In China, due to imperfect water detection methods, tens of billions of kilograms of grain are rotten during storage and transportation due to excessive moisture content, causing huge losses. Therefore, grain moisture has always been a very important quality indicator controlled by the grain sector at home and abroad.
Water in grain is divided into free water (free water) and combined water (crystal water) according to physical properties. Free water is the water that is condensed in the capillaries and molecular interstices inside grain grains through physical adsorption: bound water is the water that is adsorbed in grain cells and grain molecular structures through chemical action. Free water has the general properties of ordinary water and has an important impact on the quality of grain. Grain moisture refers to the content of free water.
There are direct and indirect methods for detecting moisture.
The direct method is to directly remove the moisture in the grain by drying or chemical methods, and detect the absolute moisture content of the sample. This method has high detection accuracy, but is time consuming and not suitable for online detection.
The indirect method is to determine the water content of a substance by detecting the physical quantity (such as the electrical conductivity, dielectric constant, etc.) of the substance. Such methods are generally fast, easy to implement online detection, and have good prospects for development and utilization.
Grain moisture measurement technology by indirect method
The conductivity type moisture meter is designed based on the principle that the conductivity or DC resistance of an object changes with its water content, and detects the water content of the object based on the change in conductivity. The 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. The state at the time will also affect the accuracy of detection.
The capacitance method is designed using 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, and grain is about 2 to 5). As the water content of a substance increases, the dielectric constant also increases. Therefore, if the dielectric constant of a substance is detected, the moisture content in the substance can be calculated. Depending on the substance being measured, the electrode structure of the capacitor is also different. There are mainly electrode structures such as flat plate and cylinder. The capacitance method uses non-contact detection, which has high reliability, simple and economical, and easy maintenance. It can be used for online inspection and is suitable for detecting high water content. The disadvantages are many influencing factors and complex data. Among the grain moisture meters in the former Soviet Union, 43% were detected by the capacitance method. At present, the detection accuracy can reach 0.5%, and the detection time is less than 5min.
The theoretical basis of the infrared absorption moisture meter is Beer's law. Moisture has a strong absorption band for long infrared radiation at 1.649m or 1.94 / zm. Because the water content of a substance is different, the absorbed energy for a specific wavelength of 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 type. Used for food moisture detection is mainly reflective. It has the advantages of no contact, fast speed, continuous detection, large detection range, high accuracy, good stability, etc., and it can measure the moisture of conductive materials with the highest accuracy of 0.1%; the disadvantage is that it is affected by the sample shape, density, and thickness And other effects, it is difficult to detect the internal moisture of the substance, and the equipment price is relatively high.
The microwave method uses ultra-high frequency energy to calculate the moisture value through the change in energy loss generated by the sample. Water has a particularly high dielectric constant compared to grain, and there is a maximum dielectric loss in the UHF range. Its advantage is non-contact measurement, which can detect the absolute value of moisture content, and can continuously detect online. The disadvantage is that due to the shape, density, thickness, etc., the instrument structure is complex 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 the material containing hydrogen nuclei, and they 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 used to calculate the water content of the substance. It is a more advanced on-line moisture detector, which can accurately detect without damaging the material structure and affecting the normal operation of the material. The neutron moisture meter has the disadvantages of manual calibration and unstable scattering of hydrogen.