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Electromagnetic flowmeters have a wide range of applications. Large-diameter instruments are widely used in water supply and drainage projects. Small and medium diameter pipes are commonly used for measuring difficult-to-measure fluids with both solid and liquid phases, or in applications with high requirements. Examples include measuring pulp and black liquor in the papermaking industry, slurry in the non-ferrous metallurgy industry, coal slurry in coal processing plants, highly corrosive liquids in the chemical industry, as well as for controlling and monitoring leaks in the cooling water systems of blast furnaces in the steel industry. They are also used for measuring and controlling the flow rate of coal transported over long distances through pipelines. Small and fine diameters are commonly used in industries such as industry, the food industry, and bioengineering where hygiene standards are important. 1. Accuracy grade and functions The functions of general-purpose electromagnetic flowmeters available on the market vary greatly; some have high accuracy and numerous functions, while others have lower accuracy and simpler functions. Instruments with high precision have a basic error of (±0.5% ± 1%)R, while those with low precision have an error of (±1.5% ± 2.5%)FS; the cost difference between the two is 1 to 2 times. Therefore, it is not economical to use high-precision instruments in applications where the requirements for measurement accuracy are not very high. Some models of flow meters claim to have a higher accuracy, with a basic error of only (±0.2% ± 0.3%)R. However, they require strict installation conditions and reference specifications; for example, the operating temperature should be between 20–22°C, and the lengths of the straight sections before and after the meter must be greater than 10D or 3D (worldwide standards specify 5D or 2D). In addition, it is required that the flow sensor be integrated with the straight sections before and after it in a flow standard apparatus for actual-flow calibration, in order to minimize the impact of improper installation. Therefore, when comparing different models, one should not focus solely on high specifications; instead, it is necessary to carefully read the manufacturer’s brochures or sample documents and conduct a comprehensive analysis. The functions of electromagnetic flowmeters available on the market vary greatly. Simple models are designed solely to measure one-way flow, outputting an analog signal that drives subsequent instruments. More advanced models offer capabilities such as measuring two-way flow, range switching, upper and lower flow limit alarms, alerts for empty pipes or power loss, suppression of weak signals, flow display and total volume calculation, automatic calibration and self-diagnosis of faults, communication with higher-level systems, and parameter configuration. Some models of instruments offer serial digital communication capabilities, with various communication interfaces and dedicated chips (ASICs) available to connect to HART protocols, PROFINET, Modbus, CONFIG, FF fieldbuses, and others. 2. Flow rate, full-scale flow rate, range, and diameter The diameter of the electromagnetic flowmeter does not necessarily have to be the same as that of the pipe; it should be determined based on the flow rate. In the process industry, liquids with viscosity levels that do not meet the requirements are transported; the typical flow velocity in pipelines is the economic flow velocity of 1.5~3 m/s. Regardless of the type of pipeline in which the flow meter is used, the diameter of the sensor needs to be the same as that of the pipeline. For electromagnetic flowmeters, the liquid flow velocity at full scale can be selected within the range of 1 to 10 m/s. In theory, there is no upper limit to the flow velocity; however, it is recommended in practice not to exceed 5 m/s, unless the lining material can withstand the impact of the fluid flow. In practical applications, the flow velocity rarely exceeds 7 m/s, and it is even rarer to see values above 10 m/s. The lower limit of the flow velocity at full flow rate is commonly 1 m/s; for some model instruments, it is 0.5 m/s. In some systems where the flow rate is low or the velocity is low at the beginning of operation, from the perspective of measurement accuracy, it is necessary to use instruments with an aperture smaller than that of the pipe, connecting them via reducer fittings. For fluids containing substances that tend to adhere, accumulate, or form scale, a flow velocity of no less than 2 m/s is recommended; ideally, it should be increased to 3–4 m/s or higher, thereby achieving self-cleaning and preventing adhesion and accumulation. For highly abrasive fluids such as slurry, the recommended flow rate should be below 23 m/s in order to reduce wear on the lining and electrodes. When measuring liquids with low conductivity near the threshold value, it is advisable to choose a lower flow rate (less than 0.5–1 m/s), as an increase in flow rate introduces motion noise, resulting in fluctuations in the output. The size of electromagnetic flowmeters is relatively large; it is not less than 20. Instruments with automatic range switching capability can reach values between 50 and 100. 