With the development of the national economy and the increasing degree of automation in the modern production process, flow as one of the main process parameters has become an important indicator for judging the efficiency, working conditions and economic performance of the production process. Therefore, the position of the flow meter for measuring fluid flow is becoming increasingly prominent.
The history of development of electromagnetic flowmeter is based on Faraday's law of electromagnetic induction used to measure the volume flow of conductive liquid meter. It measures the flow of fluids under conditions of laminar flow, turbulent flow, pulsating flow, and streamline vibration. Due to its smooth measuring tube, low pressure loss, wide measuring range, and sensitive reaction, it can obtain a signal proportional to the flow rate, plus it has no dead angles for the measurement tube to pass through. It is convenient for cleaning and sterilizing, so it is used for special hygiene requirements. It has been widely used in medicine and food industry. In particular, large-diameter electromagnetic flowmeters are even more important in water supply and drainage projects, port dredging, and wastewater treatment systems. Since Faraday discovered the law of electromagnetic induction in 1831, in 1832 he hoped to use the earth's magnetic field to measure the tide and flow of the Thames in the UK, but the experiment failed for three days. This is also the earliest test of an electromagnetic flowmeter in the world. In 1932, biologist A. Kolin successfully completed a circular-pipe electromagnetic flowmeter that can be used to measure and record instantaneous arterial blood flow. In 1954, Fox bor introduced the world's first electromagnetic flowmeter product. In 1955, Japan also made electromagnetic flowmeters. At the same time, the former Soviet Union, the United Kingdom, and the former West Germany were also successfully tested. In 1957, China began to develop electromagnetic flowmeters. After decades of development, domestic and foreign manufacturers of electromagnetic flowmeters have mushroomed rapidly. At present, the main foreign manufacturers are: Yokogawa Corporation (YEWMAG, ADMAG type), Hitachi (FMP-51 type), the United States Brooks company (Maglite, May7500 series), Foxbor company (M800 type), Germany KROHNK company (KXBO, K180 , K280, K480, etc.), Endress+Hauser (Master Mag type, etc.) and other famous manufacturers. At present, there are about 20 electromagnetic flowmeter manufacturing plants in China.
Technical Focus Since the 50s electromagnetic flowmeter was applied in industry, it has been rapidly developed and the performance of electromagnetic flowmeters has been greatly improved. At present, the transmitter's caliber is 2.5 mm to 3 m, and the precision level is ±0.5%. The highest temperature resistance can be designed at 180°C and the maximum pressure is 320 Pa. The range can reach 100:1; the full-scale flow rate 0. 3～10 m /s; liquid with a measurable electrical conductivity lower than 0. 05μs /cm; lining materials are hard, soft and polyurethane rubber, PTFE, ceramics, etc.; electrode materials are acid-resistant stainless steel ICr18Ni19Ti, Hastelloy B/C, platinum, titanium, platinum-ruthenium alloy, alloy, etc. But for a long time, the calibration, zero drift, and excitation power of large-caliber electromagnetic flowmeters are still the main factors affecting the development of electromagnetic flowmeters.
Calibration technology
For a long time, electromagnetic flowmeters at home and abroad can only be calibrated with real flow. With the improvement of the accuracy, flow range and specifications of the electromagnetic flowmeter products (up to 0.2 accuracy, 1000:1 flow range, 3 m or more caliber), the cost of technical equipment for calibration of production, use, and metering management has also increased. The higher. Shanghai Everbright Special Instrument Co., Ltd. introduced a set of 2m caliber real flow calibration technology and equipment costing nearly 2 million Euros. The calibration water tower is as high as 36 m. The use of large-caliber calibration equipment is also very costly. The 1 m-caliber flow rate At a flow rate of 1 m/s, a water flow of 2827 m3/h is required. At present, the domestic metrology management structure and enterprises are difficult to perform the necessary calibration calibration for electromagnetic flowmeters. Therefore, the realization of non-real-time flow calibration of electromagnetic flowmeter has considerable significance. The following describes a first calibration method for the ion current equivalent to the actual flow rate of the ion current in a stationary electrolyte solution.
