Electromagnetic flowmeters (EMF for short) are new flow measurement instruments that developed rapidly with the development of electronic technology in the 1950s and 1960s. The following introduces the principles and characteristics of electromagnetic flowmeters for your reference.
1. Working principle of electromagnetic flowmeter
Electromagnetic flowmeter is a flowmeter that measures flow based on Faraday’s law of electromagnetic induction. The advantage of electromagnetic flowmeter is that the pressure loss is extremely small and the measurable flow range is large. The ratio of the maximum flow rate to the minimum flow rate is generally more than 20:1. It is suitable for a wide range of industrial pipe diameters, up to 3m. The output signal is linear with the measured flow rate. It has high accuracy and can measure conductivity ≥5μs/cm. Fluid flow of acids, alkalis, salt solutions, water, sewage, corrosive liquids, and mud, pulp, pulp, etc. But it cannot measure the flow of gas, steam and pure water.
When a conductor moves cutting magnetic field lines in a magnetic field, an induced electric potential will be generated in the conductor. The size of the induced electric potential depends on the effective length of the conductor in the magnetic field and the direction of movement of the conductor perpendicular to the direction of the magnetic field. Directly proportional to speed. In the same way, when the conductive fluid flows in the vertical direction in the magnetic field and cuts the magnetic induction force lines, it will also generate an induced electric potential on the electrodes on both sides of the pipe. The direction of the induced electric potential is determined by the right-hand rule, and the magnitude of the induced electric potential is determined by the following formula:
Ex=BDv-----------------Equation (1)
In the formula, Ex—induced electric potential, V;
B—magnetic induction intensity, T
D—pipe inner diameter, m
v —The average flow velocity of the liquid, m/s
However, the volume flow rate qv is equal to the product of the flow rate v of the fluid and the cross-sectional area of ??the pipe (πD?)/4. Substituting equation (1) into this equation we get:
Qv=(πD/4B)*?Ex?----------Equation (2)
It can be seen from the above equation that when the pipe diameter D is fixed and the magnetic induction is maintained When the intensity B remains unchanged, the measured volume flow rate and the induced potential have a linear relationship. If an electrode is inserted on both sides of the pipe, the induced potential Ex can be introduced. By measuring the magnitude of this potential, the volume flow rate can be obtained.
According to Faraday's principle of electromagnetic induction, a pair of detection electrodes are installed on the tube wall perpendicular to the axis of the measuring tube and the magnetic field lines. When the conductive liquid moves along the axis of the measuring tube, the conductive liquid cuts the magnetic field lines to generate an induced electric potential. , this induced potential is detected by two detection electrodes, and the value is proportional to the flow rate. Its value is:
E=B·V·D·K
Where:? E-induced electric potential;
K-coefficient related to magnetic field distribution and axial length;
B-magnetic induction intensity;
V-average flow velocity of conductive liquid ;
D-electrode spacing; (measuring the inner diameter of the tube)
The sensor transmits the induced potential E as a flow signal to the converter. After signal processing such as amplification, transformation and filtering, it is processed with Dot matrix LCD with backlight displays instantaneous flow and cumulative flow. The converter has 4~20mA output, alarm output and frequency output, and is equipped with RS-485 and other communication interfaces, and supports HART and MODBUS protocols.
Note: The parameters of different electromagnetic flowmeters are slightly different. Please be sure to read the instructions when using them.
According to Faraday's law of electromagnetic induction, in a uniform magnetic field with a magnetic induction intensity B, a non-magnetic pipe with an inner diameter D is placed perpendicular to the direction of the magnetic field. When the conductive liquid flows in the pipe at a velocity v, The conductive fluid cuts the magnetic field lines. If a pair of electrodes are installed at both ends of the diameter of the pipe cross section perpendicular to the magnetic field, it can be proved that as long as the flow velocity distribution in the pipe is axially symmetric, an induced electromotive force will be generated between the two electrodes:
< p>e=KBDv?(3-36)In the formula, v is the average flow velocity on the pipe section, and k is the instrument constant. From this, the volume flow rate of the pipeline can be obtained:
qv=?πeD/4KB?(3-37)
From the above formula, it can be seen that the volume flow rate qv and the induced electromotive force e are measured The inner diameter D of the tube has a linear relationship, which 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 in order for equation (3-37) to be strictly established, the measurement conditions of the electromagnetic flowmeter must meet the following assumptions:
① The magnetic field is a uniformly distributed constant magnetic field;
②The flow velocity of the measured fluid is axially symmetrically distributed;
③The measured liquid is non-magnetic;
④The conductivity of the measured liquid is uniform and isotropic homosexual.
