Factors causing instability in electromagnetic flowmeters!
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Our facility has several electromagnetic flowmeters. In production, problems with inaccurate measurement often occur, and despite trying various solutions, the issue has not been completely resolved! The medium in question is a mixture of water and corn flour, in a ratio of around 100:6. These malfunctions occur irregularly – sometimes the accuracy is high and at other times it’s low. During production, sterilization is sometimes required, with temperatures reaching around 180 degrees. This could potentially be related to such conditions; however, when malfunctions occur during this period, it can take a whole day for the situation to improve, while at other times it resolves on its own. We have replaced the converters, and the sensors are functioning properly. Since this problem doesn’t occur in all of the flowmeters, it’s unlikely that it’s due to the operating environment. The flowmeters worked fine at first, but started having issues after some time, and the grounding is proper. I’m not sure what other factors might be involved. Please offer some suggestions! Thank you! Another possibility is that there might be a quality issue with the electromagnetic flowmeters themselves!! Last edited by yame1996 on 2007-12-17 08:15]
At the core of the quality issue, it is likely caused by the measurement medium wearing down the electrode or by deposits on the electrode. Is your signal unstable, noisy, or completely off from the actual value?
After removing the sensor for inspection, it was found that the electrodes showed no signs of wear or scaling. The signal was unstable and significantly deviated from the actual values; most of the time it was much lower than the correct value
I’m not sure whether your flow meter is two-wire or four-wire type. Two-wire meters have weaker interference resistance, and unstable flow rates are related to the conductivity of the fluid. Foams generated by the fluid, proper grounding, and scaling on the internal electrodes can all affect the flow rate. Also, check if there are any high-power devices operating in the vicinity, as they may cause electromagnetic interference.
This is also a reason, and it’s very important!
They are four-wire models; both units are like this. We spent a long time trying to figure out what was going on, and last time we just called the manufacturer’s technicians. They repaired a converter, and then it stopped working again. It’s strange – after those technicians left, the problem reappeared after about a week. Both of these units are products from the same manufacturer
It is related to the conductivity of the medium; foam generated by the fluid, grounding issues, scaling on the internal electrodes, and also the fact that the medium does not fill the pipes properly can all affect flow rate. It is recommended to remove one of the island meters for testing. If the meter was just purchased, it is likely not a quality issue but rather improper use.
I think many reasons for the issue you mentioned have already been discussed; I’d like to add two points: First, is there a high current flowing through your medium? II. Is there a clear distinction between system grounding and electrical grounding, and is single-point grounding achieved?
There is no current; I checked specifically the issue of single-point grounding!
Are there any branch pipes, control valves in front of the electromagnetic flowmeter, or large motors or similar devices nearby? These can be used to identify potential sources of interference through testing.!lol
If it’s not allowed, it’s recommended to take it to a metrology institute for verification first
Indeed, electromagnetic flowmeters suffer from large measurement errors in areas with strong electromagnetic interference; therefore, it is recommended not to use them in such situations
First, take the process medium and let it stand for 24 hours to observe whether any precipitation occurs, as well as to check if there are any deposits on the electrodes. Observe through a sight glass to check for bubbles in the fluid. Miscellaneous items. Build a test bench to test the flow meter using water. Observe its condition.
Are there insulation measures in place for the pipelines? What is the temperature?
Errors caused by non-axisymmetric flow: When the flow velocity of the fluid within a pipe is axially symmetric and it is in a uniform magnetic field, the electromotive force generated at the electrodes of the flow meter is independent of the velocity distribution of the fluid, and is proportional to the average flow velocity. In the case of a non-axisymmetric velocity distribution, that is, when each fluid particle is at a different geometric position relative to the electrodes, the induced electromotive force generated at the electrodes also varies; the closer a particle is to an electrode, the greater the induced electromotive force generated by that particle with higher velocity. Therefore, it is necessary to ensure that the fluid flow velocity is axially symmetric. If the flow velocity within the pipe has a non-axisymmetric distribution, it will cause errors. Therefore, when selecting an electromagnetic flowmeter, it is necessary to ensure the requirements for straight pipe sections as much as possible in order to reduce the errors caused by them. Regarding the issue of fluid conductivity, the lower limit for the measured fluid conductivity is typically 5µS/cm to 10µS/cm. Therefore, if the diameter of the electrode is 1 cm, the output impedance of the electrode will be 1/Kd, which ranges from 100kΩ to 200kΩ. To keep the impact of this output impedance below 0.1%, the input impedance of the converter should be around 200MΩ. The impact of electrode liner deposits: When measuring fluids with deposited precipitates, the electrode surface becomes contaminated, which often leads to variations in the zero point. These aspects should be taken into consideration
Is the electromagnetic flowmeter equipped with normal excitation, or dual-frequency or AC excitation? Corn flour is non-conductive, and the same situation occurs in paper mills where the pulp contains non-conductive fibers; in such cases, using an electromagnetic flowmeter with ordinary excitation will result in fluctuating readings. There are too many factors at play, making it difficult to identify them.
Yes, it’s very troublesome when non-conductive materials are mixed together…… Our company has encountered such problems before…… The medium being measured is a mixture of organic substances and water; technically speaking, it contains water, and the proportion of water remains relatively stable. My predecessor chose an EMERSON electromagnetic flowmeter for this purpose. Just a few days after it was installed, production processes became unstable. After various checks, the flowmeter was sent back to the manufacturer for re-inspection, and the flowmeter itself was found to be fine…… The problem lay in the instability of the medium itself. Once, a sample was taken for testing, and its conductivity was extremely low, not meeting the requirements of an electromagnetic flowmeter at all…… So sometimes normal flow rates could be measured, while other times no flow rate was detected at all.
Flow meters generally don’t have problems. Most issues arise from improper use...
The issue lies in the selection process – there was no in-depth consideration of the process medium, and that’s why electromagnetic methods cannot be used for measurement.
I think there might be something in the medium that hinders the use of electromagnetic flowmeters. In our company, electromagnetic flowmeters have worked quite stably, with basically no problems occurring. The issues mentioned above are worth paying attention to; other aspects also need to be checked, such as whether the output of the electromagnetic flowmeter matches what is displayed by the secondary instruments. By conducting more checks, it should be possible to identify the cause of the problems. It’s also possible to contact the manufacturer to work together in finding the root cause of the issues.