Electrical Resistance Technique

The electrical resistance technique utilizes the principle of electrical conductance. The sample is compressed by two plates made of a conducting material. The plates are connected in series to an electrical current source. The conductivity of the sample increases with an increase in the moisture content.

Conductance of the sample is dependent upon a number of factors. It is affected by moisture content, pressure applied to the sample by the plates, and temperature.

There are at least three problems encountered with the electrical resistance method. First, at low moisture contents the water in many biological materials, such as grains and seeds, is held tightly so that changes in conductance due to moisture are very small. Secondly, at higher moisture contents, greater than 23% in the case of grains and seeds, the conductivity is very high and there are very small increases in conductivity as the moisture increases. Finally, when moisture is unevenly distributed in the sample particles, surface moisture has greater influence on the reading than the moisture in the interior, and thus makes the readings too high or low.

In the United States, conductance type meters are not usually used for measuring grain moistures. However, several meters are manufactured for moisture measurements on samples of silage or hay.