Experimental study of resistance type wood moisture sensors for monitoring wood drying process above fibre saturation point/ Takistus-tüüpi puidu niiskuse andurite eksperimentaalne uurimine puidu kuivatamise monitooringul niiskussisaldustel üle kiu küllas

Abstract In wood moisture contents above fibre saturation point (FSP) the measuring accuracy of resistance type sensors starts decreasing and several side effects related to measuring wood moisture begin to occur. These side effects can be taken into account in the measuring process and can be eliminated. This research examined wood charging and discharging in the process of measurements, phenomena related to repeated measurements in both a single electrode insertion spot and different insertion spots in the specimen made of pine (Pinus sylvestris L.) sapwood and black alder (Alnus glutinosa (L.) Gaertn.). The research presented approaching equations separately for the voltage and current of the polarization and depolarization process for the initial phase of the process. Empirical equations were found for wood electrical resistance and electrical capacitance in the polarization process. For black alder possibilities for calibration of a resistance meter and resistance type electrodes were explored. It was found that to improve the calibration accuracy of resistance type measuring electrodes an additional individual calibration with regard to the average moisture content of the specimen may be carried out with the electrodes above FSP. Research results can be implemented in monitoring the wood drying process using resistance type wood moisture sensors, but also in determining the electrical parameters of growing trees.