1 Introduction Because electromagnetic flowmeters are characterized by high reliability, strong corrosion resistance, and easy change of measurement range, many industries at home and abroad have been widely used and focused on its development. 2 Usage In our projects, such as light industry, electromagnetic flowmeters are often used in industries such as food, paper, fine chemicals, and sewage treatment to measure emulsions, ketchup, beer, pulp, fillers, black liquor, and other chemicals. Such as liquid or gas liquid, liquid-solid two-phase liquid flow. In addition to clean liquids, most of these liquids are easily sticky, easily blocked, contain bubbles, contain solid particles, and some are also highly corrosive. At present, electromagnetic flowmeters have two types of sensor, transducer-integrated and split-type constructions, up to IP68 protection grade, a variety of optional corrosion-resistant wear-resistant electrodes and electrode materials, accuracy of up to 0.2, which makes electromagnetic The flow meter can basically meet the flow measurement needs of the above conditions. 3 electromagnetic flowmeter features and deficiencies 3.1 Features (1) The structure is simple, there are no moving parts and disturbing parts or throttling parts that hinder the flow of the measured medium, and the easy adhesion and solid-liquid two-phase medium are not prone to clogging, abrasion and other problems. It can make up for the lack of measurement of mass media. 3.2 Inadequacies (1) Electromagnetic flowmeters cannot be used to measure gases, vapors, and liquids containing large amounts of gas. 4 The influence of excitation on the measurement The effect of electromagnetic flowmeters in use is affected by many factors. The trueness, reliability and accuracy of the measurement are not only related to the converter, but also mainly depends on the sensor, and the excitation technology of the sensor has a great influence on the flow detection. Understanding the development process of excitation technology is of practical significance for us to select and maintain electromagnetic flowmeters. 4.1 DC excitation At the beginning of the last century, European countries had developed electromagnetic flowmeters using DC excitation and started their industrial applications. DC excitation technology uses a permanent magnet or a DC power supply to power the field winding of an electromagnetic flow sensor to form a constant excitation magnetic field, as shown in FIG. 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 electrolysis of the fluid to be measured, resulting in polarization of the electrode surface. 4.2 Power frequency sine wave excitation In order to eliminate the drawbacks of electrode polarization in DC excitation, in the following decades, the developed countries proposed the use of industrial frequency sine wave excitation, as shown in Figure 2. Is the use of power frequency 50Hz sine wave power supply to the electromagnetic flow sensor excitation winding. Its main characteristic is that it can basically eliminate the polarization phenomenon of 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 last seven or eight decades of the past century, China's papermaking industry used electromagnetic magnetic flowmeters made in China and found that electromagnetic flowmeters often exhibited zero points and instability. Strict grounding and sources far away from electromagnetic interference can not be completely resolved, affecting production. This is related to the weakness of power frequency excitation. 4.3 Low frequency rectangular wave excitation In order to completely solve the problem of electromagnetic frequency flow frequency interference and improve the accuracy of flow measurement, a low-frequency rectangular wave excitation technique between DC excitation and AC excitation is proposed. This excitation technology, as shown in Fig. 3, not only has the characteristics of no eddy current effect and transformer effect (quadrature interference) in the DC excitation technology, but also has no polarization effect due to the power-frequency sine wave excitation, and is easy to amplify signal processing. The advantage of avoiding problems such as zero drift, noise, and stability of the DC amplifier and better anti-interference performance can be widely used in the electromagnetic flowmeter. Due to these advantages of rectangular wave excitation, the joint venture product with the introduction of this excitation technology was widely used in the mid-1980s. 4.4 Dual Frequency Square Wave Excitation In the late 1980s, foreign manufacturers introduced dual-frequency square-wave excitation technology. Successfully solved the zero stability of electromagnetic flowmeter and the adaptability to liquid-solid two-phase conductive fluid and low conductivity fluid flow, opened up new application fields of electromagnetic flowmeter and started industrial application of dual-frequency rectangular wave excitation technology. . Although the low-frequency rectangular wave excitation has excellent zero stability, when the liquid-solid two-phase conductive fluid flow containing the fibers and solid particles such as mud and pulp is measured, the solid particles rub across the electrode surface and the contact potential