How does an electromagnetic flowmeter measure?

Thread Content

Hi, I’m the Industrial Control Assistant. Today, I’ll explain how electromagnetic flowmeters work to measure flow rates. Electromagnetic flowmeters have a large measurement range, typically 20:1 to 50:1, offering a wide selectable flow range ； The diameter range of electromagnetic flowmeters is wider than that of other types of flow meters, ranging from a few millimeters to 3 meters ； It can measure flow in both forward and reverse directions, as well as pulsating flow, provided that the pulsation frequency is much lower than the excitation frequency ； The instrument output is essentially linear ； It is easy to select the material types for fluid contact parts, and it can be used with corrosive fluids among other advantages. Since electromagnetic flowmeters are far more likely to measure fluids containing suspended solids or contaminants than other flowmeters, the probability of failures caused by deposits forming on their inner walls is relatively higher. 　If the conductivity of the coating layer is similar to that of the liquid, the instrument can still output signals properly; it is only the change in the flow area that causes a latent fault resulting in measurement errors ； If it is a high-conductivity adhesion layer, the electromotive force between the electrodes will be short-circuited ； If it is an insulating adhesive layer, the electrode surface becomes insulated, breaking the measurement circuit. Both of the latter phenomena will prevent the instrument from functioning. Requirements for measuring with electromagnetic flowmeters: 1. The accuracy class and functions – Select the accuracy class of the instrument based on the measurement requirements and the application environment, in order to achieve cost-effectiveness. For applications such as trade settlement, product transfer, and energy measurement, it is advisable to choose instruments with a higher accuracy grade, such as 1.0 grade, 0.5 grade, or even higher grades. In cases of process control, different accuracy grades should be selected based on the control requirements. For situations where only the process flow needs to be monitored without precise control or measurement, lower accuracy grades can be used, such as 1.5 grade, 2.5 grade, or even 4.0 grade; in these cases, inexpensive insert-type electromagnetic flowmeters can be employed. 2. Measurement of medium flow velocity, instrument range, and diameter: When measuring ordinary media, the full-scale flow rate of electromagnetic flowmeters can be selected within a range of 0.5–12 m/s for the medium flow velocity, providing a fairly wide range of options. The selection of instrument specifications (diameter) does not necessarily have to match those of the process pipeline; it should be determined based on whether the flow rate to be measured falls within the acceptable velocity range. That is, when the flow velocity in the pipeline is too low to meet the requirements of the flow meter, or when the measurement accuracy cannot be ensured at such a velocity, it is necessary to reduce the diameter of the instrument in order to increase the flow velocity inside the pipe and obtain satisfactory measurement results. 3. Try to avoid ferromagnetic objects and devices with strong electromagnetic fields, so as to prevent the magnetic field from affecting the operating magnetic field of the sensor and the flow signal. 4. It should be installed in a dry and well-ventilated area, away from direct sunlight and rain. The ambient temperature should range from -20 to +60°C, with a relative humidity of less than 85%. 5. There should be sufficient space around the flow meter to facilitate inspection and maintenance. Only periodic visual inspections of the instrument are required: check the environment surrounding it, remove dust and dirt, ensure that no water or other substances get in, verify that the wiring is in good condition, and check whether there are any newly installed devices that generate strong electromagnetic fields nearby or new wires running across the instrument. If the medium being measured tends to contaminate the electrodes or precipitate and scale inside the measurement tube, cleaning and descaling should be carried out regularly. Well, that’s all for this little piece of knowledge. It will be updated regularly, so feel free to follow!