Analysis of the advantages and disadvantages of electromagnetic flowmeter

Case introduction

The electromagnetic flowmeter is an instrument for measuring conductive liquids based on Faraday's law of electromagnetic induction. The electromagnetic flowmeter has a series of excellent characteristics and can solve problems that are difficult to apply with other flowmeters, such as the measurement of dirty flow and corrosive flow. In the 1970s and 1980s, electromagnetic flow made a major breakthrough in technology, making it a widely used type of flow meter, and its usage percentage in flow meters continues to increase. Advantages: (1) The measurement channel is a smooth straight tube that will not block and is suitable for measuring liquid-solid two-phase fluids containing solid particles, such as pulp, mud, sewage, etc.; (2) There is no pressure loss caused by flow detection. , good energy-saving effect; (3) The measured volume flow is actually not significantly affected by changes in fluid density, viscosity, temperature, pressure and conductivity; (4) The flow range is large and the caliber range is wide; (5) Corrosion can be applied sexual fluids.

Disadvantages: (1) The application of electromagnetic flowmeter has certain limitations. It can only measure the liquid flow of conductive media, but cannot measure the flow of non-conductive media, such as gas and water treatment. Better water for heating. In addition, its lining needs to be considered under high temperature conditions. (2) The electromagnetic flowmeter determines the volume flow rate under working conditions by measuring the speed of conductive liquid. According to the measurement requirements, for liquid media, the mass flow rate should be measured, and measuring the medium flow rate should involve the density of the fluid. Different fluid media have different densities, and they change with temperature. If the electromagnetic flowmeter converter does not consider the fluid density, it is inappropriate to only give the volume flow rate at room temperature. (3) The installation and debugging of electromagnetic flowmeters are more complicated than other flowmeters, and the requirements are more stringent. The transmitter and converter must be used together, and two different types of instruments cannot be used together. When installing the transmitter, from the selection of the installation location to the specific installation and debugging, the product instructions must be strictly followed. There should be no vibration or strong magnetic field in the installation location. During installation, the transmitter and pipeline must have good contact and good grounding. The potential of the transmitter is the same as that of the fluid being measured. During use, the gas remaining in the measuring tube must be exhausted, otherwise it will cause large measurement errors. (4) When the electromagnetic flowmeter is used to measure viscous liquids with dirt, the sticky matter or sediment adheres to the inner wall of the measuring tube or the electrode, causing the output potential of the transmitter to change, causing measurement errors. The dirt on the electrode reaches a certain level. Thickness may cause the meter to be unable to measure. (5) Scale or wear of the water supply pipeline changes the inner diameter, which will affect the original flow value and cause measurement errors. For example, a 1mm change in the inner diameter of a 100mm diameter instrument will bring about an additional error of about 2%. (6) The measurement signal of the transmitter is a very small millivolt level potential signal. In addition to the flow signal, it also contains some signals that have nothing to do with the flow, such as phase voltage, quadrature voltage and common mode voltage. In order to accurately measure flow, it is necessary to eliminate various interference signals and effectively amplify the flow signal. The performance of the flow converter should be improved. It is best to use a microprocessor-type converter. Use it to control the excitation voltage. Select the excitation mode and frequency according to the properties of the fluid to be measured, so that coherence can be eliminated.interference and orthogonal interference. However, the improved instrument structure is complex and the cost is high. (7) High-priced application overview: Electromagnetic flowmeters have a wide range of application fields, and large-diameter instruments are mostly used in water supply and drainage projects; medium- and small-diameter instruments are often used in high-demand or difficult-to-measure situations, such as blast furnace tuyere cooling water control in the steel industry and measurement in the papermaking industry. Paper pulp liquid and black liquor, strong corrosive liquid in the chemical industry, ore pulp in the non-ferrous metallurgical industry; small diameter and micro diameter are often used in the pharmaceutical industry, food industry, biochemistry and other places with hygienic requirements. ?Since EMF entered industrial application in the early 1950s, its use areas have been increasingly expanded. Since the late 1980s, it has accounted for 16% to 20% of the sales of flow meters in various countries. ?my country has developed rapidly in recent years, with sales estimated to be 6,500 to 7,500 units in 1994. China has produced ENFs with large diameters of 2~6m, and has equipment capabilities for real-flow verification of 3m diameter.