Structure of Electromagnetic Flowmeter

Structure of Electromagnetic Flowmeter

The structure of an electromagnetic flowmeter mainly consists 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 DC or AC magnetic field. The DC magnetic circuit is implemented using permanent magnets, which has the advantages of a relatively simple structure and less interference from AC magnetic fields. However, it can easily polarize the electrolyte liquid inside the measuring tube, causing the positive electrode to be surrounded by negative ions and the negative electrode to be surrounded by positive ions, resulting in electrode polarization and an increase in internal resistance between the two electrodes, which seriously affects the normal operation of the instrument. When the diameter of the pipeline is large, the permanent magnet is also large, bulky, and uneconomical. Therefore, electromagnetic flow meters generally use alternating magnetic fields and are excited by a 50HZ power supply.

Measurement catheter: Its function is to allow the measured conductive liquid to pass through. In order to divert or short-circuit the magnetic flux when the magnetic field lines pass through the measuring conduit, the measuring conduit must be made of non-magnetic, low conductivity, low thermal conductivity, and materials with certain mechanical strength, such as non-magnetic stainless steel, fiberglass, high-strength plastic, aluminum, etc.

Electrode: Its function is to generate an induced potential signal proportional to the measured value. The electrode is generally made of non-magnetic stainless steel and is required to be flush with the lining so that fluid can pass through without obstruction. Its installation position should be in the vertical direction of the pipeline to prevent sediment from accumulating on it and affecting measurement accuracy.

Shell: Made of ferromagnetic material, it is the outer shell of the distribution system excitation coil and isolates the interference of external magnetic fields.

Lining: There is a complete layer of electrical insulation lining on the inner side of the measuring conduit and the flange sealing surface. It directly contacts the liquid being measured, and its function is 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 materials are mostly corrosion-resistant, high-temperature resistant, wear-resistant polytetrafluoroethylene 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.