overview

Electromagnetic flowmeters (EMF) are a new type of flow measurement instrument that rapidly developed in the 1950s and 1960s with the advancement of electronic technology.

The electromagnetic flowmeter measures the unobstructed flow of components inside the tube, without pressure loss, with a range ratio of 100:1 and a flow rate of 0.1-10m/s; The applicable industrial pipe diameter range is wide, with a maximum of 3m, and the requirements for straight pipe sections are relatively low; Measurable total positive, negative, and differential amounts; The output signal is linear with the measured flow rate, with high accuracy, reaching levels 0.2 and 0.5; Measurement is not affected by changes in fluid density, viscosity, temperature, pressure, and conductivity; Can measure the fluid flow rate of acid, alkali, salt solutions, water, sewage, corrosive liquids, as well as mud, slurry, pulp, etc. with conductivity ≥ 5 μ s/cm; But it cannot measure the flow of gas, steam, and purified water.

Working principle

Electromagnetic flowmeter is a flowmeter that measures flow based on Faraday's law of electromagnetic induction. When a conductor cuts magnetic field lines in a magnetic field, an induced potential is generated in the conductor. The magnitude of the induced potential is proportional to the effective length of the conductor in the magnetic field and the speed at which the conductor moves perpendicular to the magnetic field direction. It is determined by the following formula: Ex=BDV (Formula 1)

Ex - Induced potential, V; B - Magnetic induction intensity, T

D - Inner diameter of pipeline, m; V - Average flow velocity of liquid, 0.1-10m/s

The volumetric flow rate Q is equal to the product of the fluid velocity V and the pipeline cross-sectional area 3.14D ²/4. Substituting formula 1 into this formula yields:

Q=(3.14D/4B) * Ex (Formula 2)

As can be seen from the above equation, when the diameter D of the pipeline is fixed and the magnetic induction intensity B remains constant, the measured volume flow rate is linearly related to the induced potential. If an electrode is inserted on each side of the pipeline, the induced potential Ex can be introduced, and the magnitude of this potential can be measured to obtain the volume flow rate. The induced potential Ex is used as a flow signal and transmitted to the converter. After signal processing such as amplification, conversion, and filtering, the instantaneous flow rate and cumulative flow rate can be displayed. The converter has 4-20mA output, alarm output, frequency output, RS-485 communication interface, and can also support HART protocol as needed.

Composition of electromagnetic flowmeter

An electromagnetic flowmeter is composed of a magnetic circuit system, a measuring conduit, electrodes, a housing, a lining, and a converter.

Magnetic circuit system: Its function is to generate a uniform magnetic field, usually using an alternating magnetic field, and it is excited by a 50HZ power supply;

Measurement catheter: Its function is to allow the measured conductive liquid to pass through, and non-magnetic stainless steel materials are often used; The outer shell is made of ferromagnetic material and serves as the outer cover of the excitation coil, isolating the interference of external magnetic fields;

Electrode: Its function is to generate an induced potential signal proportional to the measured value. Depending on the measurement medium, there are various non-magnetic electrode materials to choose from, with 316L electrode being commonly used;

Lining: There is a complete layer of electrical insulation lining on the inner side and flange sealing surface of the measuring conduit, which is used to increase the corrosion resistance of the measuring conduit and prevent the induced potential from being short circuited by the metal measuring conduit wall. The lining material is mostly corrosion-resistant, high temperature resistant, wear-resistant polytetrafluoroethylene plastic, rubber, ceramics, etc;

Converter: converts induced electromotive force into useful signals and displays them, outputting various secondary signals. The converter can be integrated or separated from the sensor.