NEW INTEGRITY TEST DEVELOPED
The binary gas test was developed to increase the level of defect detection sensitivity, and has been found to be particularly
useful for evaluating virus filtration membranes. It is based on the principle of differing gas permeabilites of a gas mixture's
two components through the liquid layer of a wetted membrane.
Unlike the single gas in the gas–liquid diffusion test, the binary gas test relies primarily on the measurement of downstream
gas composition rather than downstream flow rate. In an integral membrane, the permeate gas is depleted of the slower permeating
gas. However, if a defect is present, the leak through the membrane will contaminate the permeate stream, resulting in an
elevated concentration of the slower permeating gas.
Figure 2: Binary gas diffusion through a wetted membrane: (a) integral and (b) non-integral membrane.
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A key advantage of the binary gas test is that the permeate concentration expected through an integral membrane is well defined
and its sensitivity is not compromised by most of the background noise factors cited for the gas-liquid diffusion test. Figures
2a and 2b illustrate how the test works.
USING GASES TO MEASURE DEFECTS
Diffusion can be specified for each component in a gas mixture permeating across a membrane. Assuming that the gas is completely
mixed on both sides of the membrane, the composition of the permeate gas can be calculated from the ratio of diffusive flow
rates of the two components, and the inlet side composition.
The composition of the permeate gas is independent of membrane thickness, tortuosity, porosity, and area. It is also independent
of the pressure difference across the membrane but instead is dependant on the pressure ratio. The permeate composition does,
of course, depend on the feed side composition. To maintain a constant feed side composition, a constant sweep flow must be
applied.
A measured binary gas composition can be used to estimate a defect size in a device. The calculated defect size can, in turn,
be used to estimate the liquid flow rate through the defect and the total volume of liquid passing through the defect for
a given time period. Therefore, the permeate gas concentration can be used to predict the loss in virus log reduction value
(LRV) due to the defect.
Figure 3: Normalized permeability of common gases in water at 25 °C.
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The sensitivity of the binary gas test is related to the selectivity (ratio of permeabilites) of the gases through the liquid
layer. To maximize the sensitivity of the binary gas test, the gas pair should have a high selectivity. Figure 3 shows normalized
permeabilites of some common gases in water. For this study, researchers selected a carbon dioxide/ hexafluoroethane (CO2/C2F6)
pair, with a selectivity of about 1000.
The selection of the concentration of the gases in the mixture was influenced by a number of factors, including the ease of
composition measurement, gas flow rate through the membrane, and economic considerations. The concentration selected enables
convenient flow and composition measurement for even relatively small membrane areas (as low as 3 cm2 membrane area).
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