chamber was closed and the test gas was allowed to permeate across the specimen for a
sufficient time to purge any residual air at downstream chamber. At this time, only test
gas filled the chambers. When the outlet valve of the upstream chamber was closed, the
upstream pressure increased slowly. The upstream pressure can be adjusted by
controlling the pressure regulator. Sufficient time was allowed for attaining steady state
of moving rate of the liquid indicator before beginning to take readings. The distance of
rise of the liquid indicator was measured while the ambient pressures were recorded.
Calculations
The volumetric methodology is used to calculate the permeability by measuring gas
volume transmitted through a specimen. The rate of rise of the liquid indicator is
measured using the capillary tube. The volume flow rate is calculated as follows.
V = slope xa (4.1)
where a, is the cross-sectional area of a capillary tube. The slope is the rate of rise
of the liquid indicator in the capillary tube. The gas transmitted rate (GTR) is calculated
using the ideal gas law as follows.
GTR = o (4.2)
ART
where po is ambient pressure, A is transmitting area of a specimen, R is the
universal gas constant (8.3143 x 103 L-Pa/{mol-K}) and T is ambient temperature. The
permeance P is defined by the ratio between the gas transmission rate and pressure
differential across the thickness of a specimen. Therefore, the permeance P is calculated
as follows.
P G (4.3)
P-Po