Digital Aerosol Photometer Internal Reference

Purpose:
Using the internal reference “factor” causes the unit to display 100% when exposed to the light level equivalent to the chosen reference concentration.

Choosing a reference of 100 results in a displayed reading of 100% when 100-µg/l is sampled. In this manner, the % Leakage display of a digital photometer can be used to interpret the measured concentration directly as mass concentration per volume (µg/l or mg/m³).

Example:
Using varying reference factors will result in the photometer displaying 100% when sampling aerosol concentrations corresponding to the selected value. Choosing a reference of 65 would cause the unit to display 100% when the photometer samples 65-µg/l of aerosol concentration (relative to the response for the factors base reagent). The lower observed scattered light intensity is increased by a factor of 1.54 to display 100%.  (i.e., 100 ÷ 65 = 1.54)

Internal Reference “Out of Tolerance” Conditions
A low reference response condition is where the photometer reports a concentration value less than physically present. This has the effect of under reporting leakage levels and potentially passing filters that have failing characteristics.

Conditions of Example:

• Photometer has a reference value set to 100
• Reference response is low. (i.e., the photometer reads 75 µg/l when 100 µg/l is sampled)
• The 100% set point for response is not obtained by sampling a real-time aerosol upstream of the filter under test.

A leakage rate of 0.010% would be reported as 0.0075% due to the 25% reduction in response.

A high reference response condition is where the photometer reports a concentration value greater than physically present. This has the effect of over-reporting leakage levels and potentially failing filters that have passing characteristics.

Conditions of example:

• Photometer has a reference value set to 100
• Reference response is high. (i.e., the photometer reads 125 µg/l when 100 µg/l is sampled)
• The 100% set point for photometer response is not obtained by sampling a real-time aerosol upstream of the filter under test.

A leakage rate of 0.010% would be reported as 0.0125% due to the 25% increase in response.

Relevant Notes:
Several factors can impact the accuracy of the internal reference response.

The optical detection chamber condition gradually degrades during usage. This degradation causes the gain level established using the internal reference feature, and unit response to aerosol, to shift depending upon the severity. Oil aerosol exposure, sampling debris and unit handling all play a role in response degradation of the optical chamber.

Tolerances of the unit’s internal reference and the equipment used for internal reference calibration play an important role in the photometer response. The ATI calibrated aerosol sources utilize a Laskin III-A nozzle generator to create a controlled aerosol. This aerosol is used to set the internal reference response.  ATI sources have an output tolerance of 100±5 µg/l.

Consider the following:  If a photometer's internal reference were set while the source was operating at one end of the specified operating tolerance, and “As Found” results were taken while the source was operating at the other extreme of the specified operating tolerance, the resulting tolerance “stack” would move the internal reference response to the 10% tolerance bound excluding any other factors.

When utilized for filter leakage certification, every effort should be taken to use an actual sample of the “Upstream” aerosol concentration. If an actual sample of the “Upstream” aerosol challenge concentration is used to set the 100% response, an out of tolerance condition has no effect on the leakage values obtained. Sampling the Upstream aerosol concentration for the 100% setting is typically referred to ratio-metric mode. The unit reports a ratio of the detected leakage directly as a percentage of the available span at published levels of instrument accuracy. For this reason, the Internal Reference feature should be used for obtaining a working 100% set point only when an upstream sample cannot be achieved.

Displayed % Leakage values may be limited to 135% of the chosen 100% set point regardless of whether an aerosol sample or Internal Reference was used. A digital photometer having a value of 65 chosen as the reference factor has a reading ceiling of up to 135% of the 65 chosen as the reference factor, or 87.8 µg/l of the reference base. The 135% display limit is caused by range limitations of the amplifier.