Loss detection means that during the measurement process, the material to be tested is crushed or chemically changed so that it cannot maintain its original shape, structure, or composition. In these two categories, the non-destructive testing method is more economical, faster, and the most rapid development. It is the mainstream of moisture detection in the world today.
Direct Drying The direct drying method refers to placing the sample to be tested in an oven. According to the ASAE standard, it is maintained at a temperature of 130° C. for 19 hours. The difference in quality before and after the measurement is the moisture content.
Capacitance method is based on the principle that the dielectric constant of moisture is much larger than the dielectric constant of other components in food. The change in moisture content is bound to cause the principle of capacitance change. The food can be known by measuring the change in capacitance corresponding to the change in moisture in the sample. The moisture content. The representative instrument is SCY-1A. The measurement accuracy is ≤0.3%, the measurement time is 5s, and the water measurement range is 10%~20%. The main influencing factors are temperature, variety, and compactness. The law can be measured online. The measurement principle of the above two methods is very simple, and the technology is also relatively mature, but there are deficiencies: the direct drying method has a long measurement cycle, many human interference factors, and can not be measured online; the influencing factors of the capacitance method More often than not, it is difficult to meet national standards in accuracy and repeatability. With the development of artificial intelligence and data fusion technologies, new approaches have been provided for comprehensive data processing, and some gratifying results have been achieved.
Microwave heating The microwave heating method uses the 2450 MHz or 915 MHz ultra-high frequency microwaves generated by the microwave oven's magnetron to rapidly oscillate the water molecules in the grain, causing the molecules to collide and rub against each other, thereby removing the moisture in the grain. The representative instrument is MMA30, the measurement accuracy is ≤0.01%, the measurement time is 100s, and the water measurement range is 12%~100%. The main influencing factors are microwave power, grain quality, density and dielectric properties. This method cannot be measured online. Compared with traditional drying methods, these two methods shorten the measurement cycle and reduce energy consumption. Among them, the infrared method does not require heating medium, improving the thermal energy utilization; microwave method is easy to operate, and can simultaneously measure a variety of samples, but it has a warm layer effect and angular effect, resulting in non-uniform microwave, which affect the measurement accuracy.
The study of dielectric loss angle method shows that the grain moisture content is different, and the dielectric loss angle is also different, and it is a piecewise piecewise linear relationship. The method is economical and practical, with high measurement accuracy, and is particularly suitable for measuring high moisture grains. The representative instrument is MSA6450. The measurement time is 0.1s, and the water measurement range is 1%~30%. The main influencing factors are temperature and variety. The law can be measured online.
Complex impedance split capacitor method complex impedance split capacitor method through the complex impedance separation circuit design, effectively eliminate the influence of the resistance parameters, and only retain the changes in the capacitance parameters. This method is of great significance for improving the measurement accuracy of the capacitive moisture meter.
High-frequency impedance method The high-frequency impedance method is based on the theory that the moisture content of grain and its AC impedance show a logarithmic relationship under the applied electric field in the sensitive frequency band (100k~250kHz). The representative instrument is LSK-1, the measurement accuracy is ≤0.5%, and the measurement time is 1.2s. The main influencing factors are temperature, variety, compactness and electrode spacing. This method cannot be measured online.
The frictional resistance method has a linear relationship between the dynamic frictional resistance of the grain and the moisture content, high water content, and high frictional resistance. The method has few interference factors, low interference intensity, stable and reliable sensing technology, convenient calibration, flexible adjustment, long service life, low price, and easy realization of automatic control.
Acoustics In 1986, Harrenstein and Brusewitz studied the measurement of collision noise in mobile grains. Studies have shown that the elasticity and vibration characteristics of grain grains depend on the grain moisture, and the sound pressures produced by different moisture grains when they hit the surface of objects in the flow process are different. Acoustic measurements are reproducible, but noise signal shielding is a challenge. The representative instrument is an acoustic moisture tester with a measurement accuracy of ≤0.25% and a measurement time of 0.007 seconds. The main influencing factors are noise, grain size and shape. The law can be measured online. The above three methods are currently waiting for further development and have great potential. Friction resistance method and acoustic method are expected to achieve on-line measurement in theory, but there are many interference factors at present, and some problems still need further discussion. High-frequency impedance method has developed a smart plug-type rapid moisture analyzer, the product has passed the test of grain and oil industry inspection and promotion in the grain and oil system, and was rated as national key new products.
