The working principle of electromagnetic flowmeter.

The working principle of electromagnetic flowmeter.

The electromagnetic flowmeter (EMF) is a new type of flow measuring instrument which developed rapidly with the development of electronic technology in the 1950s and 1960s. The electromagnetic flowmeter is based on Faraday's electromagnetic induction law and is used to measure the volume flow of the conducting liquid. Due to its unique advantages, the electromagnetic flowmeter has been widely used in the flow measurement of various conductive liquids in industrial processes, such as acid, alkali, salt and other corrosive media. The measurement of various slurry flow of electromagnetic flowmeter has formed a unique application field.

In structure, electromagnetic flowmeter is composed of electromagnetic flow sensor and converter. Sensor installed on the industrial process piping, its role is to flow into the pipe of liquid volume flow values into a linear change of induced electric potential signal, and through the transmission line will be sending the signal converter. Converter is installed in place not far away from the sensor, it will flow from sensor signal amplifier, translates into electrical signal proportional to the standard traffic signal output, for display, accumulation and adjustment control.

The basic principle of electromagnetic flowmeter.

(I) measurement principle.

According to Faraday's law of electromagnetic induction, when the conductor in magnetic field movement cutting lines, commutation in the ends of the conductor is e, its direction to be determined by the right-hand rule, its size and the magnetic induction intensity of the magnetic field B, the length of the conductor in magnetic field and a conductor is directly proportional to the speed of movement of the u, if B, L, u three perpendicular to each other, then

E = Blu (3-35)

Like this. In the uniform magnetic field of magnetic induction intensity for B, perpendicular to the direction of the magnetic field with a diameter of D is not permeable pipe, when conducting liquid flow in pipe flow velocity u, conductive fluid cutting lines. If on both ends of the diameter of the pipe cross section perpendicular to the magnetic field on the installation of a pair of electrodes (FIG. 3-17) can be proved that the velocity distribution for axisymmetric distributions, as long as the pipe between the two electrodes and generate induced electromotive force:

E = BD (3-36)

In the formula, is the average velocity of the pipe section. The volume flow of the pipeline can be:

Qv = PI DU = (3-37)

It can be seen from the above that the volume flow qv is linear with the induction electromotive force e and the inner diameter D of the tube, which is inversely proportional to the magnetic induction intensity B of the magnetic field, which has nothing to do with other physical parameters. This is the measurement principle of the electromagnetic flowmeter.

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

The magnetic field is a constant magnetic field with uniform distribution;

The flow velocity axial symmetry distribution of the measured flow body;

The liquid is nonmagnetic;

The conductivity of the liquid is uniform and isotropic.

1 - pole; 2 - electrode; 3 - pipe

(2) excitation mode.

Excitation mode is the way to generate magnetic field. It is known from the foregoing that in order to make the formula (3-37) strictly true, the first requirement to be satisfied is to have a uniform and constant magnetic field. Therefore, a suitable excitation method is needed. At present, there are three types of excitation, namely dc excitation, ac excitation and low-frequency square wave excitation. We will introduce them separately.

1. Dc excitation

Dc excitation mode generates a magnetic field or permanent magnet by direct current, which can produce a constant uniform magnetic field. The main advantage of this dc excitation transmitter is that the interference of ac electromagnetic field is very small, so it can ignore the influence of the self-inductance in the liquid. However, the use of a dc magnetic field makes it easy to polarize the electrolyte liquid that measures the pipe, that is, electrolyte is electrolyzed in the electric field, producing positive and negative ions. Under the action of electric field force, the negative ion runs to the positive pole and the positive ion runs to the negative pole. See figure 3-18. In this way, the positive and negative electrodes are surrounded by ions of opposite polarity, which seriously affect the normal work of electromagnetic flowmeter. Therefore, dc excitation is generally used to measure non-electrolyte liquids, such as liquid metal.

2. Ac excitation

At present, the industrial use of electromagnetic flowmeter, most used (50 hz) power frequency ac excitation power supply mode, that is, its magnetic field is produced by sinusoidal alternating current, so the magnetic field is also an alternating magnetic field. The main advantage of the alternating magnetic field transmitter is to eliminate the polarization of the electrode surface. In addition, since the magnetic field is alternating, the output signal is also an alternating signal, and it is much easier to amplify and convert the low level ac signal than the dc signal.

If the magnetic induction intensity of the ac magnetic field is.

B = Bm sint (3-38)

The induced electromotive force generated on the electrode is.

E = Bm Dsint (3-39)

The measured volume flow is.

Qv = D (3-40)

Bm -- the maximum value of magnetic field magnetic induction;

-- the angular frequency of the excitation current = 2f;

T - time.

F -- power frequency.

It can be seen from equation (3-40) that when the inner diameter D of the measuring tube is constant, and the magnetic induction intensity Bm is a certain value, the induced electromotive force e output on the two electrodes is proportional to the flow qv. This is the basic working principle of ac magnetic flux transmitter.

It is worth noting that using ac magnetic field can cause a series of electromagnetic interference problems. For example, orthogonal interference. Such interference signals are mixed with useful traffic signals. Therefore, how to correctly distinguish the traffic signals from the interference signals and how to effectively suppress and eliminate all kinds of interference signals becomes an important subject for the development of ac excitation electromagnetic flowmeter.

3. Low frequency square wave excitation.

Dc excitation method and the way of communication LiZi each have advantages and disadvantages, in order to give full play to their advantages, try to avoid their disadvantages, since the 70 s, people began to adopt low frequency square wave excitation. Its excitation current waveform is shown in FIG. 3-19, and its frequency is usually 1/4-l / 10 of the working frequency.

Square wave excitation current waveform.

In half a period, the magnetic field is a steady dc magnetic field, which has the characteristics of dc excitation and is very little affected by electromagnetic interference. From the whole time process, the square wave signal is an alternating signal, so it can overcome the polarization which is easily produced by the dc excitation. Therefore, low-frequency square wave excitation is a better excitation method, which has been widely used in electromagnetic flowmeter. To sum up, electromagnetic flowmeter has the following advantages:

The electromagnetic flowmeter can avoid the orthogonal electromagnetic interference of the ac magnetic field.

The electromagnetic flowmeter eliminates the interference caused by distributed capacitance.

The electromagnetic flowmeter is used to suppress the eddy current caused by the alternating magnetic field in the pipe wall and the fluid.

The polarization of dc excitation is eliminated by electromagnetic flowmeter.