Ultrasensitive moisture content characterization of wood samples by a cylindrical cavity resonator

https://doi.org/10.1016/j.sna.2020.112298Get rights and content

Highlights

  • An ultrasensitive, non-destructive moisture content sensor for wood samples.

  • The sensor utilizes a cylindrical cavity resonator with enhanced field homogeneity.

  • A theoretical relation between moisture content and resonance frequency is derived.

  • Measurements are performed for drying wood samples and a freshly cut tree branch.

  • The sensor has a potential to be employed towards the prevention of forest fires.

Abstract

Ultrasensitive moisture content sensing in wood samples is demonstrated employing a modified cylindrical cavity resonator geometry. A vertical sensing channel is introduced at the middle section of the cavity for sample placement. Field homogeneity inside the sensing channel is improved by a thin hollow quartz inset. A relation between the moisture content of wood and the resonance frequency for the dominant TM010-mode is extracted for the operation frequency range. Full-wave simulations are used to characterize and optimize the sensing technique. For experimental verification, a wood sample with 24% moisture content is allowed to dry in the cavity at room temperature for 24 h and its resonance frequency is monitored. The experiment is repeated for a sample which partially fills the sensing channel. Finally, the variation of the moisture content of a freshly cut tree branch is monitored in vitro. The experiments and simulations reveal that the proposed technique offers a very high sensitivity for characterization of the moisture content of wood samples of different radii, owing to the very high quality factor of the cylindrical cavity resonator. The technique is thus a promising candidate for the non-destructive monitoring of the moisture content of trees towards the prevention of forest fires and for the evaluation of products made from wood in industry.

Introduction

Determination of the moisture content in organic and synthetic materials is essential for many reasons. For materials with a commercial value, relative exposure to humidity and the resulting moisture content have important implications in terms of the quality of the end product [1], [2]. Primarily, two types of portable moisture meters (resistance and capacitance based sensors) are the most commonly used products in the industry with varying levels of accuracy and reliability [3]. In academic literature, a wide range of methods have been proposed for determination of the moisture content and relative humidity, such as capacitance-based techniques [4], [5], infrared and near-infrared spectroscopy [3], ultrasound [6], radio frequency identification (RFID) [7], [8], [9], [10], [11] and polyimide-based thin-films [12], [13] among many others. All of these methods have advantages and vulnerabilities in terms of accuracy, sensitivity, and practicality; and one or the other can be preferred depending on the specific application for which the moisture content measurement is made.

Recent years have seen an increase in the number of forest fires, probably in relation with climate change [14]. Monitoring the moisture content within the tree branches can be an essential effort towards the prevention of forest fires. In addition to the aforementioned moisture sensing methods, resistive probes [15], piezoceramic transducers [16] and passive microwave probes [17] have also been shown for specifically determining the wood moisture content. In particular, resistive measurement techniques which directly contact the wood sample can provide very good accuracy, but the contacting electrodes are problematic in terms of reproducibility. For stem water content monitoring, the measurement results depend on the contacting pressure and for long-term measurements at living trees, the contact resistance changes due to corrosion of the electrodes and resin production of the tree. Contactless measurements are, therefore, the preferable choice for such applications. Also, a non-destructive measurement method is preferable over a destructive technique, since such techniques are both harmful for the tree and require more labor. There is a long list of non-destructive methods used for this purpose. Among the most accurate techniques are the gamma ray densitometry and magnetic resonance imaging (MRI); but the former provides risks in terms of human health while the latter is costly and impractical [18].

In addition to helping prevention of forest fires, determination of the wood moisture content is also important in evaluation of products made of wood. Wood is an invaluable natural material with a high level of versatility, which makes it suitable for many applications. The moisture content of wood is critical in terms of its value as a product; the nominal weight and the price of wood are directly related with its water content [19]. In addition, exposure to high humidity makes wood more susceptible to fungal decay [15]. It is known that the moisture content of wood directly affects its dielectric properties [20]. Therefore, methods that aim to analyze the moisture content generally depend on this relation.

Microwave sensing methods offer significant advantages compared to other techniques, such as having a higher precision and speed, allowing for non-destructive or non-invasive measurements, and not causing radiation [21]. Cavity resonator technique is a commonly used microwave technique for material characterization owing to the high sensitivity enabled by the high microwave energy storage and low loss of the cavities. This makes them ideal for detecting very small changes in certain properties of the sample under test, such as water density or thickness of continuously streaming dielectric materials [22], [25]. The moisture content of industrial materials has been analyzed using cavity resonator techniques. For example, Despres and Akyel studied the moisture content of wine corks at 929 MHz and 1.53 GHz [23]. Also, a cylindrical cavity resonator at 5 GHz was employed by Bourdel et al. to monitor the moisture content of cigarettes in a fabrication line [24]. Finally, the granule moisture was investigated by Tian et al. by using a rectangular cavity at 0–1.5 GHz [21]. These works have shown that microwave resonant cavity sensors offer an important advantage in material characterization due to their high sensitivity and accuracy.

