Measurements of moisture content gradients were performed during drying of Scots pine (Pinus sylvestris). The measurements were done in heartwood and sapwood separately and in all three directions to grain separately. The aim of these measurements was to evaluate Kirchhoff flow potentials and diffusion coefficients in a moisture content range from about fibre saturation to end-use dried. A new method to evaluate flow coefficients from transient measurements is presented. This new method, called the Kirchhoffian moment method, gave good results in a moisture content range from about 30 % and down to 8 % moisture content.

Keywords SummaryMoisture measurement Magnetic resonance MRI Stray field imaging Wood dryingThe moisture content profile in the surface layer of Scots pine (Pinus sylvestris) sapwood has been measured using the MRI (Magnetic Resonance Imaging) technique. A novel high-gradient permanent magnet was used for the measurement of the moisture profile in the wood during the drying process, giving a depth resolution of better than 20 lm. This paper demonstrates for the first time the possibility of measuring the moisture content depth profile in the surface layer from raw state down to 4 % moisture content without removing the wood sample from the apparatus. The main advantage of this technique and apparatus is a high resolution over a large field of view. It is possible to measure the moisture content in the wood surface layer during drying up to a depth of 300 lm. A single moisture content profile may be obtained in less than 5 min. Hence, more accurate dynamic drying information, on a single sample, may be achieved than with current techniques. In this preliminary study we observed that the wood cells at the surface had a moisture content higher than the equilibrium moisture content for wood in the surrounding bulk air. We conclude that for a wood moisture content of above approximately 50 % the resulting profiles were almost flat from a depth of 90 lm and further into the wood and in between the surface and 90 lm deep there was a steep gradient. Below 50 % there was a gradient in moisture content to at least 300 lm depth.

Moisture content profiles with high spatial pixel resolution, 21 mm, were measured in Scots pine heartwood while drying from wet conditions to near equilibrium in the surface layer, (0-300 mm). The measuring technique used was Magnetic Resonance Imaging. In order to obtain magnetic resonance profiles with a high pixel resolution at both high and low moisture contents, a highgradient permanent magnet was used. The results presented show one-dimensional moisture content profiles and mean moisture content development in the surface layer. In Scots pine heartwood, almost flat moisture content profiles were observed when the moisture content was above the fibre saturation level. When the mean moisture content in the surface layer decreases to the fibre saturation level, gradients start to build up from the surface towards the bulk of the sample although the bulk moisture content is far above the fibre saturation. Furthermore, the results presented in this study imply that a dry shell forms in the surface layer shortly after drying has begun. This dry shell controls the drying rate until the mean moisture content in the bulk decreases to the fibre saturation level.

Sorption experiments with wood based material were performed under conditions similar to those in industrial wood drying kilns. The main purpose of these experiments was to demonstrate the influence of the surface interaction with the ambient air stream on the moisture flow. Part of this work has been published earlier (Söderström and Salin 1993; Samuelsson and Sötlerslröm 1994). The analysis was done with a simple mathematical method for a directly measurable quantity and constant transport coefficients. For industrial applications the results showed good correspondence between measured and calculated values.