A photometer can be used regardless of aerosol liquid substitution because by setting 100% by sampling actual upstream aerosol, the photometer is “calibrated” to the system under test. In normal testing, 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% baseline.
Digital units require pressing the Zero key to set the 0% baseline. Establishing the 100% and 0% baselines constitutes calibrating the photometer to the filtration system under test (ratiometric mode).
In some applications (i.e. biological safety cabinet certification) an upstream sample of the aerosol air mixture cannot be obtained. Upstream concentration must be 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 reference must use the specific aerosol reagent in use or the result will not be accurate. This is because DOP and various substitutes used in the field have different refractive indexes and give different concentrations and photometric responses for the same aerosol weight per sample 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 also wrote an article published in Performance Review, the technical journal of the Controlled Environment Testing Association, which details various factors and corrections to compensate for substitute DOP liquids. A copy of this article can be obtained by calling CETA, telephone (202) 737-0204.
The TDA-2GA and later analog photometers, and all digital models, have PAO-4 and DOP Internal Reference values by default. ATI Service can calibrate older, single reference, photometers using either DOP or PAO-4. A customer returning a single-reference analog photometer for recalibration must specify the desired liquid.