Detection technology and development trend of grain moisture Abstract: The existing methods, principles and characteristics of grain moisture detection at home and abroad are comprehensively and systematically expounded, the commonly used grain moisture measuring instruments and their main performance indexes are analyzed, the test errors are analyzed and discussed, and the development direction of grain moisture detection technology is pointed out, which provides theoretical basis for grain moisture research and related personnel. Keywords: agronomy; Grain moisture detection; Overview; Development trend 0 Foreword The detection methods of grain moisture can be divided into two categories: nondestructive detection and destructive detection. Non-destructive testing refers to measuring the water content of grain through its physical, optical and chemical properties without destroying its original state and chemical properties [1]; Destructive detection means that the measured object is crushed or chemically changed during the measurement process, so that it cannot maintain its original shape, structure or composition. Among these two types, nondestructive testing is more economical, faster and fastest developing, and it is the mainstream of moisture detection in the world today. 1 main methods for nondestructive testing of grain moisture 1. 1 direct drying method and capacitance method 1. 1 direct drying method refers to placing the sample to be tested in an oven and keeping it at130℃ according to ASAE standard [2]. The principle of 1. 1.2 capacitance method is that the dielectric constant of water is much greater than that of other components in grain, and the change of water content will inevitably lead to the change of capacitance. By measuring the capacitance change corresponding to the moisture change in the sample, the moisture content of grain can be known [3]. The representative instrument is SCY- 1A, its measuring accuracy is less than or equal to 0.3%, the measuring time is 5s, the measuring range of water is 10%~20%, and the main influencing factors are temperature, variety and compactness. This method can be used for on-line measurement. The measurement principles of the above two methods are simple and the technologies are mature, but both of them have disadvantages: the direct drying method has a long measurement period and many human interference factors, so it can not be used for online measurement; Capacitance method has many influencing factors, and its accuracy and repeatability are difficult to meet the national standards. With the development of artificial intelligence and data fusion technology, it provides a new way for comprehensive data processing, and has achieved some gratifying results [4]. 1.2 infrared heating drying method, microwave heating method, dielectric loss angle method and complex impedance separation capacitance method 1.2. 1 infrared heating drying method dehydrates samples by infrared heating, so as to achieve the purpose of measuring water content. The representative instrument is FD-600, the measuring accuracy is 0. 1%, the measuring time is 1200s, the measuring range of water is 0~ 100%, and the main influencing factors are temperature and heating time. This method cannot be used for on-line measurement. 1.2.2 microwave heating method microwave heating method is to use the ultra-high frequency microwave of 2450MHz or 9 15MHz generated by the magnetron of microwave oven to make the water molecules in the grain oscillate rapidly, so that the molecules collide and rub against each other, and then remove the water in the grain [5]. The representative instrument is MMA30, the measuring accuracy is ≤0.0 1%, the measuring time is 100s, and the measuring range of water is 12%~ 100%. The main influencing factors are the power, particle quality, density and dielectric characteristics of microwave oven. This method cannot be used for on-line measurement. Compared with traditional drying methods, these two methods shorten the measurement period and reduce energy consumption. Among them, infrared method does not need heating medium, which improves the utilization rate of heat energy; The microwave method is simple to operate and can measure a variety of samples at the same time, but there are temperature layer effect and angle effect, which cause uneven microwave, thus affecting the measurement accuracy. 1.2.3 The dielectric loss angle method shows that the dielectric loss angle varies with the water content of grain, and it shows a single-value piecewise linear relationship. This method is economical and practical, with high measurement accuracy, especially suitable for the measurement of high moisture grain. The representative instrument is MSA6450, the measuring time is 0. 1s, the measuring range of water is 1%~30%, and the main influencing factors are temperature and variety. This method can be used for on-line measurement. 1.2.4 complex impedance separation capacitance method Through the design of complex impedance separation circuit, the influence of resistance parameters is effectively eliminated, and only the change of capacitance parameters is reserved [6]. This method is of great significance to improve the measurement accuracy of capacitive moisture meter. 1.3 high frequency impedance method, frictional resistance method, acoustic method 1.3. 1 high frequency impedance method are based on the theory that there is a logarithmic relationship between grain moisture and its AC impedance when an external electric field is applied in the sensitive frequency band (100k~250kHz) [7]. The representative instrument is LSK- 1, the measurement accuracy is ≤0.5%, and the measurement time is1.2s. The main influencing factors are temperature, variety, compactness and electrode spacing. This method cannot be used for on-line measurement. 1.3.2 friction resistance method The dynamic friction resistance of grain has a linear relationship with water content, with high water content and large friction resistance [8]. This method has the advantages of few interference factors, low interference intensity, stable and reliable sensing technology, convenient calibration, long service life, low price and easy automatic control. 