II. Characteristics
1. The measurement of flow rate is not affected by changes in density, viscosity, temperature, pressure and conductivity of the fluid. The sensor induced voltage signal is linear with the average flow rate. relationship, so the measurement accuracy is high.
2. There are no flow obstructions in the measuring pipe, so there is no additional pressure loss; there are no moving parts in the measuring pipe, so the sensor has an extremely long life.
3. Since the induced voltage signal is formed in the entire space filled with magnetic fields and is an average value on the pipe surface, the straight pipe section required by the sensor is shorter, with a length of 5D in front and 3D in back. of pipe diameter.
4. Only the lining and electrodes of the sensor part are in contact with the liquid being measured. As long as the electrode and lining materials are properly selected, they can be wear-resistant and wear-resistant, ensuring long-term use.
5. The converter is the crystallization of various patented technologies of the American Forward Company and can be applied to various conductive fluid situations. It adopts the latest and most advanced international single chip (MCU) and surface mount technology (SMT), with reliable performance, high precision, low power consumption, stable zero point and convenient parameter setting. Dot matrix English display LCD displays parameters such as cumulative flow, instantaneous flow, flow speed, flow percentage, etc.
3. Advantages of FFM62 series electromagnetic flowmeter
1. The biggest advantage of FFM62 series electromagnetic flowmeter is that it is very stable, has no drift and has good repeatability. It uses low-frequency rectangular wave excitation, and the excitation current is stable. This excitation method has the characteristics of being less affected by electromagnetic interference, not producing eddy current effects, small orthogonal interference and in-phase interference, and not producing polarization phenomena, so that the electromagnetic flowmeter has It has the advantages of small current, low power consumption, no heat, stable zero point, no drift, and little impact of electrode pollution.
2. The output of the electromagnetic flowmeter has nothing to do with the flow state under symmetrical distribution (laminar flow and turbulent flow), so the range ratio is extremely large, which can reach 100:1, and the measurable flow rate range is 0.3- Fluid flow rate within 12m/s (extended range 0.1-15?m/s).
3. It can measure the flow of conductive fluids such as dirty media, corrosive media and suspended liquid-solid two-phase media. There are no parts in the tube that hinder the flow of fluid. Only the electrodes and lining are in contact with the fluid. They are not blocked or sticky. According to the corrosiveness, abrasion and temperature of the medium, different lined electrodes are selected to measure the flow of dirty, corrosive and suspended liquid-solid two-phase media.
4. No additional pressure loss and energy consumption.
5. No mechanical inertia, sensitive response, can measure instantaneous pulsating flow, and can also measure flow in both forward and reverse directions.
6. It is not affected by changes in temperature, viscosity, density and conductivity (within a certain range) during the measurement process, and the measurement is stable.
7. The converter has an LCD display and buttons for menu settings. The standard output signal is a frequency signal. 4-20mA current output and HART protocol communication can also be selected.
8. The converter uses patented functional magnetic field analysis technology, which completely eliminates the influence of bias current and ensures accurate and stable measurement.
9. For measuring slurry flow with "slurry noise" interference, the FFM62 series has a converter with a patented technology of "noise suppressor", which can completely eliminate clutter and ensure the stability and accuracy of the measurement. .
10. The converter circuit has lightning protection components and is suitable for outdoor installation and use. ?
11. Unique multi-electrode structure, electrical noise interference is completely eliminated, which is more conducive to improving measurement accuracy.
12. The unique electrode lead method completely eliminates orthogonal interference and zero drift, making the FFM62 series electromagnetic flowmeter measurement more stable. Has good linearity and repeatability.
The above has introduced the principles, characteristics and advantages of electromagnetic flowmeters. Do you understand electromagnetic flowmeters now? Please continue to pay attention to Tubatu Decoration Network for more information.