowtube.

Over time, the calibration of these meters can drift due to temperature, vibration, and electrode coating to name a few causes. The traditional way to re-calibrate the device required:

…the flowmeter to be powered down, and the wiring had to be disconnected before the simulator could be connected. Then electronics needed time to warm-up. The configuration of the transmitter had to be changed for the check, and then returned to operations settings. This process was disruptive and time consuming.

Jonas highlights a second traditional method of calibration using a:

…mobile prover rig and skilled technicians. This procedure is labour intensive and disruptive. The outside contract work is costly, increasing dramatically with line size.

Smart meter verification technology makes it possible to check the calibration while the magnetic flowmeter remains in place. Jonas shares how this technology works:

The current characteristics for magnetic field strength and electrode resistance are compared to the baseline parameter values when the magnetic flowmeter was last fully calibrated. The baseline parameters for coil resistance are captured at installation. Test criteria can be set to the levels necessary to meet compliance requirements for the application. Deviation between the current values and the baseline values that exceed the test criteria indicate full calibration will be needed.

Smart meter verification does not eliminate the need for calibration, but does identify when it needs to occur. Instead of doing these calibration procedures on a time schedule, it alerts operations and maintenance when calibration needs to be performed based on current performance versus baseline performance.

Jonas notes how the combination of digital bus technologies such as Foundation fieldbus, HART, and WirelessHART, combined with Electronic Device Description Language (EDDL) provides a way to perform the smart meter verification process remotely from the control room or maintenance shop. He notes:

EDDL wizards created by the flowmeter manufacturer guide the technician step-by-step; no specialised training is required. Percent completion is displayed throughout the procedure so technicians know how much longer they need to wait. The meter verification wizard makes this a simple pass or fail test.

Give the article a read if you want to find out more about how these diagnostics help spot and alert the maintenance team about electrical noise, grounding issues, empty pipe detection, and issues with the meter and surrounding process.

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