Level detector

Abstract

Disclosed are apparatus and methods for detecting the extent to which material is present at a predetermined sensing area. An antenna probe is extended throughout the sensing area, and is loaded with a signal from an oscillator. A reference oscillator provides a constant-frequency signal of the same base frequency as the antenna oscillator signal. The presence of impedance-affecting material in the sensing area reduces the impedance load of the antenna and, therefore, the frequency of the output signal of the antenna oscillator in proportion to the extent of immersion of the antenna probe in such material. The output signals from both oscillators are compared in a frequency differencer circuit which produces output signals whose frequency, equal to the difference in frequencies of the two oscillator output signals, is an indication of the amount of material present in the sensing area. A testing circuit provides an indication of the frequency of the differencer output signal, and is used in adjusting the sensitivity of the detector circuitry with the antenna probe free of impedance-affecting material. With the oscillator sensitivities adjusted, the detector circuitry is zeroed, again with reliance on the testing circuit to indicate when both oscillators are generating the same output frequency. The differencer output signals may be converted to voltage levels proportional to the extent material is present at the antenna probe. Such voltage levels may be used, ultimately, to operate one or more devices to indicate the amount of material present in the sensing area and/or to control the amount of material in the sensing area.