1.3.3 acoustic method 1986, Harrenstein and Brusewitz studied the measurement method of collision noise of flowing particles [9]. The research shows that the elasticity and vibration characteristics of grain seeds depend on the grain moisture, and the sound pressure produced by the grain with different moisture is different when it collides with the surface of the object during the flow. Acoustic measurement has good repeatability [10], but noise signal shielding is a difficult problem. The representative instrument is an acoustic moisture tester, the measurement accuracy is less than 0.25%, the measurement time is 0.007s s, and the main influencing factors are noise, particle size and shape. This method can be used for on-line measurement. The above three methods are all potential methods that need further development at present. Both friction resistance method and acoustic method are expected to realize online measurement in theory, but there are many interference factors at present, and some problems need to be further discussed. An intelligent plug-in rapid moisture analyzer was developed by high frequency impedance method, which passed the test of grain and oil industry and was popularized and used in grain and oil system, and was rated as a national key new product. 1.4 NMR * * * vibration method, ray method and neutron method1.4.1NMR * * vibration method reorients the nuclear spins under certain conditions, so that the grains can absorb the energy of electromagnetic field with a certain frequency, and the absorbed energy is directly proportional to the number of nuclei contained in the sample [65438]. Its disadvantage is that the instrument is expensive, the maintenance cost is high, and it needs accurate calibration. The representative instrument is a nuclear magnetic vibration water analyzer, with a measurement accuracy of ≤0.5% and a water measurement range of 0.05%~ 100%. The main influencing factors are material flow, bulk density and temperature, which can be measured online. 1 .4.2 x-ray method1) Near infrared reflectance spectroscopy (NIRS) was applied to the determination of grain moisture in 1964. Because different molecules have different absorption characteristics for near-infrared light with different wavelengths, when the sample is irradiated with near-infrared light (wavelength is 1940nm), the intensity of diffuse reflection light is related to the component content of the sample and obeys Lambert-Beer law. This method is fast and simple, and can be detected only by flowing in front of the instrument without drying the grain, but it is only a surface measurement technique, which is difficult to reflect the volume moisture (internal moisture) of the whole material, and the measurement accuracy is affected by the size, shape and density of the grain. The representative instrument is XY6 17-B, the measuring accuracy is ≤0.2%, the measuring time is 0.04s, and the measuring range is 0~45%. The main influencing factors are particle size, shape and density. This method can be used for on-line measurement. 2) The microwave absorption method began in the 1940s of 19, which indirectly measures the moisture content by using the moisture in grain to absorb microwave energy or the change of parameters such as resonant frequency and phase of microwave cavity with moisture [13]. Its advantages are high sensitivity, high speed, safety, no damage to materials, on-line continuous measurement, and easy on-line digitization and visualization of measurement signals; The disadvantage is that the lower detection limit is not low enough, which is easy to cause standing wave interference. The measured value is related to the material composition, and different varieties need to be calibrated separately. The representative instrument is an on-line microwave moisture meter, with a measuring accuracy of 0.65438 0%, a measuring time of 0.5s and a moisture measuring range of 0-40%. The main influencing factors are variety, material, shape and density, which can be measured online. 1.4.3 neutron moisture meter Since the United States successfully developed the neutron moisture meter in the 1940s, all countries in the world have developed neutron moisture meters for various purposes and commercialized them. It measures the moisture content of grain by measuring the number of voltage pulses generated by the slow neutron detector. Neutron moisture analyzer has the advantages of high linearity, high moisture slicing instrument sensitivity, measurement of frozen grain moisture, no damage to grain structure, no impact on normal operation of grain and so on. The disadvantage is that the scattering characteristics of hydrogen are unstable and the theory is not perfect, which requires manual calibration. In addition, particle density and measurement volume have great influence on its accuracy. The representative instrument is Model 503, the measuring accuracy is 0.5%, the measuring range of water is 0 ~ 20%, and the main influencing factors are density and volume. This method can be used for on-line measurement. 2. Main methods for detecting grain moisture loss 2. 1 oven method 2. 1. 1.005℃ constant weight method uses the temperature slightly higher than the boiling point of water (1.05 2℃) to evaporate all the water in the quantitative grinding mode, and calculates the water content according to the quality of the lost water. This method is one of the most commonly used standard methods for moisture detection. 2. 1.2 Constant temperature and regular drying method This method is also called130 2℃ electric oven method. Its principle is: weigh the crushed sample, put it into a drying box with a certain specification, and dry it in an oven with a specified heating temperature for a certain period of time. The difference in quality before and after drying is the moisture content. 2. 1.3 Double drying method Double drying method is mainly used to measure high water content grain. When measuring, first weigh 20 ~ 30g of the whole sample, put it in the oven at 105℃ for 30min, take it out, cool it, weigh it, then crush it, and then dry it by the constant weight method at 105℃. 2. 1.4 tunnel oven method the tunnel oven method is also a constant temperature timing method, which combines the image limit calibration with the oven. After the sample is dried, it is directly weighed with a quadrant scale without cooling, and the water content of the sample can be directly read out on the quadrant scale. 