Talking about the application of grain moisture detector
Abstract: In the process of acquisition, storage, transportation and processing, the detection and control of moisture is very important. The current grain moisture testing program is collected and comprehensively analyzed, and the various existing grain moisture in China is systematically described. Common methods, principles, and measurement characteristics of measurements.
1, introduction
The moisture content of grain is an important indicator for evaluating food quality and is the basic item for food testing. Normal foods contain the right amount of water, and the moisture content is usually kept within a certain range, which is necessary for food to sustain life and maintain its inherent quality and quality. The moisture content of the grain is a variable value due to the early and late harvest, maturity and climatic conditions. The method of detecting grain moisture can be divided into two categories: non-destructive testing and lossy testing. Non-destructive testing refers to the measurement of water content by the physical, optical and chemical properties of the grain itself without destroying the original state and chemical properties of the object to be tested. Lossy detection means that the object to be tested is pulverized or chemically changed during the measurement, making it impossible to protect
Hold the original shape, structure or composition. In these two categories, the non-destructive testing method is more economical, faster, and the fastest growing, and is the mainstream of moisture detection in the world today.
Now I will talk about the main methods commonly used for grain moisture detection.
2. The main method of non-destructive testing of grain moisture
2.1 Direct drying method
Direct drying method refers to placing the sample to be tested in an oven and maintaining it at 130 °C for 19 h according to the ASAE standard. The difference in mass before and after measurement is its moisture content.
2.2 Capacitance method
The capacitance method is based on the dielectric constant of water far greater than the dielectric constant of other components in the grain. The change in moisture content is bound to cause the change in capacitance. The moisture of the grain can be known by measuring the change in capacitance corresponding to the change in moisture in the sample. content.
The measurement principle of the above two methods is very simple, and the technology is relatively mature, but there are some shortcomings: the direct drying method has a long measurement period, many human interference factors, and can not be measured online; the influencing factors of the capacitance method are more Many, it is difficult to meet national standards in terms of accuracy and repeatability. With the development of artificial intelligence and data fusion technology, it provides a new way for data comprehensive processing, and has achieved some gratifying results.
2.3 Infrared heating drying method
The infrared heating drying method uses infrared rays to heat the sample to make it lose water, thereby achieving the purpose of measuring the moisture content. The main influencing factors are temperature and heating time, which cannot be measured online.
2.4 Microwave heating method
The microwave heating method uses the ultra-high frequency microwave of 2 450 MHz or 915 MHz generated by the magnetron of the microwave oven to rapidly oscillate the water molecules in the grain, causing the molecules to collide and rub against each other, thereby removing moisture from the grain. These two methods shorten the measurement cycle and reduce energy consumption compared to traditional drying methods. Among them, the infrared method does not need heating medium, which improves the utilization rate of heat energy; the microwave method is convenient to operate, and can measure a plurality of samples at the same time, but it has a temperature layer effect and an angular effect, which causes unevenness of microwaves, thereby affecting measurement accuracy.
2.5 ray method
Near-infrared reflectance spectroscopy (NIRS) was applied to grain moisture determination in 1964. Since different molecules have different characteristics of absorption of near-infrared light of different wavelengths, when the sample is irradiated with near-infrared light (wavelength of 1 940 nm), the intensity of diffuse reflection light is related to the composition content of the sample, subject to Lambert-Bill law. The method is fast and simple, and does not need to dry the grain. It only needs to flow before the instrument to detect, but it is only a surface measurement technology, it is difficult to reflect the volume of water (internal moisture) of the whole material, and the measurement accuracy is affected by the grain grain. Size, shape and density effects.
2.6 Microwave absorption method
The microwave absorption method began in the 1840s. It uses the moisture in the grain to absorb the microwave energy or the microwave cavity resonance frequency and phase parameters to indirectly measure the moisture content. The utility model has the advantages of high sensitivity, high speed, safety, no damage to materials, continuous measurement on line, and easy measurement of on-line digitization and visualization; the disadvantage is that the detection limit is not low enough, and it is easy to cause standing wave interference, and the measured value is related to material composition, different varieties Need to be calibrated separately.
3 The main method of grain moisture damage detection
3.1 105 °C constant weight method
The water in the pulverized quantitative pattern is completely vaporized and evaporated by a temperature slightly higher than the boiling point of water (105 ° ± 2 ° C), and the moisture content is calculated based on the mass of the lost water. This method is one of the most commonly used standard methods for moisture detection.
3.2 fixed temperature timing drying method
This method is also called 130 ° ± 2 ° C electric oven method. The principle is as follows: the pulverized sample is weighed in an oven of a certain specification, and dried in an oven of a prescribed heating temperature for a certain period of time, and the difference in quality before and after drying is the moisture content.
3.3 double baking method
Double drying is mainly used to measure high water content. When measuring, first weigh 20 g ~ 30 g of the whole sample, put it in an oven at 105 °C for 30 min, take out the cooling weighed mass, then pulverize it, and then dry it with 105 °C constant weight method.
3.4 tunnel oven method
The tunnel oven method is also a kind of constant temperature timing method. It combines the quadrant scale with the oven. After drying the sample, it can be weighed directly by the quadrant without cooling, and the moisture content of the sample can be read directly on the quadrant scale. .
Of course, we can also measure the water content of the grain according to the senses. By comparing with the standard sample, it is possible to identify the sensory and identify the impurities by hand; the hand touches the eye, the eye, the bite, and the sound to judge the moisture.
For example, the moisture content of soybean is identified. The sensory identification of soybean water content is mainly the application of the method of tooth breaking, and it depends on different seasons. The water is the same and the season is different, and the feeling of tooth broken is different. Winter: When the water content is below 12%, it can be 4 to 5 pieces after the tooth is broken; when the water content is 12% to 13%, although it can be broken, it cannot be broken into many pieces; when the water is about 14% to 15%, After the tooth is broken, the bean is not broken and forms a shoulder. The bean is cracked into many small mouths. The traces of the teeth will remain on the beans, and the part of the bean that has been bitten by the teeth will be transparent. Summer: When the moisture is below 12%, the beans can be broken and squeaked; when the moisture is above 12%, the broken surface is not easy to break when the teeth are broken. Good quality soybeans - below 12% moisture, and sub-quality soybeans - more than 12% moisture.
4 Conclusion
Moisture is an important quality indicator that must be measured during grain storage, acquisition, processing, and transportation. In recent decades, grain moisture detection technology has developed rapidly, forming a variety of moisture detection methods. Generally speaking, it can be divided into two major categories: direct method and indirect method. The direct method is to directly detect the absolute water content in the grain by the drying method and the chemical method, and the detection precision is high, but it is time-consuming and is not suitable for online and on-site detection. The indirect method is to indirectly measure the moisture of a substance by detecting the physical quantity related to moisture (for example, the conductivity of the substance, the dielectric constant, etc.), and the speed is generally fast, and the on-line detection is easy. At present, the oven method is more commonly used, and of course, other methods are applied to actual production.