The electromagnetic flowmeter gives inaccurate readings, with the displayed values being on the low side.

Generally not; it’s better to look for other reasons! Focus mainly on the manufacturing process and valves

Please have the original poster carefully check the grounding situation of the table :) There is a possibility of scaling; it can be soaked in a weak acid

This post was last edited by denghl on 2012-3-30 at 13:02. This is a normal phenomenon; we have one machine that behaves in this way. Over time, as the scale builds up, the reading gradually becomes lower. There’s no good way; one can only wait for an opportunity to stop and clean it up. I haven’t figured out why the value is so low (only 20% of the true measured value). According to E=Blv, none of the three variables are likely to change much. The scaling substances are also conductive, and a scaling thickness of 1 mm is negligible compared to a pipe diameter of 200 mm. Who can solve this puzzle? Could it be that scaling has a certain “shielding” effect on the magnetic field?

The grounding is fine; we clean off the dirt every time we park, as it’s brackish water, which causes severe scaling.

If an electromagnetic flowmeter was functioning normally before, but starts to give lower measurement values after operating for some time, the issues to consider are generally grounding problems and problems with the electrodes. Additionally, it is also possible to compare with other instruments to see if there have been any changes in the actual operating conditions during production.

This post was last edited by huio1983 on 2012-3-31 at 12:26. I don’t think so; electromagnetic flowmeters generate a magnetic field, and the media being measured usually contain water. When water is magnetized, a series of physical and chemical changes occur in its properties – the hydrogen bond angle changes from 105° to 103°, and the water transforms from large molecular clusters of 13–18 molecules into smaller clusters of 5–6 molecules. The permeability, solubility, and surface tension of water increase; moreover, CaCO3 and MgCO3 in water decompose during the cooking process to form softer compounds such as Ca(HCO3)2 and Mg(HCO3)2, which do not tend to accumulate on the walls. In many cases, the property of magnetized water is utilized to achieve descaling effects. Brine should be treated separately. Isn’t the pipeline used for electromagnetic flowmeters as smooth as the inner walls of ordinary pipes? Is scaling only present there, or does it occur in other pipelines as well?

This post was last edited by huio1983 on 2012-3-31 at 16:34. The original poster can analyze the situation using the information below; this is taken from a paper. (3) The impact of electrode scaling and fouling layers. When measuring non-clean fluids such as pulp and sewage, the electrode surface is prone to contamination, which causes variations in the zero point. However, it is difficult to conduct a quantitative analysis of the relationship between these zero-point variations and the degree of electrode contamination. Based on experience, the smaller the diameter of the electrode, the less affected it will be. It is important to keep the electrodes clean during use in order to prevent fouling. The error Δ caused by deposits adhering to the lining can be calculated using equation (3) if the thickness of the deposits is constant: Δe = [1 – 2/w/kf] + [(1 – kw/kf) * (1 – 2t/D)]². In equation (3), kw and kf represent the conductivities of the fouling substances and the fluid being measured, respectively； t is the thickness of the attachment ； D is the diameter. If k in equation (3) is… When the sum and k are equal, the error is zero. The above formula still holds when the conductivity of the deposits is low, but it increases the output impedance of the electrode, thus imposing limitations; this is the case when insulating deposits are immersed in a fluid. On the other hand, with attachments such as metal powders, the high conductivity of these deposits causes the induced potential to be short-circuited, resulting in a lower electrode output and a negative deviation. When measuring fluids containing deposited substances, measurement errors can be reduced by properly selecting the lining material of the sensor. In addition to using linings such as glass or polytetrafluoroethylene that are resistant to deposit formation, it is also necessary to increase the flow rate of the fluid; rapid flow helps to scour and clean the electrodes, thereby reducing errors

Obviously, scaling is more likely to cause a smaller size. Due to dirty media, dirt gradually accumulates on the inner walls of the pipes over time, and as a result, the flow measurement values also become progressively lower (the dirt acts like an additional layer of material attached to the pipe walls and probes, affecting the magnetic field). For example, circulating water used in general industrial applications may experience a 5% reduction in capacity due to scaling after one year of use, and in severe cases, this reduction can be as much as 15%. If an electromagnetic flowmeter is installed vertically above a valve and not used for an extended period of time, it is likely to accumulate more impurities, to the point where flow measurement becomes impossible. For tubular electromagnetic flowmeters, it is generally recommended to clean them every 1–2 years; for insert-type electromagnetic flowmeters, cleaning should be done at shorter intervals depending on the circumstances.

It could be due to scaling; depending on the process conditions, it might also be caused by damage to the flow meter’s lining

Another possibility is that there may be bubbles in the medium or that the pipe is not always full, which is not a problem with the electromagnetic flowmeter. It has no direct relation to scaling.