First, the working principle of electromagnetic flowmeter
Electromagnetic flowmeter is a flowmeter for flow measurement according to Faraday's law of electromagnetic induction. The advantages of electromagnetic flowmeter are extremely small pressure loss and large measurable flow range. The ratio of maximum flow rate to minimum flow rate is generally above 20: 1, and it is applicable to a wide range of industrial pipe diameters, up to 3m. The output signal has a linear relationship with the measured flow, and the accuracy is high. It can measure the flow of acid, alkali, salt solution, water, sewage, corrosive liquid, mud, pulp, pulp and other fluids with conductivity ≥5μs/cm. But it cannot measure the flow of gas, steam and pure water.
When a conductor cuts the magnetic field lines, it will produce an induced potential, which is proportional to the effective length of the conductor in the magnetic field and the speed of the conductor moving perpendicular to the magnetic field. Similarly, when the conductive fluid flows vertically in the magnetic field and cuts the magnetic induction force line, the induced potential will also be generated on the electrodes on both sides of the pipeline. The direction of the induced potential is determined by the right-hand rule, and the magnitude of the induced potential is determined by the following formula:
Ex = BDV formula (1)
Where ex- induced potential, v;
B- magnetic induction intensity, t
D—— internal diameter of pipeline, m
V—— average flow rate of liquid, m/s.
But the volume flow qv is equal to the flow velocity v of the fluid and the cross-sectional area of the pipeline (πD? ) /4, substitute the formula (1) into the formula to get:
Qv=(πD/4B)*？ Former? Formula (2)
As can be seen from the above formula, when the pipe diameter d is fixed and the magnetic induction intensity b remains unchanged, the measured volume flow is linearly related to the induced potential. If electrodes are inserted on both sides of the pipeline, the induced potential Ex can be introduced, and the volume flow can be obtained by measuring the potential.
According to Faraday electromagnetic induction principle, a pair of detection electrodes are installed on the tube wall perpendicular to the axis and magnetic field lines of the measuring tube. When the conductive liquid moves along the axis of the measuring tube, the conductive liquid cuts the magnetic field lines to generate an induced potential, which is detected by two detection electrodes, and its value is proportional to the flow rate.
E=B V D K
Where:? Evoked potential;
K coefficient is related to magnetic field distribution and axial length;
B—— magnetic induction intensity;
V—— average velocity of conductive liquid;
D- electrode spacing; (Measure the inside diameter of the pipe)
The sensor takes the induced potential e as a flow signal and transmits it to the converter. After signal processing such as amplification, transform filtering, etc., the instantaneous flow and cumulative flow are displayed by dot matrix liquid crystal with backlight. The converter has 4~20mA output, alarm output and frequency output, and is equipped with communication interfaces such as RS-485, which supports HART and MODBUS protocols.
Note: The parameters of different electromagnetic flowmeters are slightly different. Please refer to the instruction manual when using.
According to Faraday's law of electromagnetic induction, in a uniform magnetic field with a magnetic induction intensity of B, a non-magnetic pipe with an inner diameter of D is placed perpendicular to the direction of the magnetic field. When the conductive liquid flows in the pipeline at the velocity of V, the conductive liquid cuts the magnetic field lines. If a pair of electrodes are installed at both ends of the diameter perpendicular to the magnetic field on the cross section of the pipeline, it can be proved that as long as the velocity distribution in the pipeline is axisymmetric, electromotive force will be induced between the two electrodes:
e=KBDv？ (3-36)
Where v is the average flow velocity on the cross section of the pipeline and k is the instrument constant. The volume flow of the pipeline can be obtained by the following formula:
qv=？ πeD/4KB？ (3-37)
As can be seen from the above formula, the volume flow qv has a linear relationship with the induced electromotive force E and the inner diameter D of the measuring tube, and 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 measuring principle of electromagnetic flowmeter.
