| 摘要 |
A member in the form of an overfilm or molded housing affixed to the pouch wall has a plurality of isolated vent passages. A separate filter assembly is situated in each of the passages. Adhesive coated covers are situated to obstruct the flow through all but a selected one of the passages. After the filter in the unobstructed passage has lost its effectiveness, the cover is removed from another passage and placed over the passage containing the used filter to obstruct same. The filter in each passage is used, in turn, by appropriate movement of the cover, until all of the filters have been used.
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