From Water Vapor Transmission Rate (WVTR) to Intrinsic Water Vapor Permeability (WVP)

ABSTRACT Water vapor transmission rate (WVTR) is widely used to assess breathability in skin‐interfaced electronics. In gravimetric wet‐cup tests, however, the measured flux depends on transport through both the membrane and the apparatus. Apparatus resistance can dominate measurements of highly permeable membranes. Evaporation, headspace transport, membrane permeation, and external vapor spreading are modeled as resistances in series under a common chemical‐potential driving force. The apparatus coefficients are calibrated using open‐cup measurements in the same setup, independently of the reference permeability. For poly(dimethylsiloxane) membranes, where membrane resistance dominates, both a literature air‐gap correction and the calibrated model reduce deviations from the reference. For a highly permeable anion‐exchange membrane (AEM), the air‐gap correction becomes unstable. Relative to a dynamic sweep‐cell reference, the calibrated model reduces mean absolute relative deviation from 49.8% for classical conversion to 6.2%. It also provides model‐based criteria for geometry selection and measurement duration, together with a procedure for converting WVTR to water vapor permeability (WVP).