There are some skills and precautions in the selection of electromagnetic flowmeters

As a kind of industrial flow measuring instrument, electromagnetic flowmeter has some skills and precautions in its selection. I will discuss it with you, hoping to make a contribution to the correct selection of electromagnetic flowmeter.　1. Application overview Electromagnetic flowmeter has a wide range of applications. According to the application, there are large diameter, medium and small diameter, small diameter and small diameter. Among them, large diameter electromagnetic flowmeters are mostly used in water supply and drainage engineering. Flow measurement and control, etc., and small and tiny diameters are often used in places with hygiene requirements such as the pharmaceutical industry, food industry, and biological engineering. 2. Accuracy grades and functions There are big differences in the performance of general electromagnetic flowmeters on the market. Some have high accuracy and multiple functions, and some have low accuracy and simple functions. The basic error of the instrument with high precision is (±0.5%～±1%) R, and the meter with low precision is (±1.5%～±2.5%) FS. The price difference between the two is 1 to 2 times. Therefore, it is not economically cost-effective to select high-precision instruments in places where the measurement accuracy is not very high (for example, non-trade accounting is only for control purposes and only requires high reliability and excellent repeatability). The functions of electromagnetic flowmeters on the market are also very different. The simple one is to measure the one-way flow rate, and only output the analog signal to drive the rear-position instrument; the multi-function instrument has two-way flow measurement, range switching, upper and lower limit flow alarm, empty pipe and power supply. Cut off the alarm, small signal removal, flow display and total calculation, automatic check and fault self-diagnosis, communication with the host computer and motion configuration, etc. The serial digital communication function of some models of instruments can choose a variety of communication interfaces and special-purpose chips (ASIC) to connect to the HART protocol system, PROFTBUS, FF fieldbus, etc. 3. Flow velocity, full-scale flow, range and caliber. The caliber of the selected instrument may not be the same as the pipe diameter, it should be determined by the flow rate. In the process industry, water and other liquids with different viscosities are transported, and the pipeline flow rate is generally 1.5 to 3m/s at an economical flow rate. The electromagnetic flowmeter is used on such a pipeline, and the sensor caliber is the same as the pipe diameter.　　 The liquid velocity can be selected in the range of 1～10m/s when the electromagnetic flowmeter is full-scale, and the range is relatively wide. The upper limit flow rate is not limited in principle, but it is usually recommended not to exceed 5m/s, unless the lining material can withstand the scouring of the liquid flow, the actual application rarely exceeds 7m/s, and it is even rarer to exceed 10m/s. The lower limit of the flow velocity of the full-scale flow is generally 1m/s, and for some models, it is 0.5m/s. In some newly-built projects, the flow rate is relatively low or the flow velocity is relatively low in the early stage of operation. Considering the measurement accuracy, the diameter of the instrument should be changed to a diameter smaller than that of the pipe, and the reducer should be used to connect it. 4. Liquid conductivity 　　 The premise of using an electromagnetic flowmeter is that the measured liquid must be conductive and cannot be lower than the threshold (ie the lower limit). If the conductivity is lower than the threshold value, it will cause measurement error or even can’t be used. It can be measured even if it exceeds the threshold value. The indication error does not change much. The threshold value of general electromagnetic flowmeter is 10-4～(5×10-6)S/cm Between, depending on the model. When used, it also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The manufacturer's instruction manual usually specifies the length of the signal line corresponding to the conductivity. The non-contact capacitive coupling large-area electrode meter can measure liquids with conductivity as low as 5×10-8S/cm.　　According to experience, the actual conductivity of the liquid is preferably at least one order of magnitude greater than the threshold specified by the meter manufacturer. Because the lower limit specified by the manufacturer's instrument is the lowest value that can be measured under various conditions of use, it is restricted by some conditions of use, such as conductivity uniformity, connecting signal lines, external noise, etc., otherwise it will appear Output shaking phenomenon, etc. We have encountered many times when measuring low-degree distilled water or deionized water, the conductivity of which is close to the threshold value of 5×10-6S/cm, and the output sloshing during use. It works normally, but the measured volume flow is the mixed volume flow containing the volume of bubbles; if the gas content increases to form a bomb (lumpy) flow, the electrode may be covered by the gas and the circuit will be disconnected instantaneously, and the output will shake or even fail to work normally. There are large particles in the slurry rubbing the surface of the electrode, and peak-like slurry noise will be generated in the electromagnetic flowmeter with a lower frequency rectangular excitation, which will make the flow signal unstable. It is necessary to choose a higher frequency instrument or have a stronger suppression For instruments with slurry noise capability, AC excitation instruments or dual-frequency excitation instruments can also be used.　　 For the application of ore slurry containing ore particles, attention should be paid to the degree of wear on the sensor lining. The expansion of the inner diameter of the measuring tube will cause additional errors. Ceramic lining or polyurethane rubber lining with better wear resistance should be used in this occasion. At the same time, it is recommended that the sensor be installed on a vertical pipe to make the pipe wear evenly and eliminate the shortcomings of serious local abrasion in the lower half of the horizontal installation. It can also be installed at the inlet end of the sensor. The nozzle-shaped sheath is installed to extend the service life relatively.
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