Water Vapour Transfer through Microporous Membranes and Polyester Textiles at Combinations of Temperature and Pressure that Simulate Elevated Altitudes

The water vapour permeability resistances of a commercial polytetrafluoroethylene (PTFE) membrane and some general polyester (PET) textiles were measured at simulated altitude conditions, whose temperature and pressure were both controlled simultaneously. The experiment was carried out using a simple apparatus in which the effect of the air layer could be eliminated on either side of the test specimen. The water vapour permeability resistance of the PTFE membrane was not appreciably influenced by either the ambient temperature or the atmospheric pressure, and was found to be almost constant with a value of approximately 4 sec/m. On the other hand, the resistances of some PET textiles decreased with the increasing simulated altitude. They were roughly proportional to the inverse of the diffusion coefficient of water vapour in air. The reasons for this difference are discussed in terms of the pore sizes of the specimens and the intermolecular mean free path of water vapour. Thus, In the case of normal textile materials, it was concluded that the water vapour resistance of clothing systems decreases with increasing altitude.