Measurement of moisture content of substance with capacitative sensor - incorporates parallel oscillator circuit with built in compensation for temp. and conductivity variations

摘要

The moisture content of a material or substance, in particular, soil, is measured by means of a circuit which determines the capacity of a capacitative sensor comprising two electrodes in contact with the soil. The measuring circuit incorporates an LC-oscillator and the sensor is connected parallel to the LC circuit. The LC circuit works together with a voltage dependent resistance element such that a reduction of the value of the resistance across the electrodes is compensated by an increase in the value of the resistance element and vice versa. The voltage dependent element is pref. a Zener diode in series with a diode. The oscillator pref. includes a transistor with ground base, the collector connected via the parallel LC circuit with the input pole of the voltage source and emitter connected via a feedback coil to the second pole of the input voltage source whilst the sensor electrodes are connected to the second pole of the source and with the end of the LV circuit connected to the collector respectively. The measuring circuit enables the moisture content of the material to be measured in terms of the variation of the oscillator frequency whilst eliminating the error due to variations in the conductivity of the sample.