Electromagnetic flowmeters are less affected by the in-flow velocity distribution profile, and therefore have relatively low requests for devices such as front straight tube lengths. This is because in the early 50s and 60s of the electromagnetic flowmeter, the measuring tube of the smaller-diameter flow sensor had a longer length, and it had already adjusted the distorted activity to be fully isolated. Therefore, the straight pipe was not set in advance. The length of the request, and the accuracy of the early appearance is low, the basic error is ± (1.5 ~ 2.5)% Fs (full scale), even if the impact of activity distortion is only a fraction of the fundamental error, the question is not highlighted . Following the development of the electromagnetic sewage flowmeter, the domestic production of calibers from mid-to-small-scale development to more than 1m has reached 3m. With the optimization of flow sensor planning, it has become lighter and smaller. The length of the interface between the electromagnetic flow sensor and the pipeline at the time was only 1.25-2.5 times the diameter (D), the accuracy was increased throughout, and the fundamental error was ±0.5%R (measurement value). Therefore, everyone felt that the length of the straight pipe before the rule was set. Demand. In 1991, the World Standardization Organization published IS09104 “Closed Pipe Liquid Flow Measurement—Functional Qualification Method for Liquid Electromagnetic Flowmetersâ€. The rule is: when the flow is calibrated, the inner diameter of the connecting flowmeter shall not be less than the inner diameter of the flow sensor, and shall not be greater than 3% of the inner diameter of the sensor; In the straight pipe section where the sensor electrode shaft base is at least 10 D upstream of any upstream disturbance and 5 D before the downstream disturbance, each exterior manufacturer also has a request for a distance ≥5 D from the upstream disturber at the time of operating the apparatus. In recent years, a solid stream calibrated flow meter method is very popular, and many water flow calibration device with electromagnetic flowmeter with high accuracy as a standard table, which accuracy level is generally 0.5, there are up to ~ 0.3 by 0.2. The electromagnetic flowmeter device used as a standard meter is more stringent and cannot be treated as usual with electromagnetic flowmeter devices. Some instrument manufacturers have installed a straight pipe for 0.3 degree of precision external appearance and even before and after the regular flow sensor. After being assembled and calibrated together, if they are reassembled, they must be recalibrated.  For example, the test object is an electromagnetic flowmeter with a caliber of 50mm. The inlet is connected with three sets of pipes. The inner diameters are 50mm, 55mm and 45mm. The inlet shoulders formed between 55mm and 45mm have exceeded the rules of ISO 9104. Less than 10% of the inner diameter of the flow sensor. The flow sensor measuring tube is connected to the receiving channel, and the base line and pipeline base line are concentric, straight and horizontal, and the partial pain is 3mm (6% of external diameter). Electromagnetic flowmeter is modified in the shape of the appearance of a longer measuring tube, two horizontal and vertical direction in the measuring tube to open two 30mm × 3mm glimpse window, in order to use the laser Doppler velocimetry measuring velocity dispersion. Measuring tube rubber fabric imported with a radius of 7.5mm arc transition. 　　 In summary, (1) The effect of partial pain on the flow measurement value The receiving level of partial pain destroys the vertical symmetry of the velocity distribution in the measurement tube against the base line of the electrode. If some imports are concealed, some of the flow rate in the second half will be relatively slow, and some relative speed will increase in the first half. If it is straight and painful, some conceal the right inlet to make the right side flow rate relatively slow, and the left side relatively fast. From Fig.2, the inner diameter of the receiver and the flow sensor measuring tube together, the error changes positively when compared with concentricity, and changes to +(0.1-0.15)% when the level of pain is relatively tight. (0.45~0.6)%.     (2) Influence of the interface shoulder on the flow measurement value When the inlet diameter of the inlet is smaller than the inner diameter of the flow sensor measurement tube, it becomes a sudden expansion tube. The liquid enters the measuring tube as shown in Fig. 5 to form a stream, separated from the rest of the media by the interface, and it scatters and swirls into a violent vortex, becoming a vortex. Following the flow, the vortex disappeared gradually, and the stream expanded to all sections. If the electrode orientation is in the eddy eddy current region, the flow measurement value will be affected. 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