|
No. 11
Aerosol Photometer Calibration with DOP/PAO
A photometer can be used regardless of the
liquid substitution because the photometer is calibrated with
the system being testing. In normal situations, a sample of
the aerosol air mixture upstream of the filtration system
under test is sampled and the gain level is adjusted to obtain
the proper 100% baseline. The photometer is then set to the
CLEAR mode so the air sample is drawn through a reference
filter and all particulate is removed from the air stream.
At this point, the unit can be switched to normal operating
range which is usually 0.1% or 0.01% and the straylight can
be adjusted to obtain the 0.00% baseline. Establishing the
100% and 0.00% baselines constitutes calibrating the photometer
to the filtration system under test.
There are some instances (i.e., biological
safety cabinet certification) when an upstream sample of the
aerosol air mixture cannot be obtained. In this situation,
the concentration is calculated and the sensitivity of the
photometer adjusted to the correct level using the Internal
Reference feature of the photometer. When the Internal Reference
feature is used, the photometer must be calibrated to the
specific DOP aerosol substitute or the result will not be
accurate. This is because various DOP aerosol substitutes
used in the field have different refractive indexes and they
give different concentrations and photometric responses.
For additional technical information regarding
photometric responses and particle size distribution, please
refer to "Concentrations Produced By A Laskin Nozzle Generator"
written by David W. Crosby of ATI and "Characteristics Of
Laskin Nozzle Generated Aerosols" written by Dr. Mel First
at the Harvard Air Cleaning Laboratory, Page 109 through Page
125 in the 1990 21st DOE/NRC Nuclear Air Cleaning Conference
Proceedings.
David W. Crosby has also written an article
which was published in Performance Review, the technical journal
of the Controlled Environment Testing Association, which details
various factors and correct adjustments to compensate for
substitute liquids. A copy of this article can be obtained
by calling CETA, telephone (202) 737-0204.
The ATI Service Department will calibrate
a photometer to either DOP or PAO. A customer returning a
photometer for recalibration should specify the desired liquid.
ATI's new TDA-2G Digital Photometer is calibrated with both
DOP and PAO. It also has a third Internal Reference point
for programming a correction factor for a liquid other than
DOP or PAO.
|
