How to select electromagnetic flowmeter

　　After years of development, the performance of electromagnetic flowmeters has been continuously improved and their functions have gradually become stronger. There are also many types according to classification, such as DC excitation type, AC power frequency excitation type, dual frequency excitation type and low frequency rectangular wave excitation type according to the excitation method. They are all produced based on the principle of Faraday's law of electromagnetic induction to measure the volume flow of conductive liquids.

　　Electromagnetic flowmeters have extremely small pressure loss, have no requirements on medium density, viscosity, temperature, pressure and other parameters, and have low requirements on straight pipe sections. Here is a summary of how to choose a flowmeter.

　　1. Conductivity of the medium

　　The conductivity of the medium cannot be lower than the specified lower limit. If it is lower than the lower limit, measurement errors will occur, resulting in failure to use normally. If it exceeds the upper limit, it can still be measured even if it changes, and the indication error will not change much.

　　The conductivity of industrial water and its aqueous solution is greater than 10-4S/cm, the conductivity of acid, alkali and salt solution is between 10-4 and 10-1S/cm, and the conductivity of low-degree distilled water is 10-5S/cm. There is no problem in using these. For example, petroleum products and organic solvents cannot be used if their conductivity is too low. The aqueous solution used may be mixed with industrial water, and the conductivity will be slightly higher, which is conducive to flow measurement. The figure below lists the conductivity of several liquids at 20°C.

　　2. Electrode materials

　　In order to achieve good measurement results and longer service life during measurement, the electrode material must be selected according to whether the measured fluid is corrosive. For details, please consult the manufacturer's corrosion manual. The following figure shows common electrode materials and their corresponding corrosion resistance.

　　3. Measuring range

　　The diameter of the sensor does not have to be the same as the pipe diameter. It must be determined according to the actual flow rate and the diameter of the flowmeter is selected according to the calculation formula. For industrial water and other liquids, the pipeline flow rate is generally 1.5~3m/s. When the electromagnetic flowmeter is used in such a pipeline, the diameter of the sensor can be the same as the pipe diameter.

　　The liquid velocity of the electromagnetic flowmeter at full flow can be selected in the range of 1~10m/s. The upper limit velocity is not limited in principle, and it is recommended not to exceed 5m/s if the lining material can withstand the erosion of the liquid flow. The lower limit of the velocity at full flow is generally 1m/s, and some models are 0.5m/s.

　　For fluids with substances that are easy to adhere, deposit, scale, etc., the flow rate should be not less than 2m/s, and it is best to increase it to 3~4m/s or above to achieve self-cleaning and prevent adhesion and deposition. For highly abrasive fluids such as slurry, the common flow rate should be lower than 2~3m/s to reduce wear on the lining and electrodes. When measuring low-conductivity liquids close to the threshold, try to select a lower flow rate (less than 0.5~1m/s) as much as possible, because the flow noise will increase with the increase of flow rate, and the output will shake.

　　4. Contains mixture

　　Tiny bubbles mixed into the bubbly flow can still work normally, but what is measured is the mixed volume flow rate containing the bubble volume. The gas content increases to form a bullet (block) flow, and the electrode may be covered by the gas, causing the circuit to be disconnected instantaneously, resulting in output shaking or even failure to work normally.

　　5. The medium has precipitation

　　When the liquid is easy to attach and precipitate substances on the pipe wall, if the attached conductive material is higher than the liquid conductivity, the signal potential will be short-circuited and cannot work. If it is a non-conductive layer, the electrode pollution should be paid attention to first, such as the use of pointed or hemispherical protruding electrodes that are not easy to attach, replaceable electrodes, scraper-type descaling electrodes, etc. In places where attachment is easy, the flow rate can be increased to achieve the purpose of self-cleaning, and more convenient and easy-to-clean pipe connections can be adopted, and the sensor can be cleaned without disassembling. The contact electrode electromagnetic flowmeter can still work if a non-conductive film layer is attached, but it will not work if it is a highly conductive layer.