A new procedure to measure water adsorption isotherms of porous fibrous materials

Abstract A typical relation between accumulated liquid, temperature, and vapour pressure characterizes any surface, in particular any porous structure. This relationship is usually described by means of ‘‘adsorption isotherms’’, which are functions that correlate the amount of liquid in the porous material to the partial pressure of its vapour, for a given constant temperature. Speaking about fibrous insulation materials or building materials, the fluid of interest is simply water. The knowledge of water adsorption isotherms is of great importance in the formulation of heat and mass transfer models inside moist bodies. Typical experimental techniques for adsorption isotherm evaluation (salt solution methods, climatic chamber methods, etc.) require the conteporary control of both temperature and relative humidity of the environment surrounding the specimen. This often results in the need of dedicated and expensive experimental set-ups. In this work the possibility to use a simpler apparatus to obtain the sorption-desorption characteristics of a body is investigated from the theoretical point of view by means of simulated experiments. The proposed measurement procedure uses a closed, non permeable system in which only temperature is to be controlled, while the total water content, liquid plus vapor, remains constant. The new procedure offers an additional benefit, since both the constant water and the constant dry air mass constraints are used as further a priori information to improve the precision of the estimated results.