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Introduction to flow control of electromagnetic flowmeter

Flow Control of Electromagnetic Flowmeters Electromagnetic flowmeters are used in a wide range of applications. Large-diameter instruments are mostly used in water supply and drainage projects. Medium and small calibers are often used for solid-liquid double equal unpredictable fluids or high-demand places, such as paper pulp and black liquor in the paper industry, pulp in non-ferrous metallurgical industry, coal slurry in coal preparation plant, strong corrosive liquid in chemical industry, and blast furnace tuyere in steel industry. Cooling water control and monitoring, flow measurement and control of hydraulic conveying of long-distance pipeline coal. Small caliber and small caliber are often used in places such as the pharmaceutical industry, food industry, and bioengineering.
The basic error of the full-flow non-full-tube electromagnetic flowmeter is (±1.5%, soil 2%) Fs. The plug-in type meter itself (ie, the detecting head) is soil 2% R and soil 4% Fs. Considering the point flow velocity (or the diameter flow rate) represents the average flow velocity of the surface, the influence of the variation of the velocity distribution coefficient caused by the flow rate change may bring 2% to 5% change and flow area measurement error, etc., the overall measurement accuracy is even lower. The electromagnetic flowmeter measures the clean medium at an economical flow rate of 1.5~3m/s. For mediums that are easy to crystallize, the flow rate should be increased, preferably 3~4m/s. The main reason is that the flow rate is automatically cleaned to prevent medium deposition. Function; When measuring the abrasive liquid such as pulp, the flow rate should be appropriately reduced, 1.0~2.0m/s, to reduce the wear on the inner liner and the electrode. In practical applications, the flow rate of the electromagnetic flowmeter rarely exceeds 7 m/s, and it is even more rare to exceed 10 m/s.
If it is applied to a large-diameter long-distance water pipe, because of its good speed distribution (there is a long enough straight front pipe section), the electromagnetic flow detection head is individually calibrated in the "checking tank", the flow variation range is not large, and there is no speed. Under good conditions such as a significant change in the distribution coefficient, the basic error can be close to or slightly larger than ± 2 % R . The function of the electromagnetic flowmeter on the market is also very different. The simple one is to measure the one-way flow, and only output the analog signal to drive the rear position meter; the multi-function meter has the measurement bidirectional flow path switching, upper and lower limit flow alarm, empty tube and power cutoff. Alarm, small signal cut-off flow display and total amount calculation, automatic check and fault self-diagnosis, communication with host computer and motion configuration. The serial digital communication function of some models can be selected from a variety of communication interfaces and dedicated chips (ASIC) to connect HART protocol systems PROFIBUS, CONFIG, FF fieldbus, etc.
The performance of general-purpose electromagnetic flowmeters on the market is quite different, some have high precision, many functions, some have low precision and simple functions. The basic error of the instrument with high function is (0.5% ± 1%) R, and the instrument with low precision is (1.5% ± 2.5%) FS. Prices vary by 1 to 2 times. Therefore, where the measurement accuracy is not very high (for example, non-trade accounting is only for control purposes, only places with high reliability and excellent repeatability are required), it is economically uneconomical to select high-precision instruments. Some models claim to have higher accuracy, the basic error is only (±0.2% ± 0.3%) R, but there are strict installation requirements and reference conditions, such as ambient temperature 20-22 °C, front and rear Straight pipe length requirements are greater than 10D 3D (usually 5D, 2D), and even the flow sensor should be integrated with the front and rear straight pipes for real-flow calibration on the flow standard device to reduce the impact of poor clamping. Therefore, when comparing multiple models, do not simply look at high indicators. Read the manufacturer or the manual for a comprehensive analysis.

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