The relationship between relative humidity and wood moisture content is shown in a sorption isotherm. However, the moisture content of a material depend not only on the relative humidity but also on the moisture history; the moisture content during desorption is higher than the moisture content during absorption at the same relative humidity level. In the hygroscopic range, i.e. for moisture contents corresponding to relative humidity levels below ca. 98%, methods for determination of both desorption and absorption isotherms exist. However, rain exposed wood structures reach moisture contents above this range, i.e. moisture contents corresponding to relative humidity levels close to 100%. Sorption isotherms, or suction curves, in this high moisture range is commonly determined using the pressure plate method, but this method is originally designed for desorption experiments. Two main modifications are necessary in order to perform absorption experiments. First, the specimens need to be supplied with water which requires modifications of the pressure plate cell. Second, the specimens cannot come in contact with the water saturated ceramic plate in the pressure chamber until it has reached equilibrium with the applied pressure. Otherwise there is a risk that the lower parts of the specimens reach equilibrium state from desorption instead of absorption and there will be a moisture gradient through the specimen. This paper presents a new method for determination of absorption isotherms using the pressure plate technique and first results on Norway spruce (Picea abies (L.) Karst.) sapwood for one pressure level. In the present method, the specimens are suspended in magnetic clamps until the ceramic plate has equilibrated with the applied pressure. Modifications of the pressure plate cell were made in order to supply the specimens with water.
Keywords: pressure plate, absorption, spruce, moisture content, isotherm
Authors
Fredriksson M.
Division of Building Materials, Lund University, Lund
Johansson P.
Division of Building Materials, Lund University, Lund
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