Grain moisture sensor

Abstract

A meter is disclosed for measuring the dielectric constant of a particulate material, particularly grain, as a function of its moisture content. The meter includes a test cell for containing a sample of the particulate material to be tested. An oscillator includes a variable frequency tuned circuit for varying the frequency of the oscillator output The tuned circuit includes a test cell capacitor using a sample of material to be tested as a dielectric, a calibrated variable standard capacitor and a variable trimming capacitor. An indicator is coupled to the calibrated variable standard capacitor to display a value representing the capacitance setting of the calibrated variable standard capacitor. A frequency monitor compares the output frequency of the oscillator with a selected frequency and a null display displays the difference between the output frequency of the oscillator and the selected frequency. In the invention, the frequency monitor generates DC outputs of opposite polarity according to whether the output frequency of the oscillator is greater than or less than the selected frequency and the null display is a digital display, displaying the frequency difference as zero when the output frequency of the oscillator equals the selected frequency, as a positive value when the output frequency of the oscillator is greater than the predetermined frequency and as a negative value when the output frequency of the oscillator is less than the predetermined frequency.

1 . A meter for measuring the dielectric constant of a particulate material to be tested, the meter comprising: 
 a test cell for containing a sample of the particulate material to be tested;    an oscillator including a tuned circuit for varying the frequency of the oscillator output, the tuned circuit comprising: 
 a test cell capacitor including two electrodes in the test cell and the sample of particulate material to be tested as a dielectric between the electrodes, such that the capacitance of the test cell capacitor is a function of the dielectric constant of the sample of particulate material;  
 a calibrated variable standard capacitor; and  
 a variable trimming capacitor;  
   an indicator coupled to the calibrated variable standard capacitor to display a value representing the capacitance setting of the calibrated variable standard capacitor;    a frequency monitor for comparing the output frequency of the oscillator with a selected frequency; and    a null display for displaying a frequency difference value representing a difference between the output frequency of the oscillator and the selected frequency;    CHARACTERIZED IN THAT:    the frequency monitor comprises means for generating DC outputs of opposite polarity according to whether the output frequency of the oscillator is greater than or less than the selected frequency; and    the null display is a digital display, displaying the frequency difference value as zero when the output frequency of the oscillator equals the selected frequency, as a positive value when the output frequency of the oscillator is greater than the selected frequency and as a negative value when the output frequency of the oscillator is less than the selected frequency.    
     
     
         2 . A meter according to  claim 1  wherein the frequency monitor comprises a tuned circuit including two reactance elements connected to produce respective DC voltages of opposite polarity.  
     
     
         3 . A meter according to  claim 2  wherein the two reactance elements are inductors.