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No. 21
TDA-5A/5B Background Penetration Readings
It has been brought to the attention of ATI by some very
knowledgeable and highly experienced certifiers that high
penetration readings are obtained while scanning HEPA filter
systems with thermally generated polydispersed aerosol. If
the same systems were tested with cold generated (Laskin Nozzle)
aerosol, the penetration results would be 0.000% penetration.
The penetration readings obtained with the thermally generated
aerosol will vary from .002% to >.01% depending on the size,
media, and manufacturer. This reading is obtained anywhere
on the downstream side of the filter, which ATI calls a background
reading.
To resolve this anomaly, ATI advises certifiers to use the
straylight control to eliminate the background reading and
scan as usual on the analog photometers (TDA-2D and TDA-2E),
and to reset the 0% while scanning on the digital (TDA-2G)
photometer.
ATI is currently performing aerosol research on the thermal
generators and preliminary results show that the aerosol size
is significantly lower than the cold aerosol. As more data
is acquired, ATI will post it on their Web page at www.atitest.com.
The original comparison study was performed under an Atomic
Energy Commission (USAEC) contract generated by Humphrey Gilbert
and performed at USAEC Laboratories. The data is covered in
a paper presented at the 9th Air Cleaning Conference held
in 1967.
It is entitled "An Evaluation of Two Portable Thermal
Generators for In-Place Testing" by R. H. Knuth.
The testing was performed on 24" by 24" HEPA filters, and
no significant differences between results were detected.
In today's world, the minipleat filter is in vogue. If the
blower is not centered and a well designed plenum is not in
use, that laminar flow will not be obtained. That is, much
higher velocities will occur at one area of the filter than
another. Due to the thinness of design and manufacturing problems,
the media is quite different from that used in standard (separator
type) HEPA filters. ATI speculates that this may be part of
the problem. Customers who encounter this problem could assist
ATI in resolving it by providing ATI with the manufacturer
of the filter, the background reading, the type and size of
the filter, and the concentration of the challenge.
Contact ATI at e-mail address: info@atitest.com.
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