High Resolution Measurement of the Surface Layer Moisture Content during Drying of Wood Using a Novel Magnetic Resonance Imaging Technique

Abstract

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.

Untersuchung der Wasseraufnahme von mit aminofunktionellem Silan behandeltem Holz über den Feuchtegehalt und mittels Magnetresonanztomographie

“…This gives researchers unlimited possibility to follow processes causing changes occurring over extended periods of time. Based on this potential of MRI (Dvinskikh et al 2011;MacMillan et al 2002;Rosenkilde and Glover 2002;Meder et al 2003;Hameury and Sterley 2006) it was decided to apply this technique in studies on water uptake by wood pre-treated with an organosilicon solution.…”

A method to estimate wood surface moisture content during drying

“…Even though the surface moisture content during drying is important to the final product quality, there has thus far not been a method to measure surface moisture content industrially. There have been academic examples of measuring surface moisture content measurement on small samples with near infrared (NIR), magnetic resonance and X-ray computed tomography scanning (Baettig et al 2006, Rosenkilde and Glover 2002, Watanabe et al 2011, Watanabe et al 2013, Wiberg and Morén 1999. Electrical resistivity has also been used (Yamamoto et al 2013).…”

Within-ring movement of free water in dehydrating Norway spruce sapwood visualized by neutron radiography

“…These dimensional changes cannot solely be explained by cell wall shrinkage of the drier outer wood layer (shell), which remains quite thin during the early stages of drying (Tremblay et al 2000 ;Rosenkilde and Glover 2002 ;Rosenkilde et al 2004 ;Salin 2007 ). Cell wall shrinkage is initiated when a wood conduit reaches the species specifi c MC of fi ber saturation.…”

“…For an experimental setup, where the MD remains in a state of equilibrium during the scanning time and for a smaller fi eld of view, NMR is a good choice (Rosenkilde and Glover 2002 ;Rosenkilde et al 2004 ;Dvinskikh et al 2011 ). However, besides the combined visualization of MC, radial dimensional changes and structural features, the NI technique offers also the possibility of using electronic equipment with ferromagnetic components, such as a balance, to thus observe the bulk water loss of the specimen during the whole dehydration experiment.…”