Measurement of water vapor transmission properties using the cup method – error caused by air buoyancy

Abstract

The determination of water vapor transmission properties is commonly performed using the cup method. The method is based on periodic weighing of the cup assembly over time. If the air density surrounding the balance differs between weighings, measured change of mass is affected by the difference of ambient air buoyant force. For the correction of this effect can be used a mathematical formula presented in the literature and standards. The ASTM E96 states that this correction is important if observing changes of water vapor mass 0–100 mg. This paper therefore analyses possible errors of measured water vapor transmission properties based on a single 100 mg mass change without applying the buoyancy correction. It was found that for most common samples of building materials (sd < 3 m) and regular sizes of cup assemblies the error of a single mass change measurement can be up to ca. ±13 %, while assemblies with higher volume, higher mass, or more vapor tight samples can experience an error up to ±26 %. However, the vapor transmission properties are usually determined based on five consecutive weight changes. In that case the error can be up to five times lower (±2.5–5 %). Regarding the formula of the buoyancy correction the most important parameter was found to be the chosen volume of the cup assembly which, however, depends on particular setup and sample air-permeance. Only two extreme scenarios regarding this topic were analyzed while the full understanding of the accurate correction is still unsolved in its complexity.