Factors affecting the measurement accuracy of electromagnetic flowmeter

　　Electromagnetic flowmeter is a widely used instrument that plays an important role in the measurement of wastewater and sewage. However, errors in model selection, installation and use may affect the accuracy of the results, resulting in deviations in measurement data or even damage to the flowmeter. Why measurement errors in electromagnetic flowmeters may affect the accuracy of measurement results, and how to solve these problems.

　　1. The liquid to be measured contains bubbles. This is a common phenomenon that the liquid to be measured contains bubbles. It is caused by external air suction and internal liquid dissolution. However, the electromagnetic flowmeter cannot distinguish between liquid and bubbles, so it will cause errors when calculating the measurement together. It is not recommended to install the electromagnetic flowmeter at a high point in the pipeline, so it is best to change the installation location. If the installation location is not easy to change, install a gas collector upstream of the electromagnetic flowmeter and exhaust it regularly.

　　2. The measured liquid is not full in the pipe. A partially filled pipe is an extreme case with bubbles. That is, the liquid in the pipe is not full, and a large number of bubbles are filled above the liquid in the pipe. If the liquid does not pass through the electrode, the measurement result will be greatly depreciated, which is wrong in engineering design. There are some requirements for the correct installation of electromagnetic flowmeters in different pipelines. Please keep this in mind, which will be helpful when installing to ensure the measurement accuracy of the electromagnetic flowmeter.

　　3. Short-circuit effect of conductive deposits The flow signal is short-circuited due to the gradual deposition of conductive material. This type of fault usually does not appear during commissioning, but after a period of operation. For the above problems, please disassemble the flow meter and clean the insulation layer that is mostly brown rust, which is caused by the precipitation of large amounts of iron oxides in the electrolyte. In any case where the operation is normal at the beginning but the flow rate decreases over time, you should analyze the possibility of this fault.

　　4. Liquid produces crystallization. In the selection of electromagnetic flowmeters, we often encounter media that are easy to crystallize. Although most materials can be measured normally at room temperature, and the conduit has the function of heat dissipation and heat preservation in addition to the sensor measuring tube in the flowmeter, when the ordinary fluid flows, the inner wall will form a solid layer by cooling through the measuring tube. Therefore, please choose an electromagnetic flowmeter with a smaller measuring tube. It is not recommended to choose an electromagnetic flowmeter with too complicated installation requirements.

　　5. Liquid conductivity exceeds the allowable range. The output impedance of the electrode depends on the conductivity of the measured liquid and the electrode. When the conductivity is lower than the lower limit, the electromagnetic flowmeter will not work properly or the numerical display will become slow. In view of the above problems, after the tantalum electrode is replaced normally, the electromagnetic flowmeter can work normally.

　　6. Spatial electromagnetic interference Generally, if the cable between the sensor and the converter is long and there is strong electromagnetic interference around, the cable may introduce interference signals to form common mode interference, resulting in display distortion, nonlinearity or large-scale shaking. In response to this situation, first, try to stay away from strong magnetic fields (such as nearby large motors, large transformers, and power cables). Then, shorten the length of the cable as much as possible and take shielding measures.

　　7. Asymmetric flow of the measured fluid Under normal circumstances, the current velocity of the fluid in the pipeline is required to be axisymmetrically distributed and the magnetic field is uniform. In fact, non-axisymmetric velocity distribution usually occurs, and when cyclotron current occurs, it will affect the output and then produce a large measurement error. Therefore, some measures should be taken to avoid this situation. First, there are enough straight pipe sections upstream to ensure that the current velocity is distributed in concentric circles. In addition, the inner diameter of the electromagnetic flowmeter should be the same as the inner diameter of the pipeline within a certain range upstream and downstream, otherwise the distribution of the current velocity will be uneven. If there is not enough straight pipe section upstream, a flow conditioner can be installed, but this can only partially make up for it.

　　8. The vibration at the installation site will affect the accuracy of the electromagnetic flowmeter. The electromagnetic flowmeter has strict requirements on the vibration at the installation site, so it must be installed in a place with less vibration, otherwise measurement errors will occur.