Impedance instability of a transducer during its free filling with grain

Conclusions

• 1. An analysis of the experimentally investigated nature of variations with time of the grain-moisture transducer impedance has shown that in free pouring of moist grain the current in the transducer circuit increases with time, and that: a) a constant current is observed only in dry grain (moisture content below 14%); b) the transducer's impedance variations with time decrease with frequency, and starting at 0.1–0.2 MHz the variations of contact resistances do not produce any noticeable changes in the impedance; c) the relative current increments with time increase with a rising voltage applied to the transducer terminals; moreover, a reduction in the pollution with dust of grain decreases the effect of voltage on the grain impedance variations.

• 2. A reduction of the grain-moisture transducer's impedance with the freely pouring grain is mainly due to the self-compression of grain under its own weight and to a rise of moisture at the grain points which form contact resistances.

• 3. Current instability with time can produce considerable errors in conductometric ac measurements. The method of taking readings at a given instant after switching-in the instrument cannot be recommended as a means for eliminating this error, since the rate of change of the current is irregular and depends to a considerable extent on the grain's pollution with dust. Improved results can be obtained by adopting the following measures: lowering as far as possible the voltage across the electrodes, reducing the current density in the grain, utilizing a higher frequency (100 kHz or more), and using vibration-packing devices in the transducer.