（以下简称LDE）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。 Electromagnetic flowmeter (hereinafter referred to as LDE) is a meter that measures the volumetric flow of conductive liquids by using Faraday's law of electromagnetic induction. 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.
2. Principle and mechanism The basic principle of electromagnetic flowmeter is 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. As shown in Figure 1, a 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" and its value In the following formula, E------ 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 typical structure of the sensor is shown in Figure 2. Excitation coils are installed above and below 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 make contact with the liquid. The excitation current is provided by the converter.
3.Advantages The measurement channel of the electromagnetic flowmeter is a smooth straight tube without a flow resistance detection element. It is not easy to block and 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 Wait.
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.
的口径范围比其他品种流量仪表宽，从几毫米到3m。 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.
4. Disadvantages LDE 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 LDEs cannot be used for liquids with higher temperatures; 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.
5. The general products and special instruments on the classification field can be classified from different angles.
For example, according to the excitation current, there are DC excitation, AC (power frequency or other frequency) excitation, low-frequency rectangular wave excitation, and dual-frequency rectangular wave excitation. The waveforms of several excitation methods are shown in Figure 3.
According to the system classification of output signal wiring and excitation (or power) wiring, there are four-wire system and two-wire system.
According to the converter and sensor assembly methods, there are separate and integrated types.
According to the connection method of flow sensor and pipeline, there are flange connection, flange clamp connection, sanitary connection and thread connection.
According to whether the flow sensor electrode is in contact with the measured liquid, there are contact type and non-contact type. According to the structure of the flow sensor, there are short tube type and insertion type.
Classified by purpose, there are general-purpose, explosion-proof, sanitary, water-proof and diving.
6. Selection considerations 6.1 Application Overview LDE has a wide range of applications. Large-caliber instruments are mostly used in water supply and drainage projects. Small and medium calibers are often used in solid-liquid dual-equivalent difficult-to-measure fluids or in places with high requirements, such as the measurement of pulp and black liquor in the paper industry, mineral pulp in the non-ferrous metallurgy industry, coal slurry in coal preparation plants, strong corrosive liquids in the chemical industry, and blast furnace tuyeres in the steel industry Cooling water control and leakage monitoring, flow measurement and control of hydraulic transportation of long-distance pipeline coal. Small caliber and micro caliber are often used in places with hygienic requirements such as the pharmaceutical industry, food industry, and biological engineering.
6.2 Precision levels and functions
The performance of general-purpose LDEs on the market is quite different, some are high precision, many functions, and some low precision, simple functions. The basic error of a high-precision meter is (± 0.5% to ± 1%) R, and the low-precision meter is (± 1.5% to ± 2.5%) FS. The price difference between the two is 1 to 2 times. Therefore, in places where measurement accuracy is not very high (for example, non-trade accounting is only for control purposes, and only places with high reliability and excellent repeatability), it is not economical to choose high-precision instruments.
Some models claim to have higher accuracy, and the basic error is only (± 0.2% to ± 0.3%) R, but there are strict installation requirements and reference conditions, such as ambient temperature of 20 to 22 ° C, and length requirements of straight pipe sections before and after It is greater than 10D, 3D (usually 5D, 2D) and even proposes that the flow sensor should be integrated with the front and rear straight pipes to perform real flow calibration on the flow standard device to reduce the impact of improper clamping. Therefore, when selecting and comparing multiple models, do not just look at high indicators, but read the manufacturer's samples or instructions for comprehensive analysis.
的功能差别也很大，简单的就只是测量单向流量，只输出模拟信号带动后位仪表；多功能仪表有测双向流、量程切换、上下限流量报警、空管和电源切断报警、小信号切除、流量显示和总量计算、自动核对和故障自诊断、与上位机通信和运动组态等。 The functions of electromagnetic flowmeters on the market are also very different. The simple one is to measure the unidirectional flow and only output the analog signal to drive the rear position meter. Cut off alarm, small signal removal, flow display and total calculation, automatic check and fault self-diagnosis, communication with upper computer and motion configuration, etc. The serial digital communication function of some models of instruments can select multiple communication interfaces and application-specific chips (ASICs) to connect HART protocol systems, PROFTBUS, Modbus, CONFIG, FF fieldbus, etc.