In the conventional measurement, the compressed air flowmeter is connected with the front and rear straight pipe sections (hereinafter referred to as pipes) through flanges, pipe joints, or welding methods. The pipes provide a certain length of upstream and downstream straight pipe sections for the meter, so its inner diameter should be consistent with The inside diameter of the sensor is the same. If the inner diameter Dp of the pipe differs from the inner diameter Db of the compressed air flow meter, the difference should be in the range 0.95Db ≤ Dp ≤ 1.10Db. On the premise that this range is satisfied, the piping is concentric with the sensor, and the coaxiality should be less than 0.05Dp. However, due to the inconsistency of internal diameters of compressed air flowmeters of various manufacturers on the market, the inconsistent standards of seamless steel tubes at home and abroad, and the mixed use and misuse of protruding pipes and various pipes caused by the aging of gaskets, they led to field installation and use. Piping and sensor concentricity do not necessarily satisfy the 0.05Dp condition.
In the experiment, a repetitive compressed air flow meter was selected. Under the laboratory environmental conditions, a certain amount of deviation between the sensor and the piping occurred artificially, that is, the coaxiality of the two was changed, and the simulation of the on-site installation process may occur. Non-standard phenomena, comparing the measured meter coefficient obtained in the case of offset, the relative error in the high area of ​​the meter, and the repeatability of the high area with the data under the standard installation conditions, analyzes the effect of different offsets on the flow measurement.
First, the experimental method to select a precision level of 1.0, reproducible, nominal diameter DN50mm flowmeter, flow range (30 ~ 300) m3 / h, the use of extended uncertainty of 0.23% sonic nozzle type The gas flow standard device calibrated the meter factor and measured the vernier caliper. The inner diameter Db of the sensor is 47.36mm, and the inner diameter Dp of the pipe is 49.52mm, which satisfies the conditions of 0.95Db≤Dp≤1.10Db. Make the sensor from the top, bottom, left, right direction (observed from the entrance of the pipe) 5mm and 10mm offset (measured and marked in advance on the sensor outside the frame), change the coaxiality of the two, Measured the meter coefficient of the 5 points (Qmax, 0.4Qmax, 0.22Qmax, 0.2Qmax, Qmin) within the flow range of the compressed air flowmeter (in accordance with the JJG1029-2007 "compressed air flowmeter verification procedures" calculated high area meter coefficient) And its high area repeatability (the maximum value of each point between Qmax and 0.2Qmax), and compared with the corresponding data measured under standard installation conditions. The actual deflection diagram is shown in Figure 1 (solid line is the inner diameter profile of the pipe, and the dotted line is the sensor inner diameter profile). 
Second, the experimental data According to different offset conditions, the measured data as shown in Table 1.
Third, curve and data analysis
1.8 Offset conditions and instrument coefficient deviations under standard installation conditions In Figure 2, from the same direction, the offset of 10 mm is greater than the deviation of the meter coefficient with 5 mm offset, and the upward and downward offset ratio is to the left. The offset to the right is greater, especially when offset upward by 10mm, the meter coefficient offset reaches -23.79%. The reason is that the upward offset causes part of the airflow to pass through the underside of the vortex generating body. This part of the airflow does not generate vortex separation. The output pulse number of the instrument is reduced accordingly, and the frequency is reduced, resulting in a smaller meter coefficient. 
2.8 Offset and Gauge Repeatability of Gauge under Gauge Installation Conditions The repeatability limit of the compressed air flowmeter with an accuracy rating of 1.0 in the high zone is 0.33%. In Figure 3, the downward and leftward shifts are removed. Offset 5mm to the right for 3 cases, the repeatability of the remaining offsets is already out of tolerance. The repeatability of offset 10mm is 4 to 14 times larger than the offset of 5mm, especially the offset of 10mm or even up to 6%. , more than 17 times the repeatability limit.
3.8 Offset conditions and gauge installation conditions Single-point (Qmax) meter factor, indication error, and repeatability comparison Table 2, Qmax this point, offsets up to 5mm and 10mm in both cases of meter factor vs. specification installation Under the conditions, the offsets are the largest, and they all go to negative deviation; the absolute value of the upward deviation of the indication error is also the largest in all the offset situations, reaching -1.08% and -6.46%, respectively, and in terms of repeatability. Only 0.41% of the upward shift of 10mm clearly exceeds the repeatability limit of 0.33%, and the rest of the cases are within 1/2 of the repeatability limit.
Looking at all the offsets and comparing them with the data under the standard installation conditions, the following conclusions can be drawn:
1. The offset should be controlled as much as possible during the field installation. Only the offset of 5mm will offset the meter coefficient, relative error of the meter and repeatability to different degrees. For an offset in one direction, the greater the offset, the greater the absolute value of the meter coefficient offset, and the less desirable the repeatability.
2. The above 8 kinds of offset conditions, only the downward offset 5mm, offset 5mm to the left, offset 5mm to the right 3 cases of relative error and repeatability of the instrument meet the accuracy level of 1.0 requirements, the remaining 5 species Offset conditions do not guarantee that the relative error and repeatability of the instrument all meet the corresponding accuracy level.
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