Toward an ultrasonic sensor for pressure vessels

The focus of this paper is an ultrasonic position indication system that is capable of determining one-dimensional target location in a high-temperature steel container with gaseous medium. The combination of the very high acoustical impedance of steel (45.4 MRayl) and the very low impedance of a gas, for example, ambient air (0.0004 MRayl), causes significant reflections at the interfaces. The strategy of this investigation was to develop an ultrasonic transducer capable of replacing a small portion of pressure vessel wall. In building such a transducer, acoustic matching layers for the steel-gas interface, a mechanically and acoustically competent housing, an efficient piezoelectric element, and appropriate backing materials are developed and tested. The results include a successful housing design, high-temperature acoustic matching layers, and subsequent successful wave forms with good signal-to-noise ratio. Target location through 9.6 in. (24.5 cm) of ambient air was possible, with a steel pressure boundary 0.456 in. (1.160 cm) thick, and the use of one matching layer. Our transducer was tested repeatedly to 340° C without apparent degradation. In addition to the experimental results, this investigation includes numerical simulations. Sample wave forms were predicted one dimensionally with the coupled acoustic piezoelectric analysis, a finite element program that predicts wave forms based on Navier's equation for elastic wave propagation.