- Feb 26, 2018 -
All Absolute filters should be tested individually and guaranteed to have a certain collecting
efficiency as determined by a DOP test with 0.3 um particles.Testing involves a determination of
the total collecting efficiency of the filter measured over the whole filter. The penetration of the
filter may derive either from the actual filter paper or from a combination of this and holes or other
leaks in the filter(the pinhole effect).
The causes of leakage in the filter are normally holes in the filter paper,which arise in the course of manufacture
of the paper or the filter,and incomplete sealing of the"filter package" and its connection to the filter frame.
Absolute filters are made by folding the filter paper,with corrugated separators between the folds.The filter package
is normally glued to prevent"short-circuiting" of unfiltered air around its edges.Additional glue is used for sealing the
filter package against the filter frame.
A HEPA filter may therefore have the odd hole and still meet the requirement for 99.7% min.total collecting efficiency.
In these conditions any leakage must of course be minute in extent .
It is desirable ,sometimes essential,for filters in laminar flow benches and rooms to be entirely free from leakage. A hole may
admit unfiltered air which may, if conditions are unfavourable,come into contact with the parts being assembled or the process
taking place. The result may be the same as if there were no filter at all. For these reasons the requirement has arisen that the filters
shall in addition to having a certain minimum collecting efficiency,also be subjected to occasional testing for leakage.
The filters are normally tested for leakage by three methods:
1. DOP test with 100% and 20% of rated air flow(penetration test for filter with a guaranteed min.collecting efficiency of 99.7%).
2. Scanning (leakage test).
3. "Smoke test"-visual inspection(leakage test).
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