Effect of freezing on estimating free water distribution in wood by ultrasonic waves

A simple equation which gives the ratio of path length through air-dry to w ater-saturated regions was proposed for estimating moisture distribution in wood by ultrasonic wave. In regions with free water the ultrasonic velocit y is known to change only slightly with moisture content, It was therefore examined how freezing, which changes the free water from the liquid to the solid state, affects ultrasonic velocity. Ultrasonic velocity was found to be higher in frozen than in untreated specimens in the L, R, and T directio ns at any moisture contents. With increasing moisture content, the ultrason ic velocity of untreated specimens approached the liquid water value (1.50 km/s), whereas the velocity of frozen specimens approached the value of ice (3.23 km/s). In the R and T directions, the velocity difference between un treated and frozen specimens was significant in the region with free water, but it was not significant in the bound water region. This clearly shows t hat the freezing may indicate a quantity of free water in wood. In tests to verify the proposed equation by model specimens, values calculated for fro zen specimens were closer to the pre-determined fixed value than were those for untreated specimens. Freezing is therefore considered to be an effecti ve method for evaluating free water distribution in wood.