A Detailed Guide to Moisture Analyzers

Across every industry, a new technology goes through several stages from emergence to product to mass adoption. Want to know which stage moisture detection technology has reached? Let's take a look.

Destructive testing

refers to measurement in which the sample is crushed or chemically altered so that it no longer retains its original shape, structure, or composition. Of the two categories, non-destructive testing is more economical and faster, and is developing most rapidly—it is today's mainstream in moisture detection.

Direct Drying Method

In direct oven drying, the sample is placed in an oven and, according to theASAEstandard, held at130°C for a specified time19h. The mass difference before and after measurement is the moisture content.

Capacitance method

The capacitance method is based on the fact that the dielectric constant of water is far higher than that of other grain components, so any change in moisture content causes a corresponding change in capacitance. Measuring the capacitance change corresponding to the moisture change yields the grain's moisture content. The representative instrument is the capacitance moisture meter, with a measurement accuracy of≤0.3%, a measurement time of5s, a moisture range of10%~20%, and main influencing factors of temperature, variety, and compactness，This method allows online measurement. Both methods above have very simple principles and relatively mature technology, but each has shortcomings:。Direct oven drying takes a long time, involves many human factors, and cannot measure online; the capacitance method is affected by many factors and struggles to meet national standards in accuracy and repeatability. The development of artificial intelligence and data fusion technology has opened new paths for comprehensive data processing, and encouraging results have already been achieved.

Infrared heating and drying method

In infrared heating and drying, infrared radiation heats the sample to remove water, thereby measuring the moisture content。The representative instrument is the infrared moisture analyzer, with an accuracy of±0.1%, a measurement time of1200s, a moisture range of0~100%; the main influencing factors are temperature and heating time. This method cannot measure online.

Microwave heating method

Microwave heating uses ultra-high-frequency microwaves of2450MHzor915MHzproduced by a microwave oven magnetron to rapidly oscillate water molecules in grain; collision and friction between the molecules removes moisture from the grain. The representative instrument is the laboratory microwave moisture analyzer, with an accuracy of ≤0.01%, a measurement time of100s, a moisture range of12%~100%; the main influencing factors are microwave power, grain mass, density, and dielectric properties. This method cannot measure online. Compared with conventional drying, both methods shorten measurement time and reduce energy consumption: infrared heating needs no heating medium and improves thermal efficiency, while microwave heating is easy to operate and can measure multiple samples at once; however, it suffers from layering and corner effects that cause uneven microwave distribution and affect accuracy.

Dielectric loss angle method

Studies show that the dielectric loss angle of grain varies with moisture content in a single-valued, piecewise-linear relationship. The method is economical, practical, and highly accurate, and is especially suitable for high-moisture grain. The representative instrument is the grain moisture analyzer, with a measurement time of0.1s, a moisture range of1%~30%; the main influencing factors are temperature and variety. This method allows online measurement.

Complex-impedance separated capacitance method

By designing a complex-impedance separation circuit, this method effectively eliminates the influence of the resistance component and retains only the capacitance variation. It is of great significance for improving the accuracy of capacitance moisture meters.

High-frequency impedance method

The high-frequency impedance method measures moisture based on the theory that within the sensitive frequency band(100k~250kHz)and under an applied electric field, grain moisture has a logarithmic relationship with AC impedance. The accuracy is ≤0.5%, a measurement time of1.2s; the main influencing factors are temperature, variety, compactness, and electrode spacing. This method cannot measure online.

Frictional resistance method

The dynamic frictional resistance of grain is linearly related to moisture content: the higher the moisture, the greater the resistance. This method has few and weak interference factors, stable and reliable sensing, easy calibration, flexible adjustment, long service life, low cost, and is easy to automate.

Acoustic method

1986InHarrensteinandBrusewitzstudied methods for measuring the collision noise of flowing grain. Research shows that the elasticity and vibration characteristics of grain kernels depend on their moisture, and grain with different moisture levels produces different sound pressures when colliding with surfaces as it flows. The acoustic method has good repeatability, but shielding the noise signal is a challenge. The representative instrument is the acoustic moisture tester, with an accuracy of ≤0.25%, a measurement time of0.007s; the main influencing factors are noise, kernel size, and shape. This method allows online measurement. The above3methods are highly promising and await further development. In theory, both the frictional resistance method and the acoustic method could achieve online measurement, but many interference factors remain and some issues need further study. The high-frequency impedance method has already produced an intelligent probe-type rapid moisture analyzer, which has passed testing in the grain and oil industry, been promoted throughout that system, and been named a national key new product.

Nuclear Magnetic Resonance (NMR)

In nuclear magnetic resonance (NMR), atomic nuclear spins reorient under certain conditions so that the grain absorbs energy from an electromagnetic field at a definite frequency; the amount of energy absorbed is proportional to the number of nuclei in the sample. The method is fast, accurate, and wide-ranging, and can distinguish free water from bound water; its drawbacks are expensive instruments, high maintenance costs, and the need for precise calibration. The representative instrument is the NMR moisture tester, with an accuracy of≤0.5%, a moisture range of0.05%~100%; the main influencing factors are material flow, bulk density, and temperature. This method allows online measurement.

Ray method

Near-infrared reflectance spectroscopy (NIRS) was applied to grain moisture measurement in1964. Different molecules characteristically absorb near-infrared light of different wavelengths. When near-infrared light (of wavelength1940nm) illuminates the sample, the intensity of the diffusely reflected light relates to the sample's composition according to the Lambert–Beer law. The method is fast and simple and requires no drying—grain only needs to flow past the instrument—but it is a surface measurement technique that can hardly reflect the bulk (internal) moisture of the material, and its accuracy is affected by kernel size, shape, and density. Representative instrument, accuracy ≤0.2%, a measurement time of0.04s, a moisture range of0~45%; the main influencing factors are kernel size, shape, and density. This method allows online measurement.

