What are the excitation methods of electromagnetic flow meters, how to select and install

Abstract: What are the excitation methods of electromagnetic flowmeters, how to select and install information are provided by excellent flowmeter and flowmeter manufacturers and quotation manufacturers. Electromagnetic flowmeters are mainly used to measure conductive liquid media, and cannot be used to measure gases, vapors and liquids containing a large amount of gas. Affected by the flow velocity distribution, under the condition of axisymmetric distribution, the flow signal is proportional to the average flow velocity. So, electromagnetic flowmeter. For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following is the details of how to select and install the excitation methods of electromagnetic flowmeters. Electromagnetic flowmeters are mainly used to measure conductive liquid media, and cannot be used to measure gases, vapors and liquids containing a large amount of gas. Affected by the flow velocity distribution, under the condition of axisymmetric distribution, the flow signal is proportional to the average flow velocity. Therefore, there must be a certain length of straight pipe sections before and after the electromagnetic flowmeter. Susceptible to external electromagnetic interference. The effect in use is affected by many factors. The authenticity, reliability and accuracy of the measurement are not only related to the converter, but also mainly depend on the sensor, and the excitation technology of the sensor has a great influence on the flow detection. What are the characteristics of the electromagnetic flowmeter: (1) The structure is simple, there are no moving parts and disturbance parts or throttling parts that hinder the flow of the measured medium. It can make up for the lack of mass flow meters that are not easy to measure such media. (2) Electromagnetic flowmeter is an instrument that measures volume flow, and its measurement is not affected by changes in fluid density, temperature, pressure, viscosity, Reynolds number and conductivity within a certain range. The electromagnetic flowmeter only needs to be calibrated with water as the test medium, and can be used to measure other conductive liquids without additional correction. This is an advantage that other flow meters do not have. (3) The electromagnetic flowmeter has a large measurement range, and some products have a measurement range of 1000:1. For the same diameter sensor, its full scale can be set arbitrarily as long as the medium flow rate is within the range of 0.3~15m/s. The measurement range of the electromagnetic flowmeter can cover the two velocity distribution states of turbulent flow and laminar flow, which is incomparable with the differential pressure flowmeter, turbine type, vortex flowmeter and other flowmeters. (4) The measurement principle is linear, the measurement accuracy is high, and the complete electrical signal output, the measurement reflection speed is fast, the pulsating flow rate and the rapid accumulation total can be measured. (5) Good corrosion resistance. (6) In principle, it is to measure the average flow velocity of the water-passing section, and the requirements for the flow velocity distribution are relatively low. Therefore, the straight pipe section before and after the sensor is required to be shorter than other flow meters. (7) It can measure the flowing fluid in both forward and reverse directions. Electromagnetic flowmeter excitation mode: DC excitation At the beginning of the last century, European countries developed electromagnetic flowmeters with DC excitation and began their industrial applications. The DC excitation technology uses a permanent magnet or a DC power supply to supply power to the excitation winding of the electromagnetic flow sensor to form a constant excitation magnetic field, as shown in Figure 1. The biggest problem of DC excitation technology is that the DC induced electromotive force forms a fixed positive and negative polarity on the surface of the two electrodes, which causes the electrolysis of the measured fluid medium and leads to the polarization of the electrode surface. In order to eliminate the drawbacks such as electrode polarization effect in DC excitation, in the following decades, developed countries proposed to use power frequency sine wave excitation, as shown in Figure 2. It uses the power frequency 50Hz sine wave power supply to supply power to the excitation winding of the electromagnetic flow sensor. Its main feature is that it can basically eliminate the polarization phenomenon on the electrode surface, reduce the influence of the electrochemical potential of the electrode and the internal resistance of the sensor. However, the adoption of power frequency sine wave excitation technology will bring a series of electromagnetic induction interference and noise. In the 1970s and 1980s, in the use of domestic electromagnetic flowmeters in my country's papermaking industry, it was found that the electromagnetic flowmeter often had zero point and unstable indication. Strict grounding and keeping away from electromagnetic interference sources can not be completely solved, affecting production. This is related to the weakness of power frequency excitation. Low-frequency rectangular wave excitation: In order to completely solve the problem of power frequency interference of electromagnetic flowmeters and improve the flow measurement accuracy, a low-frequency rectangular wave excitation technology between DC excitation and power frequency AC excitation is proposed. This excitation technology, as shown in Figure 3, not only has the characteristics that the DC excitation technology does not produce eddy current effects and transformer effects (orthogonal interference), but also has the power frequency sine wave excitation that basically does not produce polarization effects, which is convenient for amplifying signal processing. The advantages of avoiding zero drift, noise, stability and other problems of the DC amplifier, as well as good anti-interference performance, can be widely used in electromagnetic flowmeters. Due to these advantages of rectangular wave excitation, the joint venture products introduced with this excitation technology have been widely used in the 1980s. Dual-frequency rectangular wave excitation: In the late 1980s, foreign manufacturers introduced dual-frequency rectangular wave excitation technology.
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