Moisture measurements in granulate materials with DKV

The determination of the moisture content is of economic importance in the manufacturing and processing of materials. This understanding can be essential for evaluating the value of raw materials, implementing control measures, and ensuring final product quality. 

Moisture determination has been traditionally measured by analyzing the weight loss of a sample during the drying process, which is very time consuming and labor-intensive method.  

Electromagnetic techniques have proven a valuable tool for moisture determination. The principle of microwave moisture content measuring method is based on the fact that permittivity of water is much higher than that of most dry substances.  

Due to the direct correlation between water content and dielectric properties, microwave sensors can be used to accurately determine moisture content.

To illustrate the dependance of dielectric properties with the moisture content of granulate materials, several samples of quartz sand with different moisture content were measured with the Dielectric Kit for Vials (DKV) developed by ITACA-DIMAS.

Quartz sand is a granular material (see Figure) often used as representative because its properties are very similar to other materials employed in chemical or pharmaceutical industries. 

 

6 samples of quartz sand were mixed with different amounts of water and introduced in vials of 1mL volume. The moisture content of each sample was determined as the mass of water added (ranging from 0% to 5%) to the total mass of dry material. Dielectric properties of mixtures at room temperature were measured by inserting the vials in the DKV and represented in Fig.2. 

The figure shows the relation between dielectric properties (dielectric constant and loss factor) and moisture content of quartz sand samples around the ISM frequency of 2.45 GHz. As expected, there is strong influence of dielectric properties on moisture content at this frequency. Typical measurement deviations (error bars in the figure) are more than adequate for practical moisture determination. 

 

Dielectric materials have arrangement of electric charges carriers that can be displaced by an external electric field, causing an induced polarization. Dielectric properties provide a measure of the materials’ specific polarization, or electric dipole moment per unit volume, under the applied electromagnetic field. Water molecules act as permanent dipoles and rotate to follow an electromagnetic electric field becoming strongly polarized at microwave frequencies, which results in a complex permittivity of water more than 10 times that of most common solid materials. A small amount of water then causes significant changes of the permittivity of the wet material which can be detected using microwave measuring equipment.

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