Log moisture content determination with quantitative magnetic resonance
University of New Brunswick
Magnetic resonance (MR) and magnetic resonance imaging (MRI) were employed to study water in black spruce (Picea mariana Mill.) and American aspen (Populus tremuloides Michx.) logs. A recently developed 4.46 MHz unilateral magnetic resonance (UMR) device was employed for non-destructive measurements of moisture content (MC) in wood. This device has a region of homogenous magnetic field located approximately 1.3 cm away from the top surface of the magnet. The radio frequency coil in combination with the static magnetic field topology gives a sensitive spot for measurement of approximately 1 cm3[centimetre cubed]. This permits measuring signal within a finite volume inside the sample, through the bark. The phase transition of water in freshly cut black spruce sapwood was investigated and it was determined that substantial unfrozen water was found to exist below 0°C. The UMR instrument worked well in monitoring water in thawed wood samples. MRI was employed to verify the UMR results and to understand freeze/thaw behavior of log samples that might be expected in the field. A ring boundary behavior during the thawing process was observed. Time-domain MR measurements easily distinguish water in different environments in wood according to the spin-spin relaxation time and provide quantitative information on water content. Both black spruce and aspen had two signal components which correspond to cell wall and lumen water at MC above the fiber saturation point. The cell wall water content was constant above 40% MC. No signal from lumen water was detected at or below 20% MC in either species. Spatially resolved measurements of MC were undertaken by UMR and MRI to understand drying behaviors of the different regions in black spruce wood logs. The MRI technique can spatially resolve, in 3D, water content in wood. Diffusive drying occurred at the ends of the log for both sapwood and heartwood. Radial drying through the bark was observed in the interior of the log. Lastly, a field study was undertaken to demonstrate a viable approach to estimating the whole sample MC of black spruce logs by measuring sapwood MC. Results indicated that sapwood measurement spots displaced from the ends gave good predictions of log MC.