The Relevance of Water Vapour Transport for Water Vapour Sorption Experiments on Small Wooden Samples

To characterise the sorption behaviour of wood or other hygroscopic materials with the use of water vapour sorption experiments, an accurate understanding of water vapour transport both to and through the sample material is necessary. Within the last decades, various modelling approaches on the sorption kinetics were developed, but there seems to be no general agreement on the relevance of water vapour transport. Using small amounts of grained wood, this study aims to estimate the influence of water vapour transport in sorption experiments on small sample sizes. A theoretical analysis based on a diffusion equation including a simplified sink/source term was carried out and compared with corresponding experiments. Nonlinear isotherm and temperature were shown to have a major impact on the effective diffusion of water vapour, whereas step size in humidity seems to mainly influence processes within the cell wall (e.g. relaxation and reorganisation of microstructure). Considering the paths of stagnant air above sample surface, the anomalous behaviour for a variation in sample thickness of grained wood can be explained. In conclusion, water vapour transport appreciably influences the sorption kinetics of small sample sizes and has to be considered in the modelling and interpretation of water vapour sorption experiments.