是利用法拉第电磁感应定律制成的一种测量导电液体体积流量的仪表。 The electromagnetic flowmeter is a meter for measuring the volumetric flow rate of conductive liquid, which is made by using Faraday's electromagnetic induction law. In the early 1950s, EMF realized industrial application. In recent years, EMF production worldwide has accounted for about 5% to 6.5% of the number of industrial flow meters. Since the 1970s, a keyed low-frequency rectangular wave excitation method has gradually replaced the earlier-used industrial frequency AC excitation method. The performance of the instrument has been greatly improved and it has been more widely used. The basic principle is the Faraday's law of electromagnetic induction, that is, when a conductor cuts magnetic lines of force in a magnetic field, an induced electromotive force is generated at its two ends. The conductive liquid flows in a non-magnetic measuring tube perpendicular to the magnetic field, and an induced potential proportional to the flow rate is generated in a direction perpendicular to the flow direction. The direction of the electromotive force is according to the "Fleming right-hand rule". －－－- Induced electromotive force, that is, the flow signal, V;
k ------- coefficient;
B----magnetic induction, T;
D --- --- measuring tube inner diameter, m;
----- average velocity, m / s.
Let the volume flow of the liquid be, where K is the meter constant, and K = 4 KB / πD.
The electromagnetic flowmeter is composed of a flow sensor and a converter. The sensor has a typical structure. An excitation coil is installed on the upper and lower sides of the measuring tube. A magnetic field is generated after the excitation current passes through the measuring tube. A pair of electrodes are mounted on the inner wall of the measuring tube to contact the liquid. The excitation current is provided by the converter.
的测量通道是一段无阻流检测件的光滑直管，因不易阻塞适用于测量含有固体颗粒或纤维的液固二相流体，如纸浆、煤水浆、矿浆、泥浆和污水等。 The measurement channel of the electromagnetic flowmeter is a smooth straight tube without a flow-blocking detection element. Because it is not easy to block, it is suitable for measuring liquid-solid two-phase fluids containing solid particles or fibers, such as pulp, coal water slurry, mineral slurry, mud and sewage. The electromagnetic flowmeter does not generate pressure loss caused by the detection flow. The resistance of the meter is only the resistance along the pipeline of the same length, which has a significant energy saving effect. It is most suitable for large-diameter water supply pipelines that require low resistance loss. The volume flow measured by the electromagnetic flowmeter is actually not significantly affected by changes in fluid density, viscosity, temperature, pressure, and conductivity (as long as it is above a certain threshold). Compared with most other flow meters, the front straight pipe section has lower requirements. The electromagnetic flowmeter has a large measurement range, usually 20: 1 to 50: 1, and a wide range of optional flow rates. Full-scale value liquid velocity can be selected within 0.5 ～ 10m / s. Some types of instruments can be expanded and reduced in accordance with the needs of the site (for example, a 4-digit potentiometer is used to set the meter constant). It does not need to be removed for offline actual flow calibration. The caliber range of electromagnetic flowmeters is wider than other types of flowmeters, from a few millimeters to 3m. It can measure forward and reverse bidirectional flow, and it can also measure pulsating flow, as long as the pulsating frequency is much lower than the excitation frequency. The meter output is linear in nature. Easy selection of fluid contact materials for corrosive fluids.
不能测量电导率很低的液体，如石油制品和有机溶剂等。 Electromagnetic flow meters cannot measure liquids with very low conductivity, such as petroleum products and organic solvents. It is not possible to measure gases, vapors and liquids with large bubbles. Due to the limitation of lining materials and electrical insulation materials, general-purpose electromagnetic flowmeters cannot be used for higher temperature liquids; some types of instruments are used for liquids below room temperature, and the insulation is damaged due to condensation (or frost) outside the measuring tube.