of the electrode suddenly changes. The output signal of the flow sensor has a spike-like pulse with a frequency spectrum characteristic of 1/f; when measuring a low-conductivity fluid flow, the electrochemical potential of the electrode periodically changes to generate noise whose amplitude is inversely proportional to the frequency (ie, 1/f noise). Causes the low-frequency square wave excitation electromagnetic flowmeter output to swing, as shown in FIG. The former is called mud disturbance and the latter is called flow noise. Research and analysis show that mud disturbance and flow noise have 1/f spectral characteristics. The amplitude at low frequencies is large, and the amplitude at high frequencies is small. If a low-frequency rectangular wave excitation with a higher frequency is used, the mud disturbance can be greatly reduced in magnitude. Therefore, increasing the excitation frequency helps to reduce mud interference and flow noise and improve the signal-to-noise ratio of the sensor output signal. In the past decade or so, the use of low-frequency rectangular wave excitation technology has increased the ability of electromagnetic flowmeters to resist interference and reduce the excitation power to improve the economics of excitation. The most obvious manifestation is that the signal flow rate per unit flow rate is reduced from 1mV/ms-1 to 0.6mV. /ms-1, 0.4 mV/ms-1, 0.2 mV/ms-1, 0.1 mV/ms-1. If the potential amplitude of the signal per unit flow rate is reduced by an order of magnitude compared to the prior art, the influence of mud disturbance and flow noise will be significantly enhanced. In order to further improve the anti-jamming capability, a higher frequency square wave excitation must be used. To sum up, to ensure zero stability of the electromagnetic flowmeter, it is best to use low-frequency rectangular wave excitation; in order to more accurately measure the flow of liquid-solid two-phase conductive fluid and low conductivity fluid, must use a higher frequency Rectangular wave excitation. Using the method of dual-frequency rectangular wave excitation as shown in FIG. 5 is the best solution. With the diversification of paper products, we once encountered the measurement of high-consistency pulps and slurries with certain impurities. The output of electromagnetic flow meters has become erratic. The dual-frequency excitation mechanism allows us to understand the reasons for this phenomenon. The emergence of dual-frequency excitation products just solves the problem of measuring this liquid flow. 4.5 In addition, some manufacturers of electromagnetic flow meters are now developing new AC excitation technology. It not only maintains the zero stability of rectangular excitation but also has excellent noise removal function of high-frequency excitation, and also has good anti-50Hz power frequency interference performance. It can well measure up to 30% solids content of two-phase liquid. The accuracy of the flow meter can reach 0.1%. Another connotation of the paper industry is the reuse of waste paper and environmental protection. In waste paper pulping and sewage treatment, electromagnetic flowmeters are required to measure the amount of waste pulp containing a large amount of silt and all kinds of dust, and the amount of sewage containing a large amount of ink, sludge, and large appliances. Electromagnetic interference. They have more stringent requirements on electromagnetic flowmeters. This novel AC excitation method also provides us with metering equipment that can adapt to these conditions. After describing various excitation technologies, it should be pointed out here that domestic electromagnetic flowmeters have achieved high levels of performance and reliability after several generations of scientific and technical personnel's arduous efforts, which account for a considerable amount of domestic industrial production. Share. 5 Problems to be aware of during use These questions are already familiar to everyone. To list them is to further explore more effective use experience. (1) The electromagnetic flowmeter is a volumetric liquid flowmeter, and it is necessary to ensure that the liquid in the flowmeter is filled with pipes during the measurement. In order to make the flow rate stable, there should be more than 5D and 2D straight pipe sections before and after the flowmeter. The size of the flowmeter aperture should be chosen so that the flow rate at the minimum and maximum flow rates is within the range of accuracy of the measurement. (2) When it has to be installed in a vertical pipeline from top to bottom, the flowmeter should be installed in the lower part of the pipeline, and a throttle valve is installed downstream of the flowmeter to generate a back pressure in the downstream. (3) Do not install the flow meter at the lowest point of the inclined pipe when measuring a medium that is liable to accumulate slag. Install the cleaning valve or blind plate at the lower end of the tube and clean the inner wall attachments regularly. (4) The sensor and the converter should be installed separately in a site where the vibration is severe. The caliber to ≥ DN350 flowmeter should be set on both ends of the bracket or hanger. (5) Grounding rings must be used on plastic