NMR NMR is the reorientation of spins of atomic nuclei under certain conditions, so that grain absorbs the energy of the electromagnetic field at a certain frequency, and the amount of absorbed energy is proportional to the number of nuclei contained in the sample. The method has the advantages of rapid detection, high precision and wide measurement range, and can distinguish between free water and combined water. The disadvantage of the method is that the instrument is expensive, maintenance cost is large, and accurate calibration is needed. The representative instrument is a nuclear magnetic resonance moisture tester with a measurement accuracy of ≤0.5% and a water measurement range of 0.05% to 100%. The main influencing factors are the material flow rate, bulk density, and temperature, which can be measured on-line.
The ray method near-infrared reflectance spectroscopy (NIRS) was applied to the measurement of grain moisture in 1964. Because different molecules have different characteristics of absorption of different wavelengths of near-infrared light, when the sample is irradiated with near-infrared light (wavelength is 1940 nm), the intensity of the diffusely reflected light is related to the content of the components of the sample, obeying Lambert-Beer law. The method is fast and simple to measure, does not need to dry food, and can be detected only by flowing in front of the instrument, but it is only a surface measurement technology and it is difficult to reflect the volumetric moisture (internal moisture) of the entire material. The measurement accuracy is affected by the grain Size, shape and density effects. The representative instrument is XY617-B, the measurement accuracy is ≤0.2%, the measurement time is 0.04s, and the water measurement range is 0 to 45%. The main influencing factors are the grain size, shape and density. The law can be measured online.
The microwave absorption method began in the 1940s, and it used the moisture in the grain to indirectly measure the moisture content of the microwave energy absorption or microwave cavity resonance frequency and phase parameters with the change of moisture. The advantages are high sensitivity, fast speed, safety, no damage to materials, continuous on-line measurement, and easy digitization and visualization of measurement signals. The disadvantage is that the detection lower limit is not low enough, and it easily causes VSWR interference. The measured values ​​are related to the material composition. Different types Separate calibration. The on-line microwave moisture analyzer is on behalf of the instrument. The measurement accuracy is ±0.1%, the measurement time is 0.5s, and the water measurement range is 0 to 40%. The main influencing factors are variety, material, shape and density, and online measurement is possible.
Neutron Moisture Analyzer Since the successful neutron-type moisture meter researched by the United States in the 1940s, various countries have successively developed and commercialized neutron moisture meters for various applications. It measures the moisture content of grain by measuring the number of voltage pulses generated in a slow-neutron detector. The neutron moisture meter has the advantages of high linearity, high sensitivity of the water segmented instrument, frozen grain moisture still detectable, does not damage the grain structure, does not affect the normal operation of the grain, etc.; the disadvantage is that the scattering characteristics of hydrogen are not stable, the theory has not yet Perfection requires manual calibration, and grain density and measurement volume size have a greater impact on its accuracy. The representative instrument is Model 503 with a measurement accuracy of ±0.5% and a water measurement range of 0 to 20%. The main influencing factors are density and volume. The law can be measured online.
The 105°C constant weight method vaporizes and evaporates the moisture in the pulverized quantitative sample at a temperature slightly higher than the boiling point of water (105°±2°C), and calculates the moisture content based on the mass of the lost moisture. This method is one of the most commonly used standard methods for moisture detection.
Vacuum Drying Weighing This method utilizes vacuum processing techniques, small quantitative determination techniques, and data processing techniques to determine moisture. It is not affected by the shape of the measured material, no special pretreatment, simple operation, high reliability, and can detect trace moisture. The representative instrument is VME type, the measurement accuracy is ≤0.01%, and the water measurement range is 0.01%~10%. This method cannot be used for on-line measurement. The direct current resistance method for drying grain has a large DC resistance, and the resistance of water is very small. The change in the moisture content of the tested sample will inevitably cause changes in its conductivity. The higher the moisture content, the smaller the resistance. By measuring the resistance of the sample, the moisture content can be determined indirectly. Due to the large resistance of the tested sample, which affects the sampling, the resistance must be reduced to obtain a larger sampling signal. Therefore, this method generally requires that the sample be crushed and then measured. 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. The law can be measured online.
Toluene Distillation This is a more common method of chemical determination of water. It uses a water-insoluble solvent (toluene, xylene) to form a binary azeotropic system with a lower boiling point, and the water in the sample is distilled off. The measurement accuracy is slightly higher than the general drying method and is mainly used for moisture measurement in oils and fats. Due to the fact that the wall of the container of this method is liable to adhere to the distilled water, certain errors will be caused.
