1. Basic principles of electromagnetic flow meter

An electromagnetic flow meter is an instrument that measures flow based on Faraday's law of electromagnetic induction. It has the advantages of small pressure loss, can measure a wide range of flow, linear output, and high accuracy. It can measure a variety of fluid flows with conductivity ≥5 μs/cm, such as acid, alkali, salt solution, water, sewage, corrosive liquid, mud, mineral pulp, pulp, etc. However, electromagnetic flow meter cannot measure the flow of gas, steam and pure water. They are suitable for a wide range of industrial pipe diameters, up to 3m, and the ratio of maximum flow to minimum flow is generally more than 20:1.

Regarding the basic principle that a conductive fluid generates an induced electric potential when it flows in a magnetic field, when a conductor moves in a magnetic field, an induced electric potential will be generated in the conductor, and its magnitude is proportional to the length of the conductor in the magnetic field and the speed perpendicular to the direction of the magnetic field. Similarly, when the conductive liquid flows vertically in the magnetic field and cuts the magnetic induction lines, an induced electric potential will also be generated on the electrodes at both ends of the pipe. The direction of the induced potential can be determined by the right-hand rule, and the magnitude of the induced potential can be determined using the following formula:

Formula (1) Ex=BDv

Ex in formula (1) represents the induced potential in volts, which is related to the magnetic induction intensity B, the inner diameter of the pipe D and the average liquid flow rate v. Substituting formula (1) into the volume flow formula Qv, formula (2) can be obtained. When the pipe diameter D and the magnetic induction intensity B remain unchanged, the measured volume flow rate has a linear relationship with the induced potential. If electrodes are inserted on both sides of the pipe and the magnitude of the induced potential is measured, the value of the volumetric flow rate can be obtained.

According to Faraday's electromagnetic induction principle, a pair of detection electrodes installed on the tube wall perpendicular to the axis of the measuring tube and the magnetic field lines can detect the induced potential generated when the conductive liquid moves along the axis of the measuring tube, and its magnitude is proportional to the flow rate. The formula of induced potential is E=B - V - D - K, where B represents the magnetic induction intensity, V represents the average flow rate of the conductive liquid, D represents the electrode spacing (measuring tube inner diameter), and K represents the coefficient related to the magnetic field distribution and axial length. The sensor transmits the induced potential E as a flow signal to the converter, and displays the instantaneous flow and cumulative flow after signal processing. The converter also provides 4~20mA output, alarm output, frequency output and RS-485 communication interfaces, supporting HART and MODBUS protocols.

According to Faraday's law of electromagnetic induction, the electromagnetic flow meter can measure the flow of conductive liquid in a magnetic field. When a conductive liquid flows at a velocity v in a pipe, it cuts the magnetic field lines and generates an induced electric potential. A pair of electrodes are installed at both ends of the diameter of the pipe section perpendicular to the magnetic field, and the induced electromotive force e generated can be detected. It can be obtained that the volume flow rate of the pipeline is qv=πeD/4KB. The volume flow rate is linearly related to the induced electromotive force, inversely proportional to the magnetic induction intensity B of the magnetic field, and has nothing to do with other physical parameters. The parameters of different electromagnetic flow meter are slightly different, so you need to read the instructions when using them.

Note: In order for equation (3-37) to hold, the following conditions must be guaranteed:

The magnetic field is constant and uniform;

The flow velocity distribution of the fluid has axial symmetry;

Fluids are nonmagnetic;

The electrical conductivity of a fluid is uniform and isotropic.

2. The product features of electromagnetic flow meter are as follows:

1. Can measure in both directions.

2. The straight pipe section of the sensor is short and only requires 5 times the pipe diameter.

3. The pressure loss is very small.

4. The measurement results are not affected by fluid density, viscosity, temperature, pressure and conductivity.

5. Electromagnetic flow meter are mainly used in sewage treatment and other fields.

3. Structural composition of electromagnetic flow meter

Generally, an electromagnetic flow meter consists of two parts: a sensor and a converter. The sensor converts the medium flow into an induced potential, and the electrode extracts the potential signal and converts it into an output signal. The sensor includes a housing, catheter and electrodes. The shell is made of ferromagnetic material, which is used to isolate interference from the external environment and ensure the normal working condition of the magnetic field of the electromagnetic flow meter; the conduit is the basic structural part, which ensures the circulation of the medium (conductivity), and is usually made of non-magnetic material; the electrode extracts the induced potential signal proportional to the measurement, usually made of non-magnetic material, and is required to be flush with the lining to avoid obstruction of fluid flow. The installation position of the electrode should be in the vertical direction of the pipe to prevent sediment from accumulating on the pipe and affecting the measurement accuracy.

The conventional structure of electromagnetic flow meter consists of two parts: sensor and converter. The sensor converts the flow of the medium into an induced potential, which is then handed over to the converter to convert it into a signal output for display or control. The specific structure includes: a shell of ferromagnetic material, a conduit of the basic structure, an electrode that extracts the induced potential signal, and a magnetic circuit system that generates a magnetic field. To avoid affecting the measurement accuracy, the electrodes are installed in the vertical direction of the pipe. The magnetic circuit system generates a uniform DC or AC magnetic field, usually using an alternating magnetic field and excited by a 50Hz power frequency power supply. The converter amplifies and converts the induced potential signal into a standard signal. The lining increases the corrosion resistance of the measurement tube and prevents short circuit of the induced potential. The materials are mainly corrosion-resistant, high-temperature-resistant, and wear-resistant polytetrafluoroethylene plastics, ceramics, etc.