Liquid-Level Sensing Utilizing Evanescent Waves Created inPiezoelectric Vibrators Operating in Trapped-Energy Modes

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The measurement of liquid level on the millimeter scale or less has become an important subject in biological and chemical fields. As an alternative to the well -known pulse-echo method, we have presented a new technique that employs a piezoelectric thickness vibrator operating in a trapped-energy mode for detecting a small-scale variation in liquid level. The variation in the electric conductance G with the liquid level has been observed by experiments and also by theoretical analysis using an equivalent-network model, and the feasibility of the sensing device is confirmed.