The Karl Fischer method is a classical method for measuring moisture and is widely used. It uses the principle of a quantitative chemical reaction between water and iodine and sulfurous acid in the presence of methanol and pyridine, and measures the moisture content based on the consumption of iodine. The Karl Fischer moisture meter is classified into the volumetric method and the Coulomb method, both of which require calibration with a moisture standard substance. This method is mainly used for trace moisture measurement and has high detection accuracy. However, the cost of the reagent is also very expensive, installation is troublesome, and the circuit is complicated. The representative instrument is MKS-500, the measurement accuracy is ±0.015%, and the water detection range is from 10% to 100%. The main influencing factor is the reagent measurement error. This method cannot be measured online.
The pressure water reacts with calcium carbide to produce acetylene. Under certain conditions, the pressure of acetylene gas is linear with the water content. The above three methods are based on chemical reactions to measure grain moisture. The pressure method is in the research phase and Karl Fischer has been used as a standard method in some countries. Toluene distillation method due to the error is large, so the current application is not a lot.
Halogen fast method is currently the most representative manufacturer in the market: Shenzhen Guanya Electronic Technology Co., Ltd.
Halogen moisture meter is a new line of products in recent years, and it has been favored by many companies once it is listed. The heating source adopts a ring halogen tube, the heating is relatively uniform, and the performance is stable. Features: Ring heating, stable performance, short test time Use industry: Except flammable, explosive, volatile industry Use environment: Development trend of moisture detection technology in the laboratory, production line, etc. Although there are many kinds of moisture meters, their market potential is Differently, the rapid development of computer technology, atomic technology and semiconductor technology has provided a broad space for the development of grain moisture detection technology. In order to achieve all-digital, real-time online measurement, fast non-destructive testing technology must be guaranteed. With the need of non-destructive testing technology, non-destructive testing instruments will gradually be standardized, generalized and serialized. The promotion of large-scale programmable logic devices and digital signal processors and the reduction of their cost will certainly accelerate their application in non-destructive testing technology. The application will not only improve the speed of signal acquisition and processing, satisfy a large amount of real-time requirements in the market, but also shorten the development time and increase the functionality and scalability of the hardware. The application of computer software and hardware in non-destructive testing technology will realize the automatic compensation of temperature and other important detection factors, which will enable the development of testing instruments from the past single-purpose to multi-purpose, suitable for non-destructive testing in many different environments.
Direct Drying The direct drying method refers to placing the sample to be tested in an oven. According to the ASAE standard, it is maintained at a temperature of 130° C. for 19 hours. The difference in quality before and after the measurement is the moisture content.
Capacitance method is based on the principle that the dielectric constant of moisture is much larger than the dielectric constant of other components in food. The change in moisture content is bound to cause the principle of capacitance change. The food can be known by measuring the change in capacitance corresponding to the change in moisture in the sample. The moisture content. The representative instrument is SCY-1A. The measurement accuracy is ≤0.3%, the measurement time is 5s, and the water measurement range is 10%~20%. The main influencing factors are temperature, variety, and compactness. The law can be measured online. The measurement principle of the above two methods is very simple, and the technology is also relatively mature, but there are deficiencies: the direct drying method has a long measurement cycle, many human interference factors, and can not be measured online; the influencing factors of the capacitance method More often than not, it is difficult to meet national standards in accuracy and repeatability. With the development of artificial intelligence and data fusion technologies, new approaches have been provided for comprehensive data processing, and some gratifying results have been achieved.
Microwave heating The microwave heating method uses the 2450 MHz or 915 MHz ultra-high frequency microwaves generated by the microwave oven's magnetron to rapidly oscillate the water molecules in the grain, causing the molecules to collide and rub against each other, thereby removing the moisture in the grain. The representative instrument is MMA30, the measurement accuracy is ≤0.01%, the measurement time is 100s, and the water measurement range is 12%~100%. The main influencing factors are microwave power, grain quality, density and dielectric properties. This method cannot be measured online. Compared with traditional drying methods, these two methods shorten the measurement cycle and reduce energy consumption. Among them, the infrared method does not require heating medium, improving the thermal energy utilization; microwave method is easy to operate, and can simultaneously measure a variety of samples, but it has a warm layer effect and angular effect, resulting in non-uniform microwave, which affect the measurement accuracy.
The study of dielectric loss angle method shows that the grain moisture content is different, and the dielectric loss angle is also different, and it is a piecewise piecewise linear relationship. The method is economical and practical, with high measurement accuracy, and is particularly suitable for measuring high moisture grains. The representative instrument is MSA6450. The measurement time is 0.1s, and the water measurement range is 1%~30%. The main influencing factors are temperature and variety. The law can be measured online.
