Stable and reliable coupling state between piezoelectric ultrasonic transducer and tested workpiece is very important for ultrasonic measurement. Traditional coupling agents need to be smeared manually or recycled separately, which cannot meet the requirement of on-machine thickness measurement by ultrasonic. Herein, a suitable coupling agent is selected for thickness on-machine measurement in industrial application of ultrasonics. The acoustic impedance and the attenuation coefficient are adopted to characterize the coupling effect of coupling agents and appropriate coupled flow field is selected theoretically. The experimental system of ultrasonic on-machine measurement is established with external circulating jet. The coupling characteristics of machining coolant liquid is calculated and analyzed. From experimental comparison with commonly used coupling agents, it is indicated that the micro-emulsion cutting fluid with volume concentration from 8% to 10% could be selected as compatible coupling agent for ultrasonic on-machine measurement, because it can not only meet the ultrasonic measurement requirements, but also perform friendly to industrial environment.

压电超声换能器与被测工件之间稳定可靠的耦合状态对于超声测量非常重要。传统偶联剂需要人工涂抹或单独回收，无法满足超声波在机测厚的要求。在此，选择合适的偶联剂用于超声波工业应用中的机上厚度测量。采用声阻抗和衰减系数表征偶联剂的耦合效应，从理论上选择合适的耦合流场。建立了外循环射流超声在机测量实验系统。计算分析了加工冷却液的耦合特性。