A New Method of Determining Moisture Flow Coefficients for both Isothermal and Nonisothermal Conditions

The calculation of non-isothermal moisture transport requires a moisture flow coefficient, Dφ(φ,T), for isothermal flow with φ as the moisture state variable. In the non-isothermal case, a second flow coefficient, DTφ(φ,T), is required to account for the temperature gradient. This means that a number of isothermal measurements are required for a few different temperature levels. It also means that a second set of measurements with temperature gradients are required. This paper presents a new method of determining these flow coefficients for a set of moisture states and temperature levels. This is accomplished by measuring the change in mass over time as the sample absorbs or desorbs moisture. The sample is sealed and thermally insulated on all but one side with a heating pad at the top of the sample (within the insulation). This assembly is hung from a balance in a climate box that has a controlled temperature and relative humidity. The sample is exposed to a step-wise change in relative humidity and the change in mass over time is recorded. Care is taken to ensure that the sample is either absorbing or desorbing moisture in order to avoid complications with hysteresis. In the isothermal case, the sample is kept at a constant temperature during the measurements. In the non-isothermal case the sample has a known temperature gradient set by the temperature of the heating pad and the temperature of the climate box. The moisture flow coefficients are obtained from an analysis of the transient changes in mass. Preliminary results for a full set of measurements of a wood sample are presented in the paper.