## 2 Materials and Methods  
### Experimental Method  
The SVUFMST is depicted schematically in Figure 1 and the tested UFMs arranged in tandem along the streamwise direction were denoted as UFM1 to UFMn (here, \(n\leq 5\)) at different testing locations. \(n\) is the number of tested UFMs, which are prefixed to indicate the total number of tested UFMs, for example, 5-UFM1. In the present study, a type of transit-time UFMs (Daniel 3400) were examined, as shown in Table 1, using the working standard facility (WSF) in NJMS, and according to the China national verification regulation of UFMs \"JJG 1030-2007\" and ISO 17089-1:2010(en), \"Measurement of fluid flow in closed conduits-Ultrasonic meters for gas--Part 1: Meters for custody transfer and allocation measurement.\" These target flowmeters, with two nominal diameters, DN250 and DN150, have two sets of four paths, namely, two cross planes for the four paths, as shown in Figure 2. The abscissa, as presented in Table 1, means the plane of Path A and D is 0.809R away from the horizontal center plane of pipeline and Path B and C is 0.309R close to the center plane of pipeline. The expanded uncertainty (k = 2) of the WSF is within 0.29% of the flowrate range of 8-12,000 m\({}^{3}\)/h, which is used to verify the DN50-DN400 flowmeters with their accuracy no higher than 1.0%. Furthermore, the experimental data measured from NJMS and the experimental environment provided has strong traceability, with the help of the established primary standard facility (the expanded uncertainty is 0.10% and k = 2) and secondary standard facility (the expanded uncertainty is 0.25% and k = 2). The layout of the WSF is illustrated in Figure 3, which is designed as a bypass conduit of the mainline of WENGTP to achieve an actual-flow calibration. Once a UFMs verification is being conducted, the natural gas flow in the mainline would be switched preliminarily to the bypass conduit (also known as WSF). By applying the \"master meter method\" [6; 7], the facility consists of three flowrate-measurement nodes: the master meters (MM), checking meters (CM), and UFMs under test; the flow is generated by the pressure drop between the upstream inlet and downstream outlet of the WSF, as a result of the driving force of WENGTPs mainline. The checking meter is used to monitor the performance of the master meter in real time and the flowrate tested by the master flowmeters are designated as the reference flowrate. In order to improve the full-scale performance of WSF, this verification system is divided into two sub-systems aligned in parallel, depending on the flowrate scales: large working standard for the high-flowrate verifications and small working standard for the low-flowrate verifications, as shown in Figure 3. Therefore, the flowmeters and straight pipes with larger inner diameters are employed in the large working standard. As for the large working standard, the master meter is composed of 11 turbine flowmeters, three of which are DN150 and eight of which are DN200. Then, the 11 corresponding pipelines of the master meter converged into two checking meter lines equipped with two UFMs (DN150 and DN400), either of which could be selected to connect with the tested flowmeter of the inner diameter ranging from DN150 to DN400. Compared with the large WSF, three smaller turbine flow meters (DN50, DN80, and DN100) as the master meters and a single DN100 UFM as the checking meter were utilized to verify the tested flowmeters with small inner diameters (DN50, DN80, and DN100) in the small WSF. FE20 and FE16, as depicted in Figure 3, represent the UFMs and the turbine flowmeters, respectively, and flow control valves are installed both upstream and downstream of each flowmeter in WSF. The straight pipe length upstream and downstream of each flowmeter under test was maintained higher than 150D and 20D, respectively, to achieve a fully developed inner flow. Figure 1 also shows the straight pipe layout and Table 2 lists the straight pipe length for different flowmeters.