Determination of time lag by accurate monitoring of pressure decay in a new generation constant volume system

Abstract

The time-lag method is the standard approach for evaluating membrane permeability, diffusivity and solubility in a single gas permeation experiment. The conventional time-lag method relies on accurately monitoring the pressure rise in a constant volume downstream from the membrane following a change in pressure upstream from the membrane. The same information could be extracted from the upstream pressure decay in the same time-lag experiment. However, accurately monitoring the pressure decay presents a challenge due to the resolution limitations of absolute pressure transducers. If the membrane was characterized based on pressure decay, a mass spectrometer could be used to simultaneously monitor the composition of the gas permeating from the membrane, opening the time-lag method to gas mixtures. Also, the simultaneous monitoring of pressure rise and decay could provide additional information about gas transport in the membrane, which is critical for more complex membrane materials. • The resolution challenge was overcome by splitting the upstream volume into the working and reference volumes and monitoring pressure decay using a differential pressure transducer between the two volumes. • The validity of the measured pressure decay was confirmed by the unique relation between the upstream and downstream time lags for a commercial PDMS membrane.

Keywords

• Determination of membrane time lag based on pressure decay in a constant volume system