Estimation of moisture content in timber using ultrasonics

Abstract

This paper presents the findings of a research programme on the estimation of moisture content in timber using ultrasonics, by developing working relationships between ultrasonic wave velocity, temperature and percentage moisture content in timber samples during kiln drying. A detailed experimental programme was undertaken to develop relationships between ultrasonic wave propagation velocity and temperature, and between velocity and percentage moisture content for a large number of timber samples. The experimental programme was primarily restricted to samples of jarrah timber, but the procedures can be applied to other species. The results show conclusively that both temperature and moisture content have a significant effect on the ultrasonic wave propagation velocity in timber, whereas material density does not. Thus, ultrasonics can be used to monitor variations in percentage moisture content during the drying process in a kiln environment. A simple field-testing procedure has been developed for monitoring moisture content in timber using ultrasonics either manually (with a hand calculator) or incorporated into a computer program for on-line monitoring. The procedure is based on simple quadratic equations derived from a statistical regression analysis of velocity and temperature relationships, and velocity and percentage moisture-content relationships. To overcome any measurement inaccuracies due to density variations or other factors, the recommended procedure involves the initial calibration of each sample board.