Structure of electromagnetic flowmeters

Structure of electromagnetic flowmeter

   The electromagnetic flowmeter mainly consists of two core parts: sensors and converters. Sensors are responsible for sensing changes in magnetic induction intensity in fluids, while converters convert the sensed signals into directly usable data such as volumetric flow rate. The synergistic effect of these two parts enables the electromagnetic flowmeter to achieve accurate measurement of fluid volume flow rate.

1. Detailed explanation of sensor structure:

Sensors are key components of electromagnetic flow meters, consisting of measuring tubes, excitation systems, electrodes, and casings. The measuring tube is used for fluid passage, while the excitation system generates a magnetic field. The electrodes are responsible for sensing changes in the magnetic induction intensity in the fluid, while the shell serves as a protective and supportive function. The collaborative work of these components enables the sensor to accurately sense the magnetic induction intensity in the fluid, providing strong support for the accurate measurement of electromagnetic flow meters.

1) The core function of measuring tubes is to detect the flow rate of conductive fluids passing through. In order to ensure that the magnetic field lines do not short-circuit due to the wall of the measuring tube, and to reduce the eddy current losses of the measuring tube in strong alternating magnetic fields, high resistivity non-magnetic materials with flanges are usually used to make straight pipes, among which stainless steel, fiberglass, high-strength plastics, and aluminum alloys are common choices. If the measuring tube is made of magnetic material, it is necessary to ensure insulation between its inner wall and the electrode to avoid induced potential short circuit. In practical applications, it is often used to cover the inner wall of the measuring tube with an insulating lining, which can prevent induced potential short circuits, protect the inner wall from corrosion, and maintain its smoothness. The selection of insulation lining materials should be based on the characteristics of the tested medium, commonly including polytetrafluoroethylene plastic, epoxy resin, and ceramics.

2) The main task of the excitation system is to generate a uniform DC or AC magnetic field. DC magnetic fields are usually generated by permanent magnets, which have a simple structure and strong anti-interference ability. However, when using a DC magnetic field, the DC potential generated on the electrode may cause electrolysis of the measured liquid, and the polarization phenomenon of the electrode may also disrupt the original measurement conditions. In addition, when the diameter of the pipeline is large, the required volume of permanent magnets also increases accordingly, which is not only bulky but may also increase costs. Therefore, electromagnetic flowmeters often use alternating magnetic fields. However, alternating magnetic fields are susceptible to orthogonal interference, generating orthogonal potentials that can affect the normal operation of electromagnetic flow meters. For this reason, the detection part of electromagnetic flow meters is usually equipped with adjustment devices to reduce this orthogonal interference potential. When choosing, the advantages and disadvantages of DC magnetic field and AC magnetic field should be balanced based on the specific working conditions of the tested medium.

The structure of the excitation coil that generates an alternating magnetic field will vary depending on the diameter of the pipeline. Usually, the laminated iron core is wound with copper wire to form two saddle shaped coils, which are placed on the upper and lower sides of the measuring tube to generate a magnetic field perpendicular to the fluid flow direction.

3) The task of an electrode is to extract an induced potential signal proportional to the flow rate. These sensor electrodes are installed on the wall of the measuring tube, perpendicular to the magnetic field, so as to be able to extract the flow signal generated by the magnetic field lines cut by the measured medium. The electrode must be made of non-magnetic materials such as Hastelloy, titanium, or platinum shovel alloy, and its end face must be flush with the lining to ensure that the fluid passes through without any obstruction. In addition, the installation position of the electrode should be carefully selected to avoid the accumulation of sediment on it, thereby ensuring that the measurement accuracy is not affected.

4) The shell is designed to resist the erosion of corrosive gases from the outside and shield the surrounding stray magnetic fields. It is usually made of cast iron material to ensure the durability and accuracy of the measuring device.

2. Converter:

The converter in the electromagnetic flowmeter is responsible for converting the weak potential signal captured by the sensor into a standard electrical signal proportional to the fluid volume flow rate after processing. In this way, the signal can be received and processed by secondary instruments or control systems. The converter plays a crucial role in electromagnetic flow meters, and its performance directly affects the accuracy and reliability of measurements.

1) Due to the high internal resistance of the sensor part of the electromagnetic flowmeter, the converter needs to have a sufficiently high input impedance to adapt.

2) Due to the relatively weak induced potential generated by flow meter sensors and their susceptibility to various interferences in practical applications, converters need to have the ability to distinguish and suppress these interferences.

3) In the measurement process of flow meters, if there are fluctuations in power supply voltage and frequency, the converter can effectively eliminate the impact of these fluctuations on the measurement results.

4) In order to meet different load demands, the output current should have excellent constant current characteristics.