Clamp-on ultrasonic flowmeters serve as an important tool for on-site testing of gas flow meters. However, its accuracy is significantly affected by the actual flow field, thus limiting its application scenarios. To address this issue, this study focuses on typical industrial disturbance structures and obtains the evolution and distribution of non-ideal flow fields downstream of disturbances through experiments and numerical simulations, as well as their effects on velocity and flow measurement errors. The results indicate that when traditional reflection or diagonal measurements were used in the downstream of disturbances, the flow deviation was largely dependent on the installation position and angle of the clamp-on ultrasonic flowmeter. This introduced significant uncertainty and bias, rendering it impossible to correct measurement results through quantitative coefficients. Utilizing a dual-channel measurement method can enhance measurement accuracy. When two sets of sensors perpendicular to each other were used to combine the reflection measurement path, the deviation fluctuation downstream of disturbances can be effectively controlled within the range of ±2%, irrespective of the installation angle. This measurement approach significantly reduced the distance limitations on the distance of the straight pipe section during the use of clamp-on ultrasonic flowmeters.

Gas flowmeters are a key reference for natural gas trade settlement. To advance on-site verification technology, this paper presents air and natural gas flow test systems employing a clamp-on gas ultrasonic flowmeter, and systematically investigates the effects of pipeline parameters, pressure conditions, transducer usage, and noise reflection on measurement accuracy. Quantitative results show that measurement errors can be controlled within ±2% by selecting appropriate transducer types and installing them beyond 20D downstream of disturbances. Moreover, the proposed dual-transducer synchronous measurement—using two sets of transducers at the same location in different acoustic directions and averaging the results—effectively suppresses radial velocity effects, reducing installation and flow-related errors to within ±1.5%. Installing a single layer of acoustic damping material (≥30 mm beyond the transducer) further improves the coherent signal-to-noise ratio above 30 dB, ensuring reliable performance under noisy conditions. These quantitative findings offer practical guidelines for on-site calibration of natural gas flowmeters, contributing to improved fairness in gas trade measurement.