Particle Counting 6-3
To measure live time, a high-speed clock and accumulator are
used. The accumulator adds up the live time and the counter adds
up pulse counts. The particle concentration is then calculated by
rate flow aerosol
1
time-live daccumulate
particles counted ofnumber
×=
a
C
Photometric Calibration
For concentrations greater than 50,000 particles per cubic
centimeter, the overall amount of light scattered from the droplets
is measured as a DC voltage from the photodetector and then
calibrated to a known concentration. For the calibration, a sodium
chloride aerosol is used, dried with silica gel and neutralized using
a radioactive source. The salt aerosol concentration is varied using
a dilution bridge to develop a series of calibration data points at
high concentrations, up to the instrument concentration limit of 10
7
particles/cm
3
. The Model 3775 CPC DC voltage is recorded at the
different concentration calibration points. To determine the actual
salt concentration used for the calibration points, a diluter and
second CPC “standard” are used. Operating in the highly accurate
single count mode, the standard CPC is first used to calibrate the
dilution ratio of the diluter. Using the diluter, the standard CPC
then measures the high test concentrations at each calibration test
point while remaining in single particle counting mode. The
corresponding relationship between the DC voltage and the known
concentration is put into the 3775 CPC for photometric
measurements.
Note: For the 3775, the maximum concentration displayed is 10
7
particles/cm
3
. At concentrations above 10
7
particles/cm
3
,
particle concentration data and the top status bar on the front
panel are in red. If this occurs, aerosol needs to be diluted
before entering the CPC.
To Next Page
Loading...
