Several reasons for errors in electromagnetic flowmeters

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(1) The pipe is not fully filled with liquid. A situation of an unfilled pipe often occurs at direct discharge points where the fluid flows by gravity or where there is no back pressure after the flow meter. There are two scenarios in which the pipe is not filled with liquid: the liquid level is above the level of the measuring electrodes, or it is below that level. When the liquid level inside the tube is above the level of the electrodes, and if the straight sections before and after the tube are ideal, the electromagnetic flowmeter can generally provide stable measurements. However, the volume of liquid measured by the flowmeter includes the volume of gas present in the tube, which results in significant measurement errors. When the liquid level inside the tube is below the electrode surface, a phenomenon of overfilling occurs. The measurement values and output of the electromagnetic flowmeter are in a random state, constantly fluctuating or reaching full scale. (2) Solid phase present in the liquid: When the liquid contains solids such as powder, particles, or fibers, the possible faults include ； ①Slurry noise ； ②Contamination on the electrode surface ； ③A conductive deposition layer or an insulating deposition layer covers the electrode or liner ； ④The lining is worn or covered by deposits, reducing the flow cross-sectional area. (3) Electromagnetic flowmeters should be used with caution for liquids that are prone to crystallization. Some chemicals that easily crystallize can be measured normally at normal temperatures; since the pipes used for transporting these fluids are well insulated, crystallization does not occur when the insulation is in good condition. However, it is difficult to provide proper insulation for the measurement tube of an electromagnetic flowmeter, and as a result, the inner wall of this tube can accumulate a layer of solid material due to temperature drops as the fluid flows through it. (4) Problems caused by improper selection of electrode and grounding ring materials: Clicking and grounding ring failures related to improper matching with liquid contacts. Apart from corrosion issues, improper matching is mainly due to electrode surface effects. Surface effects include: ① Chemical reactions (formation of a passivation film on the surface, etc.) ； ②Electrochemistry and polarization phenomena (generation of potential) ； ③Catalytic action (formation of aerosols on the electrode surface, etc.). Ground rings also have these effects, but to a lesser extent. (5) Non-uniform conductivity: In the mixing process, it is common to inject a \"chemical addition\" liquid into the fluid, and this injection is usually carried out using a reciprocating pump. If the conductivity of the injected liquid differs from that of the main liquid, and the mixture has not yet been fully mixed, installing the electromagnetic flowmeter sensor downstream will cause fluctuations in the instrument’s output due to the sharp change in conductivity. However, if the conductivity of the liquid remains above the threshold and changes gradually, it will not affect the proper functioning of the electromagnetic flowmeter. This phenomenon occurs frequently in the coagulant addition process for raw water in water treatment plants.