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No. 11
Aerosol Photometer Calibration with DOP/PAO
A photometer can be used regardless of aerosol liquid substitution because the photometer is calibrated with
the system being tested. In normal situations, a sample of
the aerosol-air mixture upstream of the filter
under test is sampled and the gain level is adjusted to obtain
a 100% baseline. The photometer is then set to the
CLEAR mode and an air sample is drawn through a reference
filter with all particulate removed from the air stream.
At this point on analog photometers, the unit can be switched to normal operating
range, usually 0.1% or 0.01%, and the straylight control used to adjust the 0.000% baseline. Digital units require pressing the Zero key to set the 0.000% baseline. Establishing the
100% and 0.000% baselines constitutes calibrating the photometer
to the filtration system under test.
There are some instances (i.e., biological
safety cabinet certification) where 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 the same weight per volume.
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
an older photometer to either DOP or PAO. TDA-2GA and later analog photometers, as well as all digital models, have PASO and DOP Internal Refernce values stored form the Fcatory. A customer returning a
pre-2GA analog photometer for recalibration should specify the desired liquid.
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