Moisture content determination of thermally modified timber by electrical and ultrasound methods

Thermal modification is an increasingly popular process that increases biological durability and dimensional stability of timber. The equilibrium moisture content of thermally modified timber (TMT) is lower than that of untreated timber, as is the fibre saturation point. For these reasons, traditional methods of moisture content determination have to be reviewed and calibrated specifically for TMT and novel methods may prove more suitable for online measurement systems. In this study, the moisture content (MC) of TMT was measured using conventional dielectric and resistance meters as well as by the air-coupled ultrasound method and electrical impedance spectroscopy (EIS). The timber was kept outdoors for a period of 10 weeks while the MC was determined every other week so that a wide range of MCs could be measured. It was seen from the results that the conventional meters were not readily calibrated for TMT and the measurement range of these methods is small due to lower fibre saturation point. The study showed the potential of EIS and air-coupled ultrasound for MC analysis of TMT. Best correlation with MC at higher MCs was achieved with the maximum amplitude of through transmission signals by the ultrasound method. Higher precision and a wider range of MCs could be measured by combining both EIS and ultrasound parameters.