2.2 rapid weight loss method, vacuum drying method and DC resistance method 2.2. 1 rapid weight loss method This method is to dry materials at extreme weight loss temperature, and the main difference from the classical oven method is that the drying temperature is different. It can measure all powdery materials, and is mainly used to measure corn moisture at present. 2.2.2 Vacuum drying weighing method This method uses vacuum processing technology, micro-quantitative determination technology and data processing technology to determine moisture. It is not affected by the shape of the measured substance, does not need special pretreatment, has simple operation and high reliability, and can detect trace moisture. The representative instrument is VME type, the measuring accuracy is ≤0.0 1%, and the measuring range of water is 0.0 1%~ 10%. This method cannot be used for on-line measurement. 2. 2. 3 DC resistance method The DC resistance of grain drying is very large, but the resistance of water is very small, so the change of water content of the tested sample will inevitably lead to the change of its conductivity. The higher the water content, the smaller the resistance. By measuring the resistance of the sample, the water content can be determined indirectly. Because the resistance of the measured sample is large, which affects the detection and sampling, it is necessary to reduce the resistance to obtain a large sampling signal, so this method generally requires that the sample be crushed before measurement [14, 15]. The representative instrument is LSKC-4B, the measurement accuracy is 0.5%, and the water measurement range is 10%~20%. The main influencing factors are temperature, variety, compactness and electrode spacing. This method can be used for on-line measurement. 2.3 Toluene distillation method, Karl Fischer method and pressure method 2.3. 1 Toluene distillation method This is a commonly used chemical water measurement method, which uses water-insoluble solvents (toluene and xylene) to form a binary * * * boiling system with low boiling point to distill the water in the sample. The measuring accuracy is slightly higher than that of general drying method, and it is mainly used for measuring moisture in oil. Because this method is easy to attach distilled water to the container wall, it will cause certain errors. 2.3.2 Karl Fischer method Karl Fischer method is a classical method for moisture determination, which is widely used. It uses the principle that water reacts quantitatively with iodine and sulfurous acid in the presence of methanol and pyridine to determine the water content according to the consumption of iodine. Karl Fischer moisture scoring method can be divided into two categories: volumetric method and coulometric method, both of which need to be calibrated with moisture reference materials. This method is mainly used for the measurement of trace moisture, with high detection accuracy, but the reagent cost is also very expensive, the installation is troublesome and the circuit is complicated. The representative instrument is MKS-500, the measurement accuracy is 0.0 15%, the water measurement range is 10%~ 100%, and the main influencing factor is the reagent measurement error. This method cannot be used for on-line measurement. 2.3.3 Pressure method Water reacts with calcium carbide to generate acetylene. Under certain conditions, the pressure of acetylene gas has a linear relationship with its water content. The above three methods are all based on chemical reaction to determine the water content of grain. The pressure method is in the research stage, and Karl Fischer method has been adopted as the standard method in some countries. Toluene distillation has not been widely used at present because of its large error. 3 Development trend of moisture detection technology Although there are many kinds of moisture meters, the market potential is different. The rapid development of computer technology, atomic technology and semiconductor technology provides a broad development space for grain moisture detection technology. In order to realize full digital and real-time online measurement, fast nondestructive testing technology must be used as a guarantee. With the demand of nondestructive testing technology, nondestructive testing instruments will gradually realize standardization, generalization and serialization. The popularization and cost reduction of large-scale programmable logic devices and digital signal processors will accelerate their application in nondestructive testing technology, which can not only improve the speed of signal acquisition and processing, meet a large number of real-time requirements in the market, but also shorten the development time and increase the function and expansibility of hardware. The application of computer software and hardware in nondestructive testing technology will realize the automatic compensation of important testing factors such as temperature, and make the testing instrument develop from simplification to multi-purpose in the past, which is suitable for nondestructive testing in many different environments. The rapid development of internet technology will bring a qualitative leap to nondestructive testing technology, realize multi-user sharing and remote control, and avoid the waste of manpower, material resources and financial resources. Reference: [1] Chen Bin. Huang Xingyi. New nondestructive testing technology for food and agricultural products [M]. Beijing Chemical Industry Press 2004: 29-30. [2] Asae standard S352. 1 moisture measurement-cereals and seeds [s]. Asae, St. Joseph, Michigan 49085, 1986. Candalat. Application of capacitance sensor in single nuclear measurement [J]. Moisture content of corn. Am Soc Ag Eng:86-88。 [4] Zhai Baofeng. Research on grain moisture detection technology based on data fusion [D]. Shenyang: Shenyang University of Technology, 2002. [5] Rebecca. Study on determination of grain moisture by microwave heating [J]. Jiangsu Flavored Food, 2003,78:18-19. [6] Li Yede. 1995,26(2):73- 77.[7] Teng Zhaosheng, Zhou, Tong Diaosheng. Conductive bathtub effect of grain and new problems: which website was this found? Can you send me the website?