Wolfgang Taute

Resonant microwave sensors for instantaneous determination of moisture in foodstuffs

Abstract Open microwave resonators are very well suited for online moisture monitoring during the production of foodstuffs. In order to allow for rapid measurements, the required information is derived from the dielectric behaviour of the material. Unlike for example the method of Karl Fischer titration, the dielectric microwave method determines the moisture content indirectly and is thus dependent on density changes. The present paper outlines special features of open resonators for density independent moisture measurement. It is demonstrated, how unfavourable design of the window for coupling out the fringing field of the open resonator leads to non-linear behaviour of the characteristics of resonant frequency and reciprocal quality factor vs density and moisture changes. Several engineered open microwave moisture sensors are presented and discussed. More in detail, a surface sensor, a surface sensor equipped with a small container on top, a self contained hand-held surface moisture sensor and a grooving-probe as well as a multi-hole sensor are shown. The electronics and the signal processing of the sensors are briefly described. Finally experimental moisture measurements and a density independent calibration are given for rolled oats. Moisture measurements on whole kernels of oats, barley and wheat demonstrate the only weak dependence of the microwave moisture measurement approach on the special type of material.

Stray field ring resonators and a novel trough guide resonator for precise microwave moisture and density measurements

The paper first describes the design and construction of two different kinds of quasi-planar stray field ring resonators for microwave moisture and density measurements, which are typical for the state of the art, and then the practical performance of such sensors. The changes in resonant frequency and resonator bandwidth are utilized for the determination of the moisture content or the materials density. Nonlinear temperature behaviour is identified as a critical property of conventional stray field resonators, which restricts their measurement accuracy. A novel trough guide resonator is then presented which is entirely manufactured from a ring of alumina oxide ceramics and overcomes the temperature problems. The ring is metallized at its surface except at the front face, which permits a stray field to come into contact with the material under test. The described resonator is capable of withstanding considerable abrasive forces without having a protective cover. At the same time it shows excellent linear and predictive temperature behaviour, which allows precise compensation of the inherent temperature dependence of the resonator. Under constant environmental conditions, the sensibility and accuracy of the old and new resonator designs are exactly the same. However, at varying temperature, an error can be removed to a much higher extent for the new resonators, restricting deviations in the moisture content caused by temperature to typically less than 0.1%, as compared to errors of up to several per cent for the previous designs over a temperature range of ~20–50 °C.

In-line moisture monitoring in fluidized bed granulation using a novel multi-resonance microwave sensor

Microwave resonance technology (MRT) is known as a process analytical technology (PAT) tool for moisture measurements in fluid-bed granulation. It offers a great potential for wet granulation processes even where the suitability of near-infrared (NIR) spectroscopy is limited, e.g. colored granules, large variations in bulk density. However, previous sensor systems operating around a single resonance frequency showed limitations above approx. 7.5% granule moisture. This paper describes the application of a novel sensor working with four resonance frequencies. In-line data of all four resonance frequencies were collected and further processed. Based on calculation of density-independent microwave moisture values multiple linear regression (MLR) models using Karl-Fischer titration (KF) as well as loss on drying (LOD) as reference methods were build. Rapid, reliable in-process moisture control (RMSEP≤0.5%) even at higher moisture contents was achieved.

Real-time process monitoring in a semi-continuous fluid-bed dryer – microwave resonance technology versus near-infrared spectroscopy

The trend towards continuous manufacturing in the pharmaceutical industry is associated with an increasing demand for advanced control strategies. It is a mandatory requirement to obtain reliable real-time information on critical quality attributes (CQA) during every process step as the decision on diversion of material needs to be performed fast and automatically. Where possible, production equipment should provide redundant systems for in-process control (IPC) measurements to ensure continuous process monitoring even if one of the systems is not available. In this paper, two methods for real-time monitoring of granule moisture in a semi-continuous fluid-bed drying unit are compared. While near-infrared (NIR) spectroscopy has already proven to be a suitable process analytical technology (PAT) tool for moisture measurements in fluid-bed applications, microwave resonance technology (MRT) showed difficulties to monitor moistures above 8% until recently. The results indicate, that the newly developed MRT sensor operating at four resonances is capable to compete with NIR spectroscopy. While NIR spectra were preprocessed by mean centering and first derivative before application of partial least squares (PLS) regression to build predictive models (RMSEP = 0.20%), microwave moisture values of two resonances sufficed to build a statistically close multiple linear regression (MLR) model (RMSEP = 0.07%) for moisture prediction. Thereby, it could be verified that moisture monitoring by MRT sensor systems could be a valuable alternative to NIR spectroscopy or could be used as a redundant system providing great ease of application.