Microwave absorption began in the19th century40s. It measures moisture indirectly through the absorption of microwave energy by water in the grain, or through changes in parameters such as the resonant frequency and phase of a microwave cavity as moisture varies. Its advantages are high sensitivity, speed, safety, non-destructiveness, continuous online measurement, and signals that are easy to digitize and visualize online; its disadvantages are a detection limit that is not sufficiently low, susceptibility to standing-wave interference, readings that depend on material composition, and the need for separate calibration for each variety. The representative instrument is the online microwave moisture analyzer, with an accuracy of ±0.1%, a measurement time of0.5s, a moisture range of0~40%; the main influencing factors are variety, material, shape, and density, and online measurement is possible.

Neutron moisture meter

Since the20th century40s, when the United States first developed the neutron moisture meter, countries around the world have successively developed and commercialized neutron moisture meters for various applications. It measures grain moisture by counting the voltage pulses generated in a slow-neutron detector. Advantages include high linearity, high sensitivity at high moisture levels, the ability to measure frozen grain, no damage to grain structure, and no disruption of normal grain flow; disadvantages include unstable hydrogen scattering characteristics, an incomplete theory, the need for manual calibration, and significant influence of grain density and measurement volume on accuracy. Representative instrument, accuracy ±0.5%, moisture range0～20%, with main influencing factors of density and volume. This method allows online measurement.

105°C constant-weight method

Using a temperature slightly above the boiling point of water (105°±2°C), all moisture in a weighed, ground sample is vaporized, and the moisture content is calculated from the mass of water lost. This is one of the most commonly used standard methods in moisture testing.

Fixed-temperature, fixed-time drying method

This method is also known as the130°±2°C electric oven method. A ground sample is weighed into a drying dish of specified size and dried in an oven at a prescribed temperature for a fixed time; the difference in mass before and after drying is the moisture content.

Double-drying method

The double-drying method is mainly used for high-moisture grain. First, a whole-kernel sample is weighed20～30gand placed in105°C oven to dry30minthen removed, cooled, and weighed, after which it is ground and dried again by the105°C constant-weight method.

Tunnel oven method

The tunnel oven method is another fixed-temperature, fixed-time method. It combines a quadrant balance with an oven: after drying, the sample can be weighed directly on the quadrant balance without cooling, and the moisture content can be read directly on the balance.

Rapid loss-on-drying method

This method dries the material at its limiting weight-loss temperature; the main difference from the classical oven method is the drying temperature. It can measure any powder material and is currently used mainly for corn moisture.

Vacuum drying gravimetric method

This method determines moisture using vacuum processing, micro-quantitative measurement, and data processing technologies. It is unaffected by sample shape, needs no special pretreatment, is simple to operate and highly reliable, and can detect trace moisture. The representative instrument is theVMEmodel, with an accuracy of ≤0.01%, a moisture range of0.01%~10%. This method cannot measure online.

DC resistance method

Dry grain has a very high DC resistance while water has a very low one, so any change in the sample's moisture content changes its conductivity: the higher the moisture, the lower the resistance. Moisture can thus be determined indirectly by measuring the sample's resistance. Because the sample's high resistance weakens the measurement signal, the resistance must be lowered to obtain a stronger signal; the sample therefore generally has to be ground before measurement. Representative instrument, accuracy±0.5%, a moisture range of10%~20%; the main influencing factors are temperature, variety, compactness, and electrode spacing. This method allows online measurement.

Toluene distillation method

This is a fairly common chemical moisture method using water-immiscible solvents(toluene and xylene)to form a low-boiling binary azeotrope that distills the water out of the sample. Its accuracy is slightly higher than ordinary drying methods, and it is used mainly for moisture in oils and fats. Because distilled water tends to adhere to the container walls, it introduces some error.

Karl Fischer method

Karl Fischer (Karl Fischer) method is a classic moisture determination technique in wide use. Based on the quantitative chemical reaction of water with iodine and sulfur dioxide in the presence of methanol and pyridine, it determines moisture content from the amount of iodine consumed. Karl Fischer moisture meters fall into two major categories—volumetric and coulometric—both of which require calibration with water reference materials. The method is used mainly for trace moisture measurement: its accuracy is very high, but reagents are expensive, installation is complicated, and the circuitry is complex. Representative instrument, accuracy ±0.015%, a moisture range of10%~100%; the main influencing factor is reagent measurement error. This method cannot measure online.

Pressure method

Water reacts with calcium carbide to form acetylene; under certain conditions, the pressure of the acetylene gas has a linear relationship with the water content. The above3methods all determine grain moisture by chemical reaction. The pressure method is still in the research stage; the Karl Fischer method has been adopted as a standard method in some countries; the toluene distillation method, with its relatively large errors, is not widely used today.

Development Trends in Moisture Detection Technology

Although there are many types of moisture meters, their market potential differs. The rapid development of computer, atomic, and semiconductor technologies has opened broad space for moisture detection. Fast, non-destructive testing is essential for fully digital, real-time online measurement. With growing demand for non-destructive testing, instruments will progressively become standardized, generalized, and serialized; the spread of large-scale programmable logic devices and digital signal processors, together with falling costs, will accelerate their application—speeding up signal acquisition and processing to meet real-time market demand while shortening development time and enhancing hardware functionality and extensibility. The application of computer software and hardware will bring automatic compensation for temperature and other key factors, evolving instruments from single-purpose devices toward multi-purpose tools suited to non-destructive testing in diverse environments. The rapid growth of the Internet will bring a qualitative leap to non-destructive testing—multi-user sharing and remote control, avoiding waste of manpower, materials, and money.