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By Dan Hebert, Senior Technical Editor
The electromagnetic flowmeter, commonly known as the magmeter, gets its name from the magnetic field generated within the flow tube that produces a signal proportional to flow. Current is applied to a coil in the magmeter to generate a magnetic excitation field in the pipe. When fluid flows through the pipe, an electromotive force that is proportional to the flow rate is produced according to Faraday's law of electromagnetic induction. This force is detected by the electrodes, and the resulting value is converted to flow rate.
An electromagnet, excited by either AC power or pulsed DC power, is needed to generate the magnetic field. Although both types of electromagnets create a magnetic field, they each have their benefits and weaknesses.
With AC-powered magnetic flowmeters, line frequency is used to generate the magnetic field, typically 60Hz. These meters work well in excessive slurry noise and low-conductivity fluid applications.
"Most slurry noise is at low frequency with about 70% of the noise occurring below 15 Hz," says Dan Peterson, the flow marketing manager at Yokogawa Corp. of America (www.us.yokogawa.com/us). Using relatively high-frequency 60Hz excitation allows the magmeter to measure flow accurately in applications with a high degree of low-frequency noise, such as slurries, but creates other issues.
"With high-excitation frequency alone, differential noise generated by a temporal change in the magnetic field strength doesn't attenuate sufficiently in the sampling interval, so it's difficult to stabilize the zero point, and the accuracy is low," explains Peterson. To deal with these issues, a pulsed DC excitation magmeter can be used with an average excitation frequency typically between 3Hz to 6Hz.
Unfortunately, in an application where slurry noise is superimposed on the true flow signal, this 3-Hz to 6-Hz sampling rate falls in the maximum slurry noise range, which then creates a significantly noisy and, therefore, inaccurate output.
A solution can be found by employing a dual frequency magmeter. "Our dual-frequency magmeters combine the benefits of AC and DC excitation, using both high 75-Hz frequency and low 6.25-Hz frequency excitation. Dual-frequency excitation superimposes high frequencies on low frequencies, and utilizes the advantages, while eliminating the disadvantages of the high- and low-frequency excitation methods," adds Peterson.
"They exhibit immunity to elevated slurry noise and the fast response of high-frequency excitation and the high zero stability of low- frequency excitation. The meters are virtually immune to slurry noise, have a 0.1-second response time due to the high sampling rate, measure with an accuracy of 0.35% of flow rate, have high immunity to pulsating flows, and exhibit no zero drift," claims Peterson.
A typical application for dual-frequency magmeters is measurement of the white and black liquor found in pulp and paper processes, a particularly difficult slurry application due to the presence of wood chips and other pulp.
"Conventional style magmeters have difficulties measuring pulp stock due to problems related to slurry noise, which can produce an unstable output with resulting loss of accuracy," notes Peterson.
"High-frequency AC magmeters do a good job of reducing slurry noise, but have problems with zero stability, and in turn poor accuracy of about 1% of span. DC magmeters can't be effectively used in this type of application due to the presence of low- frequency slurry noise," points out Peterson.
"In this and similar applications, our dual-frequency magmeters combine the best of high- and low-frequency meters. Dual-frequency excitation combined with the advanced design of the flow tube ensures accurate and reliable measurement of pulp stock and other high-slurry fluids," concludes Peterson.