Views: 3 Author: Site Editor Publish Time: 2022-05-09 Origin: Site
Electromagnetic flow meters (EMF) is a new type of flow measurement instrument developed rapidly with the development of electronic technology from 1950s to 1960s. Electromagnetic flowmeter is an instrument that uses the principle of electromagnetic induction to measure the flow of conductive fluid according to the electromotive force induced when the conductive fluid passes through the external magnetic field.
The structure of electromagnetic flowmeter is mainly composed of magnetic circuit system, measuring conduit, electrode, shell, lining and converter.
Magnetic circuit system: its function is to produce uniform DC or AC magnetic field. The DC magnetic circuit is realized by permanent magnet, which has the advantages of simple structure and less interference by AC magnetic field, but it is easy to polarize the electrolyte liquid in the measuring catheter, so that the positive electrode is surrounded by negative ions, and the negative electrode is surrounded by positive ions, that is, the polarization phenomenon of the electrode, which leads to the increase of internal resistance between the two electrodes, which seriously affects the normal operation of the instrument. When the pipe diameter is large, the permanent magnet is also large, bulky and uneconomical, so the electromagnetic flowmeter generally adopts alternating magnetic field and is excited by 50Hz power frequency power supply.
Measuring catheter: its function is to let the measured conductive liquid pass through. In order to make the magnetic flux shunted or short circuited when the magnetic force line passes through the measuring conduit, the measuring conduit must be made of materials with non-magnetic conductivity, low conductivity, low thermal conductivity and certain mechanical strength. Non-magnetic stainless steel, FRP, high-strength plastic, aluminum, etc. can be selected.
Electrode: its function is to lead out the induced potential signal proportional to the measured. Electrodes are generally made of non-magnetic stainless steel and are required to be flush with the lining so that the passage of fluid is not obstructed. Its installation position should be in the vertical direction of the pipeline to prevent sediment from accumulating on it and affecting the measurement accuracy.
Shell: made of ferromagnetic material, it is the outer cover of the distribution system excitation coil and isolates the interference of external magnetic field.
Lining: there is a complete layer of electrical insulation lining on the inner side of the measuring conduit and the flange sealing surface. It is in direct contact with the measured liquid. Its function is to increase the corrosion resistance of the measuring conduit and prevent the induced potential from being short circuited by the pipe wall of the metal measuring conduit. The lining materials are mostly corrosion-resistant, high temperature resistant and wear-resistant Teflon plastics, ceramics, etc.
Converter: the induced potential signal generated by liquid flow is very weak and greatly affected by various interference factors. The function of the converter is to amplify and convert the induced potential signal into a unified standard signal and suppress the main interference signal. Its task is to amplify and convert the induced potential signal ex detected by the electrode into a unified standard DC signal.