A Brief Discussion of Grain Moisture Testing Methods

Grain moisture testing matters greatly for grain procurement, processing, storage and transport. In China, imperfect testing methods cause tens of billions of jin of grain to mold and spoil each year in storage and transport due to excessive moisture—an enormous loss. Grain moisture is therefore a crucial quality indicator controlled by grain departments at home and abroad. Today we briefly review the grain moisture testing methods we have used:~

Understanding Grain Moisture

By physical nature, water in grain is divided into free water(unbound water) and bound water (water of crystallization). Free water is condensed by physical adsorption in the capillaries and molecular gaps inside grain kernels; bound water is chemically adsorbed within grain cells and molecular structures. Free water behaves like ordinary water and greatly affects grain quality—grain moisture refers to the free-water content.

Grain Moisture Testing Methods

Over the past20 years, grain moisture testing technology has developed rapidly, with new methods and ideas emerging constantly. The main methods are listed below.

Direct methods remove water from grain directly by drying or chemical means to determine absolute moisture content. They offer high accuracy but are time-consuming and unsuitable for online testing.

Indirect methods determine moisture by measuring related physical quantities(such as electrical conductivity or dielectric constant). Generally fast and easy to implement online, they have good prospects for development and application.

Indirect Grain Moisture Measurement Technologies

Conductance-type moisture meters are designed around the principle that an object's conductance or DC resistance varies with its moisture content, detecting moisture via changes in conductance. Advantages include simple construction, fast response and low cost; disadvantages include the usual need to grind the grain and press it into a fixed shape and size, unsuitability for trace or very high moisture, and sensitivity to electrode-sample contact conditions, which affects accuracy.

The capacitance method exploits differences in dielectric constant between substances. At room temperature, water's dielectric constant is far higher than that of other substances(water: 81; grain: about 2–5). As moisture increases, the dielectric constant rises correspondingly, so measuring the dielectric constant yields the moisture content. Electrode structures vary with the material measured—mainly flat-plate and cylindrical types. Capacitance measurement is non-contact, reliable, simple, economical, easy to maintain, suitable for online testing and well suited to high-moisture materials. Its disadvantages are many influencing factors and complex data. In the former Soviet Union, 43% of grain moisture meters used the capacitance method; current accuracy reaches 0.5% with testing times under 5 minutes.

The infrared absorption moisture meter is based on Beer's law: moisture strongly absorbs infrared radiation at wavelengths of1.64 µm or 1.94 µm. Because materials with different moisture contents absorb different amounts of energy at specific wavelengths, measuring absorbance completes the moisture determination. Specific implementations include reflection, transmission and combined reflection-transmission; the reflection type is mainly used for grain. It offers non-contact operation, speed, continuous measurement, wide range, high accuracy and good stability, can measure conductive materials, and reaches accuracy up to 0.1%. Its disadvantages are susceptibility to sample shape, density and thickness, difficulty detecting internal moisture, and relatively high equipment cost.

The microwave method calculates moisture from the change in energy loss as ultra-high-frequency energy passes through the sample. Water's dielectric constant is exceptionally high compared with grain, with a maximum dielectric loss in the UHF range. Advantages: non-contact measurement, determination of absolute moisture content, and continuous online detection. Disadvantages: susceptibility to shape, density and thickness, complex instrument structure and high price.

The neutron moisture meter works on the principle of neutron scattering. A source emits fast neutrons that collide with hydrogen nuclei in the material and slow to thermal (slow) neutrons; measuring the slow-neutron density reveals the total hydrogen content and thus the moisture content. It is a relatively advanced online moisture meter, measuring accurately without damaging the material structure or affecting material flow. Its disadvantages include the need for manual calibration and unstable hydrogen scattering.