Reasons why intelligent electromagnetic flowmeters need lining and tips for selection

The field of using intelligent electromagnetic flowmeters in modern industrial production is getting wider and wider. The measurement effect and accuracy of intelligent electromagnetic flowmeters are also constantly improving with the continuous advancement of manufacturing technology and processes. The measurement principle of electromagnetic flowmeters is based on Faraday's law of electromagnetic induction: when a conductive liquid cuts magnetic lines of force in a magnetic field, an induced potential is generated in the conductor. When measuring flow, the conductive liquid flows through a magnetic field perpendicular to the flow direction at a speed V. The flow of the conductive liquid induces a voltage proportional to the average flow rate. The induced voltage signal is picked up by two or more electrodes in direct contact with the liquid, and sent to the converter through a cable for intelligent processing, and then the LCD is displayed or converted into a standard signal of 4-20ma and 0-1khz output. In this way, the intelligent electromagnetic flowmeter can measure the flow of conductive fluids.



When we select an electromagnetic flowmeter, there is an important selection parameter, which is the choice of the lining material in the instrument. Why does the electromagnetic flowmeter need to be lined? This is determined by the measurement principle of the intelligent electromagnetic flowmeter. Electromagnetic flowmeters generally have a set of coils and two electrodes. The function of the coils is to add an electric field to the fluid. The flowing conductive liquid is equivalent to a conductor. According to Faraday's law of electromagnetic induction, when the conductor cuts the magnetic lines of force, an electromotive force proportional to the speed will be generated accordingly. The function of the electrodes is to measure this induced electromotive force. Therefore, only the electrodes in the measuring tube are connected to the conductive liquid, and the other parts are lined. To ensure insulation, the electromagnetic flowmeter can work properly. If the section of the metal pipe with a magnetic field is also in contact with the liquid, there will be a short circuit between the conductive liquid measured by the electromagnetic flowmeter and the metal pipe, and there will be conductivity, which will lead away the potential and make the electromagnetic flowmeter unable to measure the potential. Therefore, the inside of the intelligent electromagnetic flowmeter is lined.

And for this reason, we can only use electromagnetic flowmeters to measure the flow of conductive liquids, that is, the intelligent electromagnetic flowmeter has a minimum requirement for the dielectric constant of the measured medium. If the conductivity is lower than the threshold, measurement errors will occur and it cannot be used. If it exceeds the threshold, it can be measured even if there is a change, and the indication error does not change much. The threshold of the lower limit of the dielectric constant of the general electromagnetic flowmeter is between 10-4 and (5 × 10-6) S/CM, depending on the model. The conductivity of industrial water and its aqueous solution is greater than 10-4 s/cm, and the conductivity of acid, alkali, and salt solution is 10-4 ~10-1  s/cm. There is no problem with the use of low-degree distilled water of 10-5 s/cm. If the conductivity of petroleum products and organic solvents is too low, the intelligent electromagnetic flowmeter cannot be used.
From the data, it is found that some pure liquids have low conductivity and are considered unusable. However, in actual work, there are examples that can be used because they contain impurities. Impurities are beneficial to increase conductivity. For aqueous solutions, the conductivity in the data is measured in the laboratory with pure water. The actual aqueous solution may be mixed with industrial water, and the conductivity will be higher than the measured one, which is also conducive to flow measurement. Depending on the measured medium, the lining material selection of the intelligent electromagnetic flowmeter is also different. The lining materials

used for ordinary aqueous media, such as sewage, ionized water, etc., and corrosive liquid media (acid, alkali, and salt solutions) cannot be the same, including the selection of electrodes used for measurement. According to experience, the guidance method for selecting lining materials under normal circumstances is as follows. 1. Ordinary rubber, natural rubber, soft rubber, hard rubber. The operating temperature is 60°C, and its characteristics are elasticity and good wear resistance. It is generally used in urban water supply and drainage and other fields, and its corrosion resistance is relatively poor. 2. Polytetrafluoroethylene, also known as PTFE, also known as F4. One of the more commonly used lining materials, because of its stable chemical properties, it is generally used for sanitary liquids or highly corrosive liquids, such as concentrated acids and alkalis. 3. Polyperfluoroethylene, also known as F46. This material is similar to PTFE, but its wear resistance is stronger than PTFE. The medium temperature can reach up to 100℃. 4. Polyvinyl fluoride, also called Fs. It has similar characteristics to F4 material, but its temperature resistance is slightly worse. Generally, the medium temperature does not exceed 80℃. It has high cost performance and lower cost than F4 material. 5. Chloroprene, also called CR, also called Neoprene. It is characterized by good wear resistance and excellent elasticity. It is generally used in water supply and drainage, sewage treatment and other fields. Its corrosion resistance is slightly poor and it is not resistant to oxidation. 6. Polyurethane rubber, also called Polyurethane. It has excellent wear resistance, but it is not capable of corrosion, and the temperature must not exceed 80℃. It is generally used in industrial and mining environments with high wear resistance requirements, such as the measurement of ore slurry, coal slurry and other media. 7. Ceramic material Ceramic is undoubtedly the best of all materials and an absolute high-end product. The only disadvantage is that the price is not down-to-earth, the production process is complicated, the process requirements are extremely high, and the price is super high.