Design idea
A controllable current I is added to the stationary electrolyte solution so that the ions in the solution have a controlled velocity V along the sensor tube. Under the action of the vertical magnetic field B, the two signal electrodes in the sensor generate an electric field E in the axial direction. It can be concluded that the electromotive force at the two ends of the sensor signal electrode is the solution of the ion flow velocity V in the liquid under the magnetic field and the boundary condition (ie, the weight function determined by the sensor's electrode and pipe space relationship); use the electrolyte standard solution mobility as the flow reference, then The ion velocity of the calibration electrolyte liquid can be obtained, and the corresponding ion current can be specifically equivalent to the actual fluid flow value.
Theoretical basis
For a Hall effect with a width of Wm, a thickness of Hm, and a Hall coefficient of Rm3/C in a rectangular section, there is a Hall potential U:U=(RIB)/H under the ion current I and the magnetic field B. Rewritten as U = [(R/I)/(WH)] BW, when V = (R/I)/(WH), the dimension of V is m / s, that is, V is the mean cross-sectional velocity of the conductive ions. The Hall potential is U=VBW, which is the electric potential form of the sensor electrode of the rectangular electromagnetic flowmeter. Therefore, the circular pipeline electromagnetic flowmeter is the basic equation in the magnetic field, electrode and pipe spatial relationship determines the weight function of the electrode potential deionization current calibration method can achieve a variety of diameter electromagnetic flowmeter calibration, especially for large diameter, high precision The calibration of a wide range of electromagnetic flow meters is of great significance.
Zero stability
Excitation frequency is the main performance index that affects the dynamic response speed and zero point stability of electromagnetic flowmeter. Excitation frequency is low, zero stability is high, but instrument anti-low frequency interference ability is weakened, response speed is slow; Excitation frequency is high, instrument anti-interference ability is strong The response speed is fast, but the zero stability is poor. For a long time, the excitation technology restricts the development of the electromagnetic flowmeter.
Dual frequency excitation
In 1988, Yokogawa Corporation of Japan introduced a dual-frequency excitation technique that consists of a low-frequency (6.25 Hz) rectangular wave and a high-frequency (75 Hz) rectangular wave superimposed to form a waveform of the excitation current. Two frequencies are sampled to obtain both high and low frequencies. Flow signal, which can achieve zero stability, fast response (up to 0.1s), strong anti-low frequency interference requirements. In the case of a relatively stable flow, this method can achieve satisfactory results, but when the flow continuously fluctuates, it is technically difficult to ensure the linearity after superposition.
Zero dynamic correlation complementarity
With two sample holders, the zero drift of the signal is dynamically eliminated in a forward (or negative) excitation, independent of the excitation frequency. In this way, a higher excitation frequency can be used, and at the same time a stable zero point can be obtained, and flow noise can also be easily eliminated. However, it must be possible to achieve a better overall performance index when the relevant complementarity reaches a better level.
Excitation power
Excitation power has an important influence on the explosion-proof performance of electromagnetic flowmeters, the sensitivity of sensors, and the signal-to-noise ratio of inductive signals. Excitation current directly affects the excitation power. When the excitation current is small, the excitation power is small, and the electromagnetic flowmeter has good explosion-proof performance, but the sensitivity of the sensor is reduced, the signal-to-noise ratio of the sensing signal is decreased, the range becomes smaller, and the accuracy is reduced. Through the zero point automatic dynamic recognition and feedback compensation, better results can be obtained. That is, the zero point drift of the signal is firstly identified dynamically (the zero drift value and the fluid flow velocity are determined), and the drift value is fed back to the preamplifier so that the amplifier performs high gain amplification after the zero point is eliminated. In this way, even with a small excitation current, it is possible to obtain a performance index with a high response speed, a large range, and high accuracy. It opens up a good prospect for the application of electromagnetic flowmeter in the chemical industry with high explosion-proof requirements.
Electromagnetic flowmeters have also made significant advancements in micro-computerized and intelligent converters, integration of sensors and transducers, compactness, lightweighting, lining materials, electrode shapes and materials, and elimination of contaminants on the surface of electrodes, making electromagnetic flowmeters accurate. The degree, linearity, and stability are significantly improved, and an electrodeless electromagnetic flowmeter emerges, which greatly reduces the conductivity requirements of the measured liquid. With the rapid development of electromagnetic flowmeter technology, it has also been widely used in the chemical, food and pharmaceutical, papermaking, municipal engineering, minerals, construction and other industries.
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