HEPA filters are normally used
only where an extremely high level of cleanliness or purity is
required. The requirement may be due to problems caused by the
presence of particulates or physiological problems caused by
viable airborne organisms. In any event, the efficiency of every
filter is of paramount importance and must be measured in an
Mil-Std-282 is recognized as
the standard for "hot" DOP efficiency testing and is
used for compliance with many HEPA filter specifications. It
is also recognized as being "monodisperse
0.3 micron particles" as referenced in EPA and OSHA definitions
for HEPA filters.
DOP (dioctylpthalate) is an
oil commonly used with vinyl resins to make soft vinyl plastics.
It is also used by air filter manufacturers and various testing
agencies to make an aerosol to test the effectiveness of air
filters. Other oil-like materials, like DOS, can be substituted
with similar results.
The DOP aerosol used to challenge
HEPA filters to test for efficiency by this standard is known
as "hot" or "thermally generated" DOP because
it is derived from heated dioctylpthalate oil. Sophisticated
equipment is used for carefully controlling oil and air temperatures,
air flow rates and mixing conditions. This "hot" DOP
aerosol has a very narrow particle size distribution (monodisperse).
Because the only way to determine the efficiency
of a filter on a specific particle size (fractional efficiency)
is to test with particles of that size, DOP is used to produce
a high concentration of 0.3 micron particles - that which theory
indicates and has historically been considered to be the most
penetrating of filter media.
For each test, the average
aerosol concentration is measured both upstream and downstream
of the filter with a photometer. The inefficiency or penetration
in percent can therefore be determined and recorded on the filter
label. For example, a filter with a penetration of .008% would
mean it was 99.992% efficient, well above the minimum of 99.97%
for HEPA efficiency.
DOP aerosol can be generated
in the field but the equipment used, while relatively simple
and portable, cannot produce truly "hot" DOP that is
monodisperse. The DOP generated by such equipment is "cold"
DOP which having a broad particle size distribution is polydisperse.
Such an aerosol is useful for field testing for leaks and ensuring
the integrity of an installation, however, without the ability
to particle count the 0.3 micron size particles, "cold"
DOP does not provide the ultimate test of filter efficiency.
penetration or efficiency of a filter is strongly affected by
the particle size of the challenge aerosol. A small change in
particle size can have a significant effect on penetration. The
smaller the particle, the lower the efficiency until the maximum
penetrating particle size is reached.
As indicated earlier, "cold"
DOP has a broad particle size with larger average size than "hot"
DOP. Efficiencies are, therefore, higher with "cold"
DOP than with "hot" DOP. The control of temperatures
and flow rates with the equipment is critical to maintaining
a consistently tight particle distribution which allows for consistent
and reproducible efficiency measurements.
Where "cold" DOP
can be useful in determining HEPA filter efficiency is when testing
in accordance with IEST-RP-CC007.1. For each test, particle counters
are calibrated to simulaneously count the number of 0.3 micron
particles both upstream and downstream of the filter. Providing
the "cold" DOP challange aerosol contains a statistically
significant number of 0.3 particles, the inefficiency or penetration
in percent can determined. In this test, the polydisperse nature
of "cold" DOP is irrelavent because the other particle
sizes are not measured.
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