3. Liquid conductivity A prerequisite for using an electromagnetic flowmeter is that the liquid being measured must be conductive, and its conductivity must be above a certain threshold. If the conductivity is below the threshold, measurement errors occur until it can no longer be used; once the threshold is exceeded, measurements can still be taken even if there are changes, with little variation in the indicated error. Its application also depends on the length of the dark lines of the flow flag between the sensor and the converter, as well as their distributed capacitance; the manufacturer uses a length for the dark lines of the flag that corresponds to the specified conductivity in the design specifications. Meters with non-contact capacitive coupling for large-area electrodes can measure liquids with a conductivity as low as 5×10-8 S/cm. Industrial water and its aqueous solutions, acids, bases, salt solutions, etc., can be used without any problems. Petroleum products and organic solvents cannot be used if their conductivity is too low. According to available information, some pure liquids or aqueous solutions have low conductivity and are considered unsuitable for use; however, in theoretical exercises, there are examples of substances that can be used despite containing impurities. Regarding aqueous solutions, the conductivity values listed in the data were measured in a testing chamber using pure water as the base solution. For theoretical calculations, industrial water can be used as the base solution; in this case, the conductivity will be higher than the values obtained, which is also advantageous for flow measurement. The liquid conductivity for theoretical applications should be at least one order of magnitude higher than the threshold specified by the instrument manufacturer. 4. Contaminants in the liquid Fine bubbles mixed into the flow can still allow normal operation, but what is measured is the volumetric flow rate including these bubbles; when gas is present and forms a gaseous stream, the electrodes may be covered by the gas, causing an instantaneous interruption in the circuit, which results in fluctuations in output or even prevents normal operation. The volumetric flow rate of the two phases can also be measured for solid-liquid biphasic fluids containing non-ferrimagnetic particles or fibers. Fluids with a high solid content, such as drilling mud, drilling cement slurries, and pulp, are theoretically non-Newtonian fluids. Since the solids move together with the carrier fluid, there is sliding between them and a difference in speeds; therefore, instruments calibrated for single-phase fluids will introduce additional errors when used with solid-liquid two-phase flows. When larger particles in the slurry scrape against the electrode surface, peak-shaped slurry noise occurs during operation with low-frequency rectangular excitation, which causes instability in the flow rate signals. In such cases, it is necessary to use instruments with higher frequencies or those capable of effectively suppressing slurry noise. Instruments powered by AC mains or those with dual-frequency excitation can also be used. Fluids containing ferromagnetic materials present a problem, as the magnetic permeability inside the measurement tube changes due to varying amounts of ferromagnets, resulting in measurement errors. However, by incorporating a flux detection coil in the magnetic circuit for compensation, the influence of contaminants in the ferromagnet can be reduced. When using slurry containing ore particles, attention should be paid to the degree of wear on the sensor lining, as an increase in the inner diameter of the measurement tube can cause additional errors. For such applications, ceramic linings or polyurethane rubber linings with good wear resistance are recommended. It is also advised to install the sensors on vertical pipes, so as to ensure even wear across the pipe and eliminate the problem of severe wear in the lower part when the pipe is installed horizontally. A nozzle-shaped shield can also be installed at the sensor inlet to relatively extend its service life. 5. Adhesion and precipitation In fluids where substances tend to adhere to and precipitate on the tube walls during measurement, if the substances adhering are conductive ones with a higher electrical conductivity than that of the liquid, the potential signal will be short-circuited and unable to function properly. In such cases, attention should first be paid to electrode contamination; for example, sharp or hemispherical protruding electrodes that are less prone to having substances adhere to them, replaceable electrodes, or scraper-type cleaning electrodes can be used. The areas prone to attachment can have their flow rate increased to achieve self-cleaning, and they also allow for easier-to-clean pipe connections, eliminating the need to disassemble the sensors for cleaning. A contactless electrode electromagnetic flowmeter with a non-conductive film layer can still function, but if it has a highly conductive layer, it will not be able to function. For more information, please visit the company’s official website at http://www.yb1518.com/. Please retain this link when reproducing the content! http://www.yb1518.com/UploadFiles/2012717174126829.jpg