Several reasons for electromagnetic flowmeter errors

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Several reasons for electromagnetic flowmeter errors [Copy link]

        (1) Problems caused by improper selection of materials for electrodes and grounding rings. Electromagnetic flowmeters that fail due to material mismatch with the measured medium include electrodes and grounding rings. Improper matching, in addition to corrosion resistance, can also cause surface effects on electrodes. Surface effects include: ① chemical reactions (formation of blunt film on the surface, etc.); ② electrochemical and polarization phenomena (generation of electric potential); ③ catalytic effects (generation of aerosol on the electrode surface, etc.). Grounding rings also have these effects, but the degree of impact is smaller. (2) Liquids contain solids such as powder, particles or fibers. Possible faults include: ① slurry noise; ② electrode surface contamination; ③ conductive deposits or insulation, deposits covering the electrode or lining; ④ linings are worn or covered by deposits, reducing the flow area. (3) Liquids that may crystallize, electromagnetic flowmeters should be used with caution. Some chemical materials that are prone to crystallization can be measured normally under normal temperature conditions. Since the conduits that convey the fluid have good heat insulation, they will not crystallize during the insulation work. However, it is difficult to implement heat insulation for the measuring tube of the electromagnetic flow sensor. Therefore, when the fluid flows through the measuring tube, it is easy to cause a layer of solid to form on the inner wall due to cooling. Since the measurement of flowmeters based on other principles also has the problem of crystallization, in the absence of other better methods, a "ring-shaped" electromagnetic flow sensor with a very short measuring tube length can be selected, and the heat insulation of the upstream pipeline of the flowmeter can be strengthened. In terms of pipeline connection methods, the flow sensor should be easy to disassemble and assemble, so that it can be easily removed for maintenance once crystallization occurs. (4) The liquid in the pipe is not full. Due to insufficient back pressure or poor installation position of the flow sensor, the liquid in the measuring tube cannot be filled. The fault phenomenon has different manifestations depending on the degree of infilling and flow conditions. If a small amount of gas flows in a water pipe in a stratified or wavy flow, the fault phenomenon is manifested as an increase in error, that is, the flow measurement value does not match the actual value; if the flow is a bubble flow or a plug flow, in addition to the measurement value not matching the actual value, the output will also shake due to the gas phase momentarily covering the electrode surface; if the gas phase part of the flow cross-sectional area in the horizontal pipe stratified flow increases, that is, the degree of liquid underfilling the pipe increases, the output will also shake. If the liquid underfilling the pipe is serious, so that the liquid level is below the electrode, the output will be overfilled. (5) Problems caused by liquid conductivity exceeding the allowable range If the liquid conductivity is close to the lower limit, shaking may also occur. Because the lower limit value specified in the manufacturer's instrument specification is the lowest value that can be measured under various good conditions, and the actual conditions cannot be ideal, it is often encountered that low-degree distilled water or deionized water, whose conductivity is close to the lower limit value of 5 specified in the electromagnetic flowmeter specification, has output shaking when used. It is generally believed that the lower limit value of conductivity that can be stably measured is 1 to 2 orders of magnitude higher.

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Problems caused by liquid conductivity exceeding the allowable range If the liquid conductivity is close to the lower limit, shaking may also occur. This is because the lower limit specified in the manufacturer's instrument specification is the lowest value that can be measured under various good conditions of use, and the actual conditions may not be ideal.