Indirect method for grain moisture determination technology
Conductivity moisture meter is designed based on the principle that the conductance or DC resistance of the object changes with its water content, and the water content of the object is detected according to the change of conductance. Its advantage is that the mechanism is simple, the response speed is fast, and the cost is low; the disadvantage is that it is generally necessary to grind the grain and press it into a fixed size and shape resistor, which is not suitable for detecting the moisture of trace and high water content substances. In addition, the electrode is in contact with the sample. The state of the time will also affect the accuracy of the detection.
The capacitance method is designed by taking advantage of the difference in dielectric constant of different substances. At room temperature, the dielectric constant of water is larger than that of other substances (water is 81, grain is about 2-5). As the water content of the material increases, the dielectric constant also increases accordingly. Therefore, if the dielectric constant of the material is detected, the water content in the material can be calculated. According to the different substances to be measured, the electrode structure of the capacitor is also different, mainly including plate type, cylindrical type and other electrode structures. The capacitance method adopts non-contact detection, which has high reliability, is simple and economical, and is easy to maintain. It can be used for online detection and is suitable for detecting high water content. The disadvantage is that there are many influencing factors and complex data. Among the grain moisture meters in the former Soviet Union, 43% were detected by capacitance method. At present, the detection accuracy can reach 0.5%, and the detection time is less than 5 minutes.
The theoretical basis of the infrared absorption moisture meter is Beer's law, and the moisture has a strong absorption band at 1.649m or 1.94/zm by the long infrared radiation. Due to the different water content of the material, the absorption energy of specific wavelength radiation is also different, as long as the absorbance is measured, the water content can be determined. Specific methods include reflection method, projection method, and reflection-projection composite method. It is mainly reflective type used for grain moisture detection. It has the advantages of non-contact, fast speed, continuous detection, large detection range, high accuracy, good stability, etc., and can measure the moisture of conductive substances, and the highest accuracy can reach 0.1%. The disadvantage is that it is affected by the shape and density of the sample. , thickness, etc., it is difficult to detect the moisture inside the material, and the equipment price is relatively high.
The microwave method is to use ultra-high frequency energy to calculate the moisture value through the change of energy loss produced by the sample. Water has a particularly high dielectric constant compared to food, and there is a maximum dielectric loss in the ultra-high frequency range. Its advantage is non-contact measurement, can detect the absolute value of moisture content, and can detect continuously on-line. The disadvantage is that affected by shape, density, thickness, etc., the structure of the instrument is complicated and the price is high.
The neutron moisture meter works on the principle of molecular scattering. Using a neutron source capable of emitting fast neutrons, the emitted fast neutrons meet with substances containing hydrogen nuclei, and collide with each other to slow down to slow neutrons. According to the measured density of slow neutrons, it is possible to know The total amount of hydrogen is calculated to calculate the water content of the substance. It is a relatively advanced online moisture detector, which can detect accurately without destroying the structure of the material and not affecting the normal operation of the material. The neutron moisture meter has the disadvantages of manual calibration and unstable hydrogen scattering.
