The working principle of electromagnetic flow meter

Abstract: The information on the working principle of electromagnetic flowmeters is provided by excellent flowmeter and flowmeter manufacturers and quotation manufacturers. Electromagnetic flowmeter (Eletromagnetic Flowmeters, referred to as EMF) is a new type of flow measuring instrument developed rapidly with the development of electronic technology in the 1950s and 1960s. The electromagnetic flowmeter is made according to Faraday's law of electromagnetic induction, electromagnetic flow. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following is the details of the working principle of the electromagnetic flowmeter. Electromagnetic flowmeter (Eletromagnetic Flowmeters, referred to as EMF) is a new type of flow measuring instrument developed rapidly with the development of electronic technology in the 1950s and 1960s. The electromagnetic flowmeter is made according to Faraday's law of electromagnetic induction, and the electromagnetic flowmeter is used to measure the volume flow of conductive liquids. Due to its unique advantages, electromagnetic flowmeters have been widely used in the flow measurement of various conductive liquids in industrial processes, such as various acids, alkalis, salts and other corrosive media; electromagnetic flowmeters for various slurry flow measurement, forming unique application areas. Structurally, an electromagnetic flowmeter consists of an electromagnetic flow sensor and a converter. The sensor is installed on the industrial process pipeline. Its function is to linearly transform the liquid volume flow value flowing into the pipeline into an induced potential signal, and send this signal to the converter through the transmission line. The converter is installed not too far from the sensor, it amplifies the flow signal sent by the sensor, and converts it into a standard electrical signal output proportional to the flow signal for display, accumulation and adjustment control. The basic principle of electromagnetic flowmeter (1) Measurement principle According to Faraday's law of electromagnetic induction, when a conductor moves in a magnetic field to cut the magnetic field lines, an induced potential e is generated at both ends of the conductor, and its direction is determined by the right-hand rule. It is proportional to the magnetic induction intensity B of the magnetic field, the length L of the conductor in the magnetic field and the moving speed u of the conductor. If B, L and u are perpendicular to each other, then e=Blu (3-35) is similar to this. In a uniform magnetic field with a magnetic induction intensity of B, a non-magnetic pipe with an inner diameter of D is placed perpendicular to the direction of the magnetic field. When the conductive liquid flows in the pipe with a flow velocity u, the conductive fluid will cut the magnetic field lines. If a pair of electrodes is installed on both ends of the pipe section perpendicular to the diameter of the magnetic field (Figure 3—17) It can be proved that as long as the flow velocity distribution in the pipeline is axisymmetric distribution, an induced electromotive force is also generated between the two electrodes: e = BD (3-36) where, is the average flow velocity on the pipeline section. From this, the volume flow of the pipeline can be obtained as: qv=πDUˉ= (3-37) It can be seen from the above formula that the volume flow qv has a linear relationship with the induced electromotive force e and the inner diameter D of the measuring tube, and is inversely proportional to the magnetic induction intensity B of the magnetic field, and has nothing to do with other physical parameters. This is the measurement principle of electromagnetic flowmeter. It should be noted that, to make the formula (3—37) Strictly established, the measurement conditions must meet the following assumptions: ① The magnetic field is a constant magnetic field with uniform distribution; ② The flow velocity of the fluid to be measured is axisymmetrically distributed; ③ The measured liquid is non-magnetic; ④ The conductivity of the measured liquid is uniform and each Homotropic. Figure 3-17 The principle diagram of electromagnetic flowmeter 1-magnetic pole; 2-electrode; 3-pipeline (2) Excitation mode Excitation mode is the way to generate magnetic field. From the foregoing, it can be seen that in order to make the formula (3—37) Strictly established, the first condition that must be satisfied is to have a uniform and constant magnetic field. To this end, it is necessary to choose a suitable excitation method. At present, there are generally three excitation methods, namely DC excitation, AC excitation and low frequency square wave excitation. They are introduced separately. 1. DC excitation The DC excitation method uses direct current to generate a magnetic field or uses a permanent magnet, which can generate a constant and uniform magnetic field. The biggest advantage of this DC excitation transmitter is that it is little affected by the interference of the AC electromagnetic field, so the influence of the self-inductance phenomenon in the liquid can be ignored. However, the use of a DC magnetic field is easy to polarize the electrolyte liquid passing through the measuring pipe, that is, the electrolyte is electrolyzed in the electric field to generate positive and negative ions. Under the action of the electric field force, the negative ions run to the positive pole, and the positive ions run to the negative pole. Figure 3—18 shown. In this way, the positive and negative electrodes will be surrounded by ions of opposite polarity respectively, which will seriously affect the normal operation of the electromagnetic flowmeter. Therefore, DC excitation is generally only used to measure non-electrolyte liquids, such as liquid metals. Figure 3-18 DC excitation method 2. AC excitation At present, most of the electromagnetic flowmeters used in industry use the AC excitation mode of power frequency (50Hz) power supply, that is, its magnetic field is generated by sinusoidal alternating current, so the generated magnetic field is also an alternating magnetic field. The main advantage of the alternating magnetic field transmitter is to eliminate the polarization interference on the electrode surface. In addition, since the magnetic field is alternating, the output signal is also an alternating signal, and it is much easier to amplify and convert low-level AC signals than DC signals.

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