A novel approach in liquid-level sensing by trapped-energy-mode thickness vibrators

A novel approach is presented for detecting a small-scale variation in liquid level using an energy-trapping phenomenon in piezoelectric thickness-mode vibrators. In a piezoelectric plate operating in a trapped-energy mode, an evanescent field is created in the surrounding region of the plate. When this region is dipped in a liquid, a small leakage of vibration energy occurs. Because the amount of leakage varies depending on dipping depth, small variations in liquid level are detected by observing the changes in the electric characteristics of the vibrators, such as quality factor and the electric conductance G. Preliminary experiments have shown that almost linear variations in G against dipping depth can be obtained within a few-millimeter range.