Development of multi-grain capacitive sensor for determination of moisture content in grains

Abstract

The dielectric properties of a variety of Indian whole cereal grains viz. mustard, moong, masoor, chick pea, urad and wheat, with moisture content (MC) ranging from 7 to 35% (wet basis), were characterised to develop a prototype multi-grain moisture meter based on the capacitive sensing technique. The operating frequency of the multi-grain moisture meter was fixed at 230 kHz. It consisted of a calibrated oscillator circuitry, capacitive sensing cell as a sample holder, temperature sensor and microcontroller circuitry. It senses the MC of cereal grains due to variation of dielectric medium resulting in capacitance variation by using an oscillator tank circuit connected across a fixed reference capacitance value. This generates proportionate frequency modulations at the output of the oscillator circuitry. The empirical calibration equations for the prediction of moisture in a variety of grains were developed, validated and embedded in the microcontroller unit. A unique empty cylinder referencing system based on the relative frequency sensing technique associated with grain sample moisture variations was developed to compensate for empty cylinder referencing variations. This enabled the accurate measurement of frequencies. A thermistor was introduced in the oscillator circuitry to compensate for thermal drifts of the output signal. This enhanced the precision and stability of the oscillator unit. The coefficient of correlation between grain MC and capacitance variation was high and measurement accuracy was found to be 1%.