To date, no recognized national metrological institute has successfully devised a method of calibrating a forward light scattering photometer. This is primarily due to the difficulty in generating a stable and repeatable aerosol that is capable of providing input at the lowest ranges of photometric detection.
Currently available calibrated aerosol sources are limited to between 8 and 100 milligrams per cubic meter, while photometers may display values as low as 0.0001 milligram per cubic meter.
Due to the instability of available aerosol generation methods, and the difficulty in quantifying low output levels, an alternate validation method is required.
The “Media Stack” test demonstrates the linearity and repeatability of the forward light scattering technology used to measure aerosol concentration in photometric based applications. This partially alleviates the need of performing multi-point verification throughout the range of response of the forward light scattering detectors in stationary and portable photometers employed in high efficiency filter certification.
- Calibrated aerosol Penetrometer model TDA-100P (Air Techniques International)
- Penetration using forward light scattering photometer
- Resistance using an electronic pressure transducer
- Flow using an electronic mass flow meter
- 5 Glass Fiber Filters, Grade HE-1071 (I.W. Tremont Co., Inc.)
Number each filter from “1” to “5” so that they are independently identified.
Adjust the TDA-100P flow rate to be at 16 liters per minute.
Test each filter with the TDA-100P to obtain its penetration value and record results.
Filter stack test:
- Place filter “1” in the machine test fixture and test, record result.
- Place filter “2” on top of filter “1”, test and record result.
- Place filter “3” on top of filters “1-2”, test and record result.
- Place filter “4” on top of filters “1-2-3”, test and record result.
- Place filter “5” on top of filters “1-2-3-4”, test and record result.
- Remove filter “5”, test and record result.
- Remove filter “4”, test and record result.
- Remove filter “3”, test and record result.
- Remove filter “2”, test and record result.
- Repeat the previous test two times to obtain repeatability data, testing the filters in the same order each time.
Filter 1: [P]=[C]*P1
Filter 1-2: [P]=[C]*P2*P1
Filter 1-2-3: [P]=[C]*P3*P2*P1
Filter 1-2-3-4: [P]=[C]*P4*P3*P2*P1
Filter 1-2-3-4-5: [P]=[C]*P5*P4*P3*P2*P1
|3 Cycle Average (“Set E” data set sample)|
|Average||Median||Std Dev||Std Dev as % Avg Penetration|
Linearity and repeatability is excellent across all ranges and scales. Standard deviation of the reported penetration, at lowest readings, exceeds published specifications by a factor of 4, showing a worst case variation of 0.00012%.