One well known method for measuring moisture content of materials such as grain, soils and the like is by measuring the electrical capacitance of the material. The electrical capacitance of the material is directly related to the dielectric constant of the material. The dielectric constant of most soil materials and of dry grain is relatively low. The dielectric constant of water is significantly higher which substantially affects the overall dielectric constant of the materials depending on the concentration of the water in the materials. Therefore, a measurement of the dielectric constant or capacitance of a material can serve as an accurate measurement of the moisture content.
Capacitance sensitive probes are disclosed in U.S. Pat. Nos. 4,044,607 to Deal and 4,399,404 to Resh and in articles by A. M. Thomas, "In situ Measurement of Moisture in Soil and Similar Substances by Fringe Capacitance," J. Sci. Instrum., Vol. 43, 1966, Pages 21-27 and D. Wobschall, "A Frequency Shift Dielectric Soil Moisture Sensor," IEEE Transactions on Geoscience Electronics, Vol. GE-16, No. 2, Apr. 1978, Pages 112-118. These probes generally comprise a pair of electrodes which are so arranged that the test material functions as a dielectric between the electrodes. The electrodes are in an electrical circuit which measures the capacitance by suitable means such as an oscillating LC network. Changes in capacitance cause a measurable frequency shift in the oscillating circuit and thus can provide an indication of the moisture content of the soil.
Capacitance moisture sensing probes however, are by necessity precision instruments. A slight change in the capacitance of soil may represent a substantial difference in moisture content. Therefore, capacitance moisture sensing probes must be highly sensitive to the capacitance and must be carefully calibrated to assure that the measured capacitance is accurately coordinated with the moisture content.
Because of variations in manufacturing tolerances, moisture sensing probes of the same design will not necessarily sense the same capacitance. Consequently, each capacitance sensing probe must be individually calibrated and as a practical matter the probe should be periodically checked to assure that the calibration has been maintained.
The most common method to calibrate a capacitance moisture probe, particularly a probe used to measure the moisture content of soil, is to prepare calibration standards with carefully measured portions of dry sand and water. The probe is inserted into each of the calibration standards and the frequency of the oscillating circuit is recorded. The frequencies then may be calibrated to correspond to the known moisture contents of each of the soil samples. The preparation of the samples is, however, a tedious and painstaking process which is prone to errors. Therefore, the probes tend to be recalibrated infrequently, if at all.
At least one device has been developed to provide a suitable substitute to simulate a particular moisture content. U.S. Pat. No. 4,147,976 to Wang discloses a calibrating device which is compatible with a grain moisture sensor. The grain moisture sensor has the appearance of a cup with an upright electrode centered therein. A second electrode is positioned in the wall of the cup and the capacitance is measured through the grain between the electrodes. The calibrating device comprises a pair of concentric dielectric tubes which form an annular space. The annular space is sealed at both ends and filled with metal, liquid or other materials which provide the device with a predetermined capacitance simulating grain having a particular moisture content. The calibrating device is then inserted into the cup and the capacitance is measured through the annular space. This type of device is, however, unsuitable for calibrating moisture probes of the type which are inserted into the soil because of the geometry of such probes. The electrodes of soil moisture probes are typically mounted on the periphery of a cylinder and are not suitably disposed to measure capacitance through an annular space. Also, outside electrical fields have a tendency to affect the measurement of capacitance by the moisture probe. This problem is not seriously encountered while down in the soil, but out in the open it becomes a greater concern. The Wang device provides no measure of protection against outside electrical fields.
Accordingly, it is an object of the present invention to provide a capacitance standard for use to calibrate capacitance moisture probes which avoids the disadvantages of the prior art as noted above.
It is a more particular object of the present invention to provide a capacitance standard for use to calibrate capacitance moisture probes which provides an inexpensive reliable capacitance measurement and which is not affected by outside electrical fields.