The structural principle of electromagnetic flow meters

Electromagnetic flowmeter is an instrument made according to Faraday's law of electromagnetic induction, used to measure the volume flow rate of conductive liquids.

Due to its unique advantages, electromagnetic flow meters have been widely used in the flow measurement of various conductive liquids in industrial processes;

Such as various corrosive media such as acid, alkali, salt, etc; Electromagnetic flow meters have formed a unique application field for measuring various slurry flow rates.

Structurally, an electromagnetic flowmeter consists of two parts: an electromagnetic flow sensor and a converter.

Sensors are installed on industrial process pipelines, and their function is to linearly transform the volume flow rate value of the liquid flowing into the pipeline into an induced potential signal, which is then transmitted to the converter through a transmission line.

The converter is installed not too far from the sensor. It amplifies the flow signal sent by the sensor and converts it into a standard electrical signal output proportional to the flow signal for display, accumulation, and adjustment control.

measuring principle

According to Faraday's law of electromagnetic induction, when a conductor moves in a magnetic field and cuts magnetic field lines, an induced potential e is generated at both ends of the conductor;

Its direction is determined by the right-hand rule, and its magnitude is proportional to the magnetic induction intensity B of the magnetic field, the length L of the conductor in the magnetic field, and the velocity u of the conductor's movement. If B, L, and u are perpendicular to each other, then

E=Blu (3-35)

Similarly, in a uniform magnetic field with a magnetic induction intensity of B, a non magnetic pipeline 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 a velocity u, the conductive fluid 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 (Figure 3-17), it can be proven that as long as the flow velocity distribution in the pipeline is axisymmetric, there is also an induced electromotive force between the two electrodes:

E=BD (3-36)

In the formula, is the average flow velocity on the cross-section of the pipeline. From this, the volumetric flow rate of the pipeline can be obtained as:

Qv=π DU ˉ＝ (3-37)

As can be seen from the above equation, the volumetric flow rate qv is linearly related to the induced electromotive force e and the inner diameter D of the measuring tube, inversely proportional to the magnetic induction intensity B of the magnetic field, and independent of other physical parameters. This is the measurement principle of electromagnetic flow meters.

It should be noted that in order for equation (3-37) to strictly hold, the measurement conditions must satisfy the following assumptions:

① A magnetic field is a uniformly distributed constant magnetic field;

② The axisymmetric distribution of the flow velocity of the measured fluid;

③ The measured liquid is non-magnetic;

④ The conductivity of the measured liquid is uniform and isotropic.