Conventional moisture measurement methods often have problems with accuracy when the material density varies or high moisture ranges are to be determined. This paper addresses density-independent moisture measurement in moving bulk materials using microwave techniques. Four measurement methods are compared, one using a fixed frequency and the other using different frequency ranges. The characteristics, limitations, and material dependencies of these methods are examined, as well as their suitability for continuous moisture measurement in moving bulk materials. These fluctuating bulk materials occur frequently, especially in materials such as wood chips. Using the transmission method of microwave transmission with two antennas, material moisture can also be determined independently of density on the basis of permittivity. By cleverly choosing the frequency and processing attenuation and phase shifts, moisture contents of up to 20% can be measured independently of the material density. Moisture measurement methods often have problems distinguishing between high and low moisture contents at extreme moisture contents due to signal saturation. By measuring a wide frequency range, much higher moisture contents and ranges can be determined. In addition, a variant is presented that detects high moisture content with a small frequency range. Our approach allows high-resolution measurements to be performed on continuously changing materials at significantly accelerated rates. This accelerated process allows near instantaneous visualization of frequency sweep results. The measurements were performed on moving bulk materials.

Moisture measurement in industrial processes is of great importance for quality control and energy saving. One possible method for moisture measurement is microwave transmission using two antennas, where the material moisture is determined based on the permittivity. However, multiple reflections between the antennas can distort and misrepresent the measurements. In this context, this article deals with the design of tapered slot antennas (TSAs) specifically developed for moisture measurements in natural materials such as wood chips or paper. Design aspects of the antenna as a density-independent moisture sensor with respect to improved selectivity to water content in a composite natural material are explained and requirements concerning frequency range and bandwidth are given. In particular, the advantages of specially designed dual antipodal Vivaldi antennas are compared to those antenna types, which are usually used for such applications, namely horn antennas. Design aspects are discussed and new approaches are proposed to improve manufacturability by printing the antenna on a flexible substrate and wrapping it around a block of low-loss foam. A manufactured antenna is experimentally fully characterized by measurements. Low reflectivity at the free spaceports is verified, and its superiority in comparison to horn antennas is documented.

In this paper, the design of a novel multi-resonance microwave resonance sensor for moisture monitoring of pharmaceutical granulations is reviewed. This sensor allows a reliable determination of residual moisture directly in the production container. The reliably determinable measuring range could be extended from below 10% to at least 20% independent of other physical attributes and thus covers the complete moisture range typically occurring in fluid bed granulations. For this purpose, experimental prototypes with a novel electronic structure were developed. In order to enable the installation in the various granulators, prototypes with different dimensions of the sensor heads were realized. Within this publication, the measurement principle will be reviewed and prospects for the multi-resonance sensor will be provided.