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The basic principle of electromagnetic flowmeters
Published on:2014-12-21 18:36:30 Hits:

(A) measuring principle

According to Faraday's law of electromagnetic induction, when a conductor in a magnetic field motion cutting magnetic field lines at both ends of the conductor which produce induced electric potential e, whose direction is determined by the right hand, in the field of its magnetization magnitude of the magnetic field B, conductor the length L and the conductor is proportional to the velocity u, if B, L, u three mutually perpendicular, then

e = Blu (3-35)

And this is similar. In the magnetic induction B in a uniform magnetic field, perpendicular to put an inner diameter D of the magnetic field direction is non-magnetic pipe, when the conductive liquid in the pipeline to flow velocity u, the conductive fluid on the cutting magnetic field lines. If a pair of electrodes installed vertically on both ends of the pipe cross-section than the diameter of the magnetic field (Fig. 3-17) can be proved that as long as the flow velocity distribution in the pipe is axisymmetrical distribution, between the two electrodes to generate the induced electromotive force also Laid:

e=BD  (3-36)

In the formula, the average flow velocity on the pipe cross-section. This gave the volume flow of the pipeline:

qv=  =  (3-37)


Seen from the above equation, the volume flow rate qv with EMF measuring tube diameter D e and a linear relationship, independent of other physical parameters and magnetic induction B is inversely proportional to the magnetic field. This is the measuring principle of electromagnetic flowmeter.

It should be noted that, to make the formula (3-37) strictly true, measurement conditions must meet the following assumptions:

① constant magnetic field is evenly distributed;

② measured fluid velocity axisymmetric distribution;

③ measured liquid non-magnetic;

④ conductivity measured liquid homogeneous and isotropic.

14191596613410367.gif

The schematic diagram of the electromagnetic flowmeter 3-17

1- pole; 2- electrode; 3- pipeline

(B) excitation mode

Excitation mode that generates a magnetic field approach. From the foregoing, for the formula (3-37) strictly true, the first condition that must be met is to have a uniform and constant magnetic field. To do this, you need to select an appropriate excitation mode. Currently, there are three general ways Reed touch that DC excitation, AC excitation and low frequency square wave excitation. Now be introduced separately.

1. DC excitation

DC excitation mode or with direct current magnetic field is generated using permanent magnets, can produce a constant uniform magnetic field. The biggest advantage of this DC excitation transducer is subject to the exchange of electromagnetic interference little effect, thereby affecting the phenomenon of self-inductance of the liquid can be ignored. However, easy to make use of a DC magnetic field by measuring the electrolyte liquid conduit is polarized, i.e., the electrolyte is electrolyzed in the electric field, generating positive and negative ions. Under the action of the electric force, ran the positive ions, positive ions ran negative. As shown in Figure 3-18. This will lead to positive and negative electrodes are surrounded by the opposite polarity ions, seriously affect the normal operation of the instrument. Therefore, the DC excitation is generally used for measuring non-electrolyte liquid, such as liquid metals.

1.gif

Figure 3-18 DC excitation mode

2. AC excitation

Currently, electromagnetic flowmeters used in the industry, mostly using power frequency (50Hz) Power AC excitation mode, that is, its magnetic field is generated by a sinusoidal alternating current, so the magnetic field is generated by an alternating magnetic field. The main advantage of the alternating magnetic field of the transmitter is the elimination of the polarization scrambling at the electrode surface. Further, since the magnetic field is alternating, the output signal is an alternating signal, a low level of amplification and conversion AC signal is much easier than a DC signal.

If the magnetic induction of the alternating magnetic field

B = Bm sin t (3-38)

The induced electromotive force is generated on the electrode

e=Bm D  sin  t (3-39)

Measured volumetric flow rate

qv= D  (3-40)

Where Bm-- maximum magnetic induction field;

- Angular frequency of the excitation current, =2  f;

t-- time;

f-- power frequency.

By formula (3-40), when the measuring tube diameter D constant magnetic induction Bm certain value, the output of the two electrodes and flow induced emf e qv proportional. This is the basic principle of alternating magnetic field electromagnetic flow transmitter.

It is worth noting that, with the alternating magnetic field will bring a series of electromagnetic interference. For example orthogonal interference. In-phase interference, which interference signal and a useful signal mixed with the flow. Therefore, how to correctly distinguish traffic signal and interfering signals, and how to effectively suppress and eliminate interference signals, has become an important issue AC excitation electromagnetic flowmeter developed.

3. Low frequency square wave excitation

DC and AC excitation mode excitation methods have advantages and disadvantages of AIDS, in order to give full play to their advantages, try to avoid their shortcomings, since the 1970s, people began to use low frequency square wave excitation mode. It's exciting current waveform shown in Figure 3-19, the frequency of the power frequency is typically 1/4-l / 10.

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Figure 3-19 square wave excitation current waveform

As seen in Figure 3-19, in the half-cycle, the magnetic field is constant steady dc magnetic field, which has the characteristics of a DC excitation by electromagnetic interference small. From the entire time course, the square-wave signal is an alternating signal, so it can overcome the polarization of DC shunt mayonnaise is easy to produce. Therefore, the low-frequency square wave excitation is a better excitation mode, has been widely applied in the electromagnetic flowmeter. To summarize, it has several advantages as follows:

① orthogonal to the alternating magnetic field can avoid electromagnetic interference;

② eliminate frequency interference caused by the distributed capacitance;

③ suppress eddy current AC magnetic field caused by the internal wall and fluid;

④ exclude direct excitation polarization.



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