In this work, a cylindrical cavity resonator operating in TM010-mode is employed for determining the moisture content in wood samples. The cavity resonator geometry is modified by opening a vertical sensing channel at the middle section and the sample is placed inside this channel. For samples whose radii are smaller than that of the channel and/or are irregularly shaped (such as tree branches), slight movements within the cavity can take place during drying. By inserting a hollow dielectric material around the channel for improving the field homogeneity inside, the effect of these movements on the sensing performance is reduced. The cylindrical cavity resonator operates at a resonance frequency fres of approximately 2.5 GHz. When the sample is present, it leads to a frequency shift due to the modification of the effective permittivity inside the cavity. A further 16% change in the moisture content of the wood sample is observed to create a shift up to 100 MHz in fres in the experiments. To demonstrate the application of stem water content sensing, we monitor the moisture change in a freshly cut-off tree branch. This way, the very slow natural process of water transportation in woody plants is shown in an accelerated fashion. Normally, the suggested use of the cavity resonator does not involve cutting off of the tree since a significant moisture content drop would occur naturally before a fire. Since no probes are inserted into the sample, a method which achieves both high resolution and contactless and non-destructive measurement is demonstrated. This ultrasensitive technique is also shown to be very advantageous for benchtop characterization of moisture content within cylindrical samples for industrial deadwood materials. Although this work is focused on moisture sensing in an organic material, the cavity resonator sensing technique can be applied to any type of samples, including synthetic materials as well.

The organization of the paper is as follows. In Section 2, the design of the cylindrical cavity resonator is discussed and a formulation for determining fres and its connection to the moisture content of the sample is presented. Furthermore, the results of a full-wave simulation of the system are given, and the dependence of the sensing performance on the sample radius is investigated. In Section 3, the results of a set of experiments are shown and discussed where wood samples of different radii are monitored in terms of their effect on the cavity transmission while they are allowed to dry at room temperature. Finally, conclusions are drawn in Section 4.

Section snippets

Resonator design

A modified version of the classical cylindrical cavity resonator geometry is employed in this work for the detection of the moisture content within a material. The tubular sample is placed into a sensing channel introduced at the middle section shown in Fig. 1(a). This sensing channel is circumscribed by a thin quartz hollow insert of thickness ti. The purpose of this insert is to improve the electric field homogeneity through the sensing channel [25]. In this way, for samples with a diameter

Measurements

In this section, an experimental validation of the moisture content sensing technique comprising the cylindrical cavity resonator is discussed. The setup for the measurements is shown in Fig. 5. A Rohde & Schwarz ZVL Vector Network Analyzer (VNA) was used to excite and monitor the resonator. The resonator was placed on a Styrofoam platform, whose dielectric constant of 1.03 is very close to that of air. In this way, the effect of the bottom medium on the resonator was minimized. For the first

Conclusion

In this work, the performance of a moisture sensing technique using a cylindrical cavity resonator operating in TM010-mode was demonstrated. The sensing scheme is focused on the moisture content characterization of wood samples. First, a theoretical background for the relation between the cavity resonance frequency and the moisture content of the wood sample was derived. For this, an exponential fit was used to define the relation between the measured relative permittivity of a wood species and

Authors’ contributions

Burak Ozbey: Simulations, Experiments, Writing – Original draft preparation.

Usman Faz: Methodology, Resonator Design.

Bernhard Wolf: Consulting, Writing – Reviewing and Editing.

Thomas Eibert: Supervision, Methodology, Writing – Reviewing and Editing.

Conflict of interest

The authors declare no conflict of interest.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgment

The first author would like to thank the Technical University of Munich Foundation for its support under the TUFF Fellowship.

Burak Ozbey received the B.S., M.S., and Ph.D. degrees in electrical and electronics engineering from Bilkent University, Ankara, Turkey, in 2008, 2011, and 2016, respectively. From 2017 to 2019, he was with the ElectroScience Laboratory, Ohio State University, USA as a Post-Doctoral Researcher. Since 2019, he has been a Post-Doctoral Fellow at the Technical University of Munich, Munich, Germany. His research interests include microwave and THz systems, structural health monitoring, design and

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    Burak Ozbey received the B.S., M.S., and Ph.D. degrees in electrical and electronics engineering from Bilkent University, Ankara, Turkey, in 2008, 2011, and 2016, respectively. From 2017 to 2019, he was with the ElectroScience Laboratory, Ohio State University, USA as a Post-Doctoral Researcher. Since 2019, he has been a Post-Doctoral Fellow at the Technical University of Munich, Munich, Germany. His research interests include microwave and THz systems, structural health monitoring, design and testing of wireless RF sensors and computational electromagnetics.

    Usman Faz received the B.S. degree in electronics engineering from the Sir Syed University of Engineering and Technology, Karachi, Pakistan, in 2007,and the M.S., and Ph.D. degrees in microwave engineering from the Technical University of Munich, Munich, Germany, in 2009 and 2019, respectively. His current research interests include microwave cavity design and optimization for material sensing and filter applications.

    Bernhard Wolf received the B.Sc., M.Sc. and Ph.D. degrees in biology from Albert-Ludwigs-Universität, Freiburg, Germany. He is a professor emeritus at the Heinz Nixdorf-Chair of Medical Electronics, Technical University of Munich, Munich, Germany. His primary research interests lie in the areas of cell based assays, analytical electron microscopy, bioelectronics and medical electronics, bioelectronics home care systems, biohybrid devices, biosensors, tumor diagnostics and therapy, structured biological modelling, micro-bioreactors and magnetic stimulation. He has published widely in these areas and holds several patents in cell chip monitoring.

    Thomas F. Eibert received the Dipl.-Ing. (FH) degree from FachhochschuleNürnberg, Nuremberg, Germany, the Dipl.-Ing. degree from Ruhr-Universität Bochum, Bochum, Germany, and the Dr.-Ing. degree from BergischeUniversität Wuppertal, Wuppertal, Germany, in 1989, 1992, and 1997, all in electrical engineering. He is currently a Full Professor of high-frequency engineering at the Technical University of Munich, Munich, Germany. His current research interests include numerical electromagnetics, wavepropagation, measurement and field transformation techniques for antennas and scattering as well as all kinds of antenna and microwave circuit technologies for sensors and communications.

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