Understanding electromagnetic flowmeters
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Electromagnetic flow meters are the Swiss army knife in flow measurement. Quick and easy to incorporate in the design. Always right. For all the familiar reasons. But not many flow engineers know exactly how the thing works. And that is precisely what would help you make designs that measure better and cause fewer problems. This article opens the hood and pulls apart the engine block.
General working principle
Magmeters generate a magnetic field with two coils around the measuring tube. The moment a conductor moves in that field, a voltage is created and that can be measured. Exactly the same principle as a dynamo. A dynamo generates that field with permanent magnets.
So could you make a magmeter with a permanent magnet? Yes and no. It would work. Even with a permanent magnet around the measuring tube, a potential difference arises over the electrodes as soon as there is flow. But the generated potential difference is incredibly small and also very sensitive to interference.
This is why magmeters were based on electromagnetism from the very beginning. The magnetic field can be made much stronger this way. The first magmeters were equipped with AC field exit. That was back in the 1960s. Simply because they had nothing else. Just put the mains voltage on the coils. Just like in a transformer. Because electronics still consisted of electron tubes, so other waveforms were difficult.
And that worked fine. With the well-known advantages of this principle: full bore, no moving parts so insensitive to interference, insensitive to maintenance and long life and therefore a total cost of ownership that was excellent compared to what was on the market at the time.
Yet there was still much room for improvement. The signal/noise ratio was poor. Accuracy and response time to flow variations could be improved by variations in the conductivity of the medium (due to the flow profile for example).
Illustration: electromagnetic flow meter working principle
At magmeters, we're always looking for:
Illustration: AC magmeter principle
AC magmeter principle
With an AC flowmeter one uses directly the mains voltage to generate the magnetic field. The advantage is that you get a very strong magnetic field. But the disadvantage is that your zero crossing is not very stable, because it varies with all the pollution on the mains. Even if you filter out that pollution, phase shift is a problem.
Illustration: Pulsed DC magmeter principle
DC Magnetometer principle
The zero crossing must be stable. If you use electronics to make a wave out of DC, then you have everything under control. Manufacturers call that "switched DC". But as you can see in the graph, a magnetic field is not a perfect block shape, which also causes inaccuracy.
Illustration: Pulsed AC magmeter principle
Pulsed alternating current magnetometer principe
Manufacturer Mecon has come up with the following solution. By extending the zero crossing, the zero stability is enormous. And by using the AC waveform, the wave is symmetrical and the pulse is very strong. Which gives a very good signal to noise ratio.
Using a reference coil
And now we come to an interesting part. As we already saw with the DC meter, the generated electromagnetic field is not exactly of the same shape as the voltage you put on it. This has to do with the electrical properties of coils, but we won't go into that here. So the fact is that you don't know exactly which magnetic field you've generated. And you also don't know exactly what you can expect as a measuring voltage at a constant flow. The manufacturer Mecon has thought of a solution: by building in extra coils they measure the generated magnetic field. This way the measuring signal on the electrodes can be corrected for deviations in the field. It's a great solution for noise caused by solid particles in the liquid. So especially in dredging and sand mining applications you will benefit from this.
Illustration: the reference coil that Mecon came up with.
Photo: 34mm thick PU lining in a DN600 electromagnetic flow meter.
The lining of magmeters
The conductive medium must be electrically separated from the tube. There would be no field distortion caused by the flow. And then you measure nothing.
You can see this compulsory lining as a disadvantage, but it has also proven to be a huge advantage: the magmeter is uniquely capable of measuring slurries. Slurries and especially dredgings let all components wear out. In this case, you are happy with a full flow meter, but if the lining is also made of ceramic, you have a winning team.
Other typical lining-application combinations you often see:
Ceramic in polyurethane
HardOx
Tiles
Slurries.
Dredging and sand extraction.
The usability of electromagnetic flowmeters
Because the steel outer jacket of the tube is not affected, this type of flowmeter can be made for any pressure. Up to 1000 bar. Or more. If you know how to achieve it with your system.
The temperature is limited by the type of lining. You could invent all kinds of things, but in practice they go up to 180 degrees Celcius.
Downloads Electromagnetic flowmeters
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