Theory of Operation B-17
Caution
At concentrations above 10,000, the Model 3010 CPC display flashes in the
concentration mode (PT/CM
3
). If this occurs, the number of particles shown on the
display could be substantially lower than the actual concentration.
The Data Measurement Process
Aerosol Instrument Manager
®
Software for SMPS™ spectrometer
and the microcomputer tie the Electrostatic Classifier, the
impactor, and the Condensation Particle Counter (CPC) together to
form the SMPS
™ spectrometer. Refer to the Aerosol Instrument
Manager Software for SMPS
™ Instruction Manual.
When the computer executes the Run command, the computer
sends the CPC scan parameter data based on the selections
entered under the Hardware Setup menu. After parameter data is
sent, a command is sent to the CPC to begin the sample. The CPC
uses its analog output port to control the Differential Mobility
Analyzer (DMA) rod voltage in the Classifier. The CPC counts the
particles leaving the DMA and sends the count data to the
computer every tenth of a second.
The Model 3785 WCPC, 3775 and 3022A CPCs use a photometric
mode for concentrations greater than 10,000, 50,000, and 10,000
particles/cm
3
, respectively. So when the CPC is in photometric
mode, the count value returned to the computer is calculated
based on the concentration of particles measured. Since the
photometric mode of the Model 3785 WCPC, 3775 or 3022A CPC in
the SMPS setup is updated only once per second, the granularity of
the data is increased for high particle concentrations.
During the time the analyzer rod voltage is increasing (the Up
scan), particles leaving the DMA are increasing in size. Their actual
size depends on the electrical field they passed through within the
DMA. The raw particle counts detected by the CPC are stored in an
internal computer array. The computer bins (maps) the particle
count data into particle-size channels (64 channels per decade of
particle size) based on their mobility and with the assumption that
each particle has a single charge. This data is used for the raw
count data graphs and tables.
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