Complex impedance split capacitor method complex impedance split capacitor method through the complex impedance separation circuit design, effectively eliminate the influence of the resistance parameters, and only retain the changes in the capacitance parameters. This method is of great significance for improving the measurement accuracy of the capacitive moisture meter.
High-frequency impedance method The high-frequency impedance method is based on the theory that the moisture content of grain and its AC impedance show a logarithmic relationship under the applied electric field in the sensitive frequency band (100k~250kHz). The representative instrument is LSK-1, the measurement accuracy is ≤0.5%, and the measurement time is 1.2s. The main influencing factors are temperature, variety, compactness and electrode spacing. This method cannot be measured online.
The frictional resistance method has a linear relationship between the dynamic frictional resistance of the grain and the moisture content, high water content, and high frictional resistance. The method has few interference factors, low interference intensity, stable and reliable sensing technology, convenient calibration, flexible adjustment, long service life, low price, and easy realization of automatic control.
Acoustics In 1986, Harrenstein and Brusewitz studied the measurement of collision noise in mobile grains. Studies have shown that the elasticity and vibration characteristics of grain grains depend on the grain moisture, and the sound pressures produced by different moisture grains when they hit the surface of objects in the flow process are different. Acoustic measurements are reproducible, but noise signal shielding is a challenge. The representative instrument is an acoustic moisture tester with a measurement accuracy of ≤0.25% and a measurement time of 0.007 seconds. The main influencing factors are noise, grain size and shape. The law can be measured online. The above three methods are currently waiting for further development and have great potential. Friction resistance method and acoustic method are expected to achieve on-line measurement in theory, but there are many interference factors at present, and some problems still need further discussion. High-frequency impedance method has developed a smart plug-type rapid moisture analyzer, the product has passed the test of grain and oil industry inspection and promotion in the grain and oil system, and was rated as national key new products.
NMR NMR is the reorientation of spins of atomic nuclei under certain conditions, so that grain absorbs the energy of the electromagnetic field at a certain frequency, and the amount of absorbed energy is proportional to the number of nuclei contained in the sample. The method has the advantages of rapid detection, high precision and wide measurement range, and can distinguish between free water and combined water. The disadvantage of the method is that the instrument is expensive, maintenance cost is large, and accurate calibration is needed. The representative instrument is a nuclear magnetic resonance moisture tester with a measurement accuracy of ≤0.5% and a water measurement range of 0.05% to 100%. The main influencing factors are the material flow rate, bulk density, and temperature, which can be measured on-line.
The ray method near-infrared reflectance spectroscopy (NIRS) was applied to the measurement of grain moisture in 1964. Because different molecules have different characteristics of absorption of different wavelengths of near-infrared light, when the sample is irradiated with near-infrared light (wavelength is 1940 nm), the intensity of the diffusely reflected light is related to the content of the components of the sample, obeying Lambert-Beer law. The method is fast and simple to measure, does not need to dry food, and can be detected only by flowing in front of the instrument, but it is only a surface measurement technology and it is difficult to reflect the volumetric moisture (internal moisture) of the entire material. The measurement accuracy is affected by the grain Size, shape and density effects. The representative instrument is XY617-B, the measurement accuracy is ≤0.2%, the measurement time is 0.04s, and the water measurement range is 0 to 45%. The main influencing factors are the grain size, shape and density. The law can be measured online.
The microwave absorption method began in the 1940s, and it used the moisture in the grain to indirectly measure the moisture content of the microwave energy absorption or microwave cavity resonance frequency and phase parameters with the change of moisture. The advantages are high sensitivity, fast speed, safety, no damage to materials, continuous on-line measurement, and easy digitization and visualization of measurement signals. The disadvantage is that the detection lower limit is not low enough, and it easily causes VSWR interference. The measured values ​​are related to the material composition. Different types Separate calibration. The on-line microwave moisture analyzer is on behalf of the instrument. The measurement accuracy is ±0.1%, the measurement time is 0.5s, and the water measurement range is 0 to 40%. The main influencing factors are variety, material, shape and density, and online measurement is possible.