It should be noted that in order to make Equation (3-37) strictly hold, the measurement conditions of electromagnetic flowmeter must meet the following assumptions:
① The magnetic field is a constant magnetic field with uniform distribution;
② The velocity distribution of the measured fluid is axisymmetric;
③ The tested liquid is non-magnetic;
④ The conductivity of the measured liquid is uniform and isotropic.
Second, the characteristics
1、? The measurement of flow rate is not affected by the changes of fluid density, viscosity, temperature, pressure and conductivity, and the voltage signal induced by the sensor has a linear relationship with the average flow rate, so the measurement accuracy is high.
2、? Measure the unobstructed part of the pipeline, so there is no extra pressure loss; There are no moving parts in the measuring pipeline, so the service life of the sensor is extremely long.
3、? Because the induced voltage signal is formed in the whole space filled with magnetic field, and it is the average value on the surface of the pipeline carrier, the straight pipe section required by the sensor is short, and the length is the diameter of the pipeline after the first 5 days.
4. Only the lining and electrode of the sensor are in contact with the liquid to be measured. As long as the materials of electrode and lining are reasonably selected, wear resistance can be achieved and long-term use can be guaranteed.
5、? The converter is the crystallization of several patented technologies of Forward Company in the United States, and can be applied to various conductive fluids. Adopting the latest and most advanced single chip microcomputer (MCU) and surface mount technology (SMT), it has reliable performance, high precision, low power consumption, stable zero point and convenient parameter setting. Dot matrix English display LCD, which displays cumulative flow, instantaneous flow, flow, flow percentage and other parameters.
Advantages of FFM62 series electromagnetic flowmeter
The biggest advantages of 1 and FFM62 series electromagnetic flowmeters are very stable, no drift and good repeatability. Using low frequency rectangular wave excitation, the excitation current is stable. This excitation mode has the characteristics of small electromagnetic interference, no eddy current effect, small orthogonal interference and in-phase interference, and no polarization phenomenon, which makes the electromagnetic flowmeter have the advantages of small current, low power consumption, no heating, zero stability, no drift and little electrode pollution.
2. The output of the electromagnetic flowmeter has nothing to do with the flow state (laminar flow and turbulent flow) under symmetrical distribution, so the range ratio is extremely large, reaching 100: 1, and the measurable velocity range is 0.3- 12m/s (the extended range is 0.1-1. M/s).
3. It can measure the flow of conductive fluid such as dirty medium, corrosive medium and suspension solid two-phase medium. There are no parts in the tube that hinder the flow of fluid, only the electrode and the lining are in contact with the fluid, so it will not be blocked or stuck. According to the corrosiveness, abrasion and temperature of the medium, different lining electrodes are selected to measure the flow of dirty, corrosive and suspended liquid-solid two-phase medium.
4. No extra pressure loss and energy consumption.
5, no mechanical inertia, sensitive response, can measure instantaneous pulsating flow, and can also measure the flow in both positive and negative directions.
6. In the process of measurement, it is not affected by the changes of temperature, viscosity, density and conductivity (within a certain range), and the measurement is stable.
7. The frequency converter is equipped with LCD screen, and buttons are used for menu setting. The standard output signal is frequency signal, and 4-20mA current output can also be selected to communicate with HART protocol.
8. The converter adopts patented technology of functional magnetic field analysis, which completely eliminates the influence of bias current and ensures the accuracy and stability of measurement.
9. For mud flow measurement with "mud noise" interference, FFM62 series has a patented converter with "noise suppressor", which can completely eliminate clutter and ensure the stability and accuracy of measurement.
10, the inverter circuit has lightning protection elements, which is suitable for outdoor installation. ?
1 1, a unique multi-electrode structure, completely eliminates electrical noise interference, which is more conducive to improving measurement accuracy.
12, unique electrode lead mode, completely eliminating orthogonal interference and zero drift, making FFM62 series electromagnetic flowmeter measurement more stable. Has good linearity and repeatability.
The principle, characteristics and advantages of electromagnetic flowmeter are introduced above. Do you know anything about electromagnetic flowmeter now? For more, please continue to pay attention to Tuba Rabbit Decoration Network.
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