Product Overview 1-11
Flow Equalizer Assembly
Some applications of the classifier
use a flow equalizer assembly
between the DMA and the CPC in
order to run the DMA at a lower flow
rate than the CPC, enabling size
distribution measurements at the
upper end of the DMA size range
while maintaining sheath to aerosol
flow ratios 5:1 and higher. Clean
dilution air from ambient is
introduced via a HEPA filter and is
mixed with the aerosol flow
immediately downstream of the
DMA. The amount of dilution air is
controlled manually with a needle
valve. The flow equalizer assembly is
a standard accessory shipped with
purchase of a 3938 SMPS.
The flow equalizer assembly relies
upon back pressure in the aerosol
line of the 3082 to overcome the
pressure drop of the HEPA filter.
Figure 1-6
Flow Equalizer Assembly
Therefore an impactor must be installed in the inlet of the classifier when
using the flow equalizer assembly. With an impactor installed