Use of oscillators employing piezo-electric transducers as frequency determining elements in applications for detecting the presence or absence of materials is well known, as for example, detecting the presence or absence of frost on cooling fins of refrigerators, as shown in U.S. Pat. No. 4,176,524, or the presence or absence of liquid, as shown in U.S. Pat. No. 4,019,072. In devices of this type, the oscillator has a steady state condition of operation when the transducer is not subject to load, as for example the absence of the material being detected, and ceases to oscillate when the material is present and applies a load to the transducer. More particularly, the transducer ceases to resonate when subjected to any type of damping or loading. As a result, the transducer can be used to measure qualitative changes, such as the presence or absence of material, but cannot be used to measure quantitative changes such as changes in the thickness of frost or changes in liquid levels.
In contradistinction, the present invention is directed toward measuring devices using oscillators employing piezo-electric transducers which can be used to measure quantitative changes of the character indicated.