Electrical Capacitance Method

Agricultural materials act as dielectric materials when placed between two plates or concentric metal cylinders which form a capacitor. The dielectric constants for the dry components of grains (oil, starch, protein) vary between 2 and 5 while "free" water has a dielectric constant of 80. Because of the significant difference between the dielectric constants for water and dry components, changes in the moisture content will change the capacitance reading on the meter. In the range of 7 - 23% moisture content, the capacitance of the grain changes significantlly with changes in moisture content.

The Seedburo GMA 128 Grain Moisture Meter manufactured by Seedburo Equipment Company, Chicago, IL is an example of a moisture meter that utilizes the electrical capacitance method.

The moisture content readings are affected by the density of packing and temperature. When grain is dropped into a container, the method of filling may cause variations in the porosity, which is the percentage of the volume of the container occupied by air. Porosity may vary from as low as 35% to as high as 65%. Since the dielectric constant for air is slightly greater than 1, the density of packing will have a great effect on the meter reading. Therefore, the method of filling the chamber is precisely controlled.

Capacitance meters are less affected by the distribution of water within the grain kernels than conductance meters. They tend to give more accurate readings over a wider range of moistures.