The invention generally relates to microwave sensors for detecting moisture content in agricultural materials.
Moisture content of materials is a key parameter in many research and industrial applications, including the food and agriculture-related industries. The most widely used standard techniques for moisture content determination are various forms of oven drying. These techniques are based on drying samples under specific conditions, such as temperature and time, depending on the material. Besides being energy and time consuming, in some instances the representative character of the samples might be questionable compared to the whole volume or mass of material under consideration. Moreover, most industrial processes are highly automated and require real-time, on-line measurement of the moisture content.
For grains, moisture is an important factor affecting price paid for grain. Therefore, moisture content must be determined whenever grain is traded. If moisture content is too high at the time of harvest, the grain kernels can be damaged in the mechanical harvesting process, leaving them more susceptible to infection by fungi. If they are stored at moisture contents too high for the prevailing environment, they can spoil because of the action of microorganisms, and the value is degraded or completely lost for human and animal consumption. Electrical measurement methods have been developed that depend on correlations between the electrical properties of the grain and moisture content and grain moisture meters in the United States today are predominantly those operating at about 149 MHz and that sense the dielectric properties (relative permittivity) of the grain samples.
Research on sensing moisture content in grain by microwave measurements has indicated two important advantages for microwave frequencies. The inconsistency of moisture measurements by instruments operating in the low frequency range may be due, in part, to the influence of ionic conduction on the measured dielectric properties at high moisture levels. At microwave frequencies, the influence of ionic conduction is negligible, and better correlations between permittivity and moisture content can be expected.
In U.S. Pat. No. 8,629,681, Trabelsi et al provide a method that appeared to solve some of the problems associated with the measurement of density and moisture content of bulk materials. FIG. 1 shows part of the basic concept of the microwave measurement system 10 proposed by Trabelsi et al, which includes a rectangular sample holder 20 that is placed between two directional antennas. One antenna acts as a transmit (Tx) antenna 30 and the other acts as a receive (Rx) antenna 40. The phase and attenuation of the microwave beam as it traverses sample holder 20 is measured with and without peanuts in sample holder 20. Trabelsi et al appear to teach that a relative change in the phase and attenuation of the microwave beam that traverses sample holder 20 is used to estimate the moisture content of the peanuts. A correction is then applied to correct for the temperature of the peanut sample.
During the testing of the Trabelsi '681 moisture meter prototype in a grain lab, it was discovered that the moisture meter provided adequate results, however the levels of accuracy and repeatability of results demanded by users in the market are higher. Hence there is a need in the market for a microwave-based moisture meter system exhibiting higher levels of measurement accuracy with consistent repeatability of outcomes.