Methods for determination of moisture conditions in wood exposed to high moisture levels

This study is a start to investigate the relationship between micro climate, i.e. the climate at the wood surface, and the wood moisture content. The micro climate is affected by the construction design; if water gets trapped in joints and stays on wood surfaces, the time during which water is absorbed by the wood increases. To be able to measure moisture content profiles and investigate the relationship between joint design and moisture content, methods for moisture content determination, duration of surface moisture and presence of water in gaps were developed. The moisture content was determined by measuring the electrical conductance between two electrodes inserted in the wood. Unlike the electrodes commonly used for this purpose these electrodes were both small and fastened with electrical conductive adhesive so that the contact between the electrode and the wood remained. The relationship between electrical conductance and moisture content is affected by several wood and experimental variables and calibration experiments were therefore performed. Since the study concerns wood outdoors, the relationship was determined mainly for high moisture contents. Specimens were conditioned to different relative humidity (RH) levels using two different methods. Between 64% and 95% RH, saturated salt solutions were used and for high RH levels (98-100%), specimens were conditioned using the pressure plate method. The relationship between conductance and moisture content was obtained for moisture contents between 18% and 180%. Since the specimens in the pressure plate experiment were not fully equilibrated, the curve is preliminary. The pressure plate experiment also resulted in a preliminary desorption isotherm between 98% and 100% RH. A method for determination of absorption isotherms at high RH levels is also suggested