Capacitance-Based Sensing of Material Moisture in Bulk Solids: Applications and Restrictions
This paper presents applications of capacitive techniques for moisture sensing of bulk solids. Based on experimental measurement data for three test materials, theoretical considerations on moisture models in bulk solids are undertaken and the reliability
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Abstract This paper presents applications of capacitive techniques for moisture sensing of bulk solids. Based on experimental measurement data for three test materials, theoretical considerations on moisture models in bulk solids are undertaken and the reliability of these models is discussed. The sensitivity of the capacitive setup on the moisture distribution inside the particles is analyzed by means of a multilayer model and Finite Element Analysis methods. A planar sensor topology featuring spatial resolution is proposed and the cross-sensitivity on material compression as well as the influence of the inter-electrode distance are investigated. Strengths and weaknesses of capacitive moisture sensing are discussed. Keywords Moisture measurement · capacitive sensing · bulk solids · planar setup
1 Introduction Having good and reliable instrumentation for the determination of moisture content in gases and solid material is of importance for many applications in industry. For example, moisture content may have an effect on corrosion and decomposition of material, affects discharging behavior for solids, or may improve/worsen the feasibility of industrial transportation processes and their measurement [6]. Pneumatic conveying of granular material has become a frequently used method to transport granular material in pipelines [20]. In various industrial applications,
Anton Fuchs Institute of Electrical Measurement and Measurement Signal Processing, Graz University of Technology, Graz, Austria, e-mail: [email protected] Hubert Zangl Institute of Electrical Measurement and Measurement Signal Processing, Graz University of Technology, Graz, Austria, e-mail: [email protected] Gert Holler Institute of Electrical Measurement and Measurement Signal Processing, Graz University of Technology, Graz, Austria, e-mail: [email protected]
S.C. Mukhopadhyay, G.S. Gupta (eds.), Smart Sensors and Sensing Technology, c Springer-Verlag Berlin Heidelberg 2008
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such as supply of coal powder to furnaces, loading of harvest and fertilizer, or mixing of cement powder and sand, it is essential to monitor and rate the flow of material in conveyor pipes. A capacitive flowmeter, which has been developed by our research group, allows for the non-invasive determination of the absolute velocity profile and the relative concentration profile of a gas-solids flow [21]. For a reliable mass flow measurement, as needed for many applications, calibration strategies are required to obtain an absolute concentration profile as well. The main challenge for the developed capacitance-based flowmeter is its susceptibility to moisture, since the permittivity of water differs significantly from that of most dry materials [9]. Different approaches on the measurement of the moisture content in granular materials and powders have been undertaken: A standard technique is usually based on oven drying [12], which is destructive and time consuming and requires taking representative samples of the transportation medium. Microwave s
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