pipes or lined pipes. If the grounding ring and the measuring electrode are made of different materials, electrochemical corrosion may damage the electrode. If the electrode material is not properly matched, the measured medium will polarize the electrode and weaken the signal potential. Therefore, the choice of electrode material should also be noted, can refer to the instructions and manufacturers. (6) Some working conditions need to inject chemicals into the pipeline. After injection, the conductivity of the liquid will cause unevenness to the output signal of the electromagnetic flowmeter. In this regard, injection can be used downstream of the flowmeter. If injection is required upstream, the injection point should be kept at a distance from the flowmeter so that the liquid can be mixed evenly. (7) The maximum cable length between the sensor and the converter of the split electromagnetic flowmeter is related to the electrical conductivity.æ¤ æ¤ æ¤ æ¤ ä»‹ç» å¯¼ç”µ 导电 导电 导电 导电 导电 导电 导电 导电 导电 导电 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 (8) Electromagnetic flowmeters should be installed in high-power electrical equipment and switchgear cabinets to reduce electromagnetic interference. (9) Overvoltage and surge currents generated in the line during a lightning strike can enter the instrument (magneto-meter) through the meter's power cord to damage it. At present, this problem has been recognized by more and more self-control personnel. When designing at the input terminal of the instrument's main power supply, even at the power supply end of a single instrument, overvoltage and surge current absorption devices are installed to prevent them from damaging the instrument. (10) New extensions. At present, some manufacturers have taken new measures for grounding. The conductive rubber is embedded in the sensor lining, eliminating the need for grounding rings or grounding flanges, reducing costs, and facilitating installation. High-performance PEEK materials are used as the electrode insulation seal to improve the stability and reliability of the electrode insulation. To ensure that in the high humidity and harsh environment, the electrode insulation resistance can reach 200MΩ or more. According to the principle of capacitance effect, design the flow empty tube alarm function. It has high sensitivity but is not affected by residual liquid in the tube wall. The constant development of the flowmeter's excitation technology reflects the need for production to promote scientific research. The results of scientific research have also promoted the development of production. This also shows that the development of science and technology can better rejuvenate the country. 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(2) The electromagnetic flowmeter is a kind of instrument that measures the volume flow rate, and its measurement is not influenced by the density, temperature, pressure, viscosity, Reynolds number, and the change of the conductivity within a certain range of the fluid. Electromagnetic flow meters require only water as the test medium for calibration and can be used to measure other conductive liquids without the need for additional corrections. This is an advantage that other flow meters do not have.
(3) The electromagnetic flowmeter has a large measuring range, and some products have a measurement range of 1000:1. For the same caliber sensor, the full-scale range can be arbitrarily set as long as the media flow rate is in the range of 0.3 to 15 m/s. The measurement range of the electromagnetic flow meter can cover both the turbulent and laminar state of the velocity distribution, which is not comparable to differential flow meter, turbine, vortex and other flowmeters.
(4) The measurement principle is linear, the measurement accuracy is high, and the electrical signal is completely output. The measurement speed is fast, the pulsating flow can be measured and the total amount can be quickly accumulated.
(5) Good corrosion resistance.
(6) The principle is to measure the average flow velocity of the cross-section of the water, and the requirement for the flow velocity distribution is low. Therefore, straight sections before and after the sensor are required to be shorter than other flowmeters.
(7) The flow of fluid in both positive and negative directions can be measured.
(2) Electromagnetic flowmeters cannot currently be used to measure liquid media with very low conductivity. The conductivity of the liquid medium to be measured must not be lower than 10-5 S/cm (equivalent to the conductivity of distilled water). There is nothing to do with petroleum products or organic solvents.
(3) Since the insulating material of the measuring tube is limited by the temperature, the current industrial electromagnetic flowmeter cannot measure the high temperature and high pressure fluid.
(4) The electromagnetic flowmeter is affected by the flow velocity distribution. Under the conditions of axially symmetrical distribution, the flow signal is proportional to the average flow velocity. Therefore, before and after the electromagnetic flowmeter must also have a certain length of straight pipe sections.
(5) Electromagnetic flowmeters are susceptible to external electromagnetic interference. 
