Fine-pitch threads are used to pressure-couple extensional mode ultrasonic pulses at frequencies on the order of 50 to 100 kHz between four stainless steel lead wires of 1.6 mm diameter and a density or liquid level sensor of rectangular cross section (2.5 × 6.4 mm). Two of the lead wires are orthogonal to the plane of the sensor and are tightly screwed into its ends off-axis to generate and detect, by mode conversion, torsional waves in the sensor. Use of this screw-pressure coupling technique in acoustic emission and moduli studies, and in temperature and corrosion monitoring, is suggested.
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Lynnworth et al. (1984) studied this question.
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