A transient technique for determining diffusion coefficients in hygroscopic materials

Abstract

A transient technique is described to find the diffusion coefficients of moisture transfer in building materials as a function of moisture content. Measurements that take many months by steady-state methods can be done in days or even hours. The material is subjected to a staircase function of relative humidity steps, the transient weight changes noted, and a numerical algorithm used which extracts the Kirchhoff potential, one value for each relative humidity step and corresponding moisture content. Diffusion coefficients are then calculated from the Kirchhoff potential. Experimental results show fair agreement between Kirchhoff potentials found by steady-state methods and those found by this transient technique, but good agreement between the corresponding diffusion coefficients. An important limitation to the technique at this state of its development is the need to know the material sorption curve.