Neutron Moisture Analyzer Since the successful neutron-type moisture meter researched by the United States in the 1940s, various countries have successively developed and commercialized neutron moisture meters for various applications. It measures the moisture content of grain by measuring the number of voltage pulses generated in a slow-neutron detector. The neutron moisture meter has the advantages of high linearity, high sensitivity of the water segmented instrument, frozen grain moisture still detectable, does not damage the grain structure, does not affect the normal operation of the grain, etc.; the disadvantage is that the scattering characteristics of hydrogen are not stable, the theory has not yet Perfection requires manual calibration, and grain density and measurement volume size have a greater impact on its accuracy. The representative instrument is Model 503 with a measurement accuracy of ±0.5% and a water measurement range of 0 to 20%. The main influencing factors are density and volume. The law can be measured online.
The 105°C constant weight method vaporizes and evaporates the moisture in the pulverized quantitative sample at a temperature slightly higher than the boiling point of water (105°±2°C), and calculates the moisture content based on the mass of the lost moisture. This method is one of the most commonly used standard methods for moisture detection.
Vacuum Drying Weighing This method utilizes vacuum processing techniques, small quantitative determination techniques, and data processing techniques to determine moisture. It is not affected by the shape of the measured material, no special pretreatment, simple operation, high reliability, and can detect trace moisture. The representative instrument is VME type, the measurement accuracy is ≤0.01%, and the water measurement range is 0.01%~10%. This method cannot be used for on-line measurement. The direct current resistance method for drying grain has a large DC resistance, and the resistance of water is very small. The change in the moisture content of the tested sample will inevitably cause changes in its conductivity. The higher the moisture content, the smaller the resistance. By measuring the resistance of the sample, the moisture content can be determined indirectly. Due to the large resistance of the tested sample, which affects the sampling, the resistance must be reduced to obtain a larger sampling signal. Therefore, this method generally requires that the sample be crushed and then measured. 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. The law can be measured online.
Toluene Distillation This is a more common method of chemical determination of water. It uses a water-insoluble solvent (toluene, xylene) to form a binary azeotropic system with a lower boiling point, and the water in the sample is distilled off. The measurement accuracy is slightly higher than the general drying method and is mainly used for moisture measurement in oils and fats. Due to the fact that the wall of the container of this method is liable to adhere to the distilled water, certain errors will be caused.
The Karl Fischer method is a classical method for measuring moisture and is widely used. It uses the principle of a quantitative chemical reaction between water and iodine and sulfurous acid in the presence of methanol and pyridine, and measures the moisture content based on the consumption of iodine. The Karl Fischer moisture meter is classified into the volumetric method and the Coulomb method, both of which require calibration with a moisture standard substance. This method is mainly used for trace moisture measurement and has high detection accuracy. However, the cost of the reagent is also very expensive, installation is troublesome, and the circuit is complicated. The representative instrument is MKS-500, the measurement accuracy is ±0.015%, and the water detection range is from 10% to 100%. The main influencing factor is the reagent measurement error. This method cannot be measured online.
The pressure water reacts with calcium carbide to produce acetylene. Under certain conditions, the pressure of acetylene gas is linear with the water content. The above three methods are based on chemical reactions to measure grain moisture. The pressure method is in the research phase and Karl Fischer has been used as a standard method in some countries. Toluene distillation method due to the error is large, so the current application is not a lot.
Halogen fast method is currently the most representative manufacturer in the market: Shenzhen Guanya Electronic Technology Co., Ltd.
Halogen moisture meter is a new line of products in recent years, and it has been favored by many companies once it is listed. The heating source adopts a ring halogen tube, the heating is relatively uniform, and the performance is stable. Features: Ring heating, stable performance, short test time Use industry: Except flammable, explosive, volatile industry Use environment: Development trend of moisture detection technology in the laboratory, production line, etc. Although there are many kinds of moisture meters, their market potential is Differently, the rapid development of computer technology, atomic technology and semiconductor technology has provided a broad space for the development of grain moisture detection technology. In order to achieve all-digital, real-time online measurement, fast non-destructive testing technology must be guaranteed. With the need of non-destructive testing technology, non-destructive testing instruments will gradually be standardized, generalized and serialized. The promotion of large-scale programmable logic devices and digital signal processors and the reduction of their cost will certainly accelerate their application in non-destructive testing technology. The application will not only improve the speed of signal acquisition and processing, satisfy a large amount of real-time requirements in the market, but also shorten the development time and increase the functionality and scalability of the hardware. The application of computer software and hardware in non-destructive testing technology will realize the automatic compensation of temperature and other important detection factors, which will enable the development of testing instruments from the past single-purpose to multi-purpose, suitable for non-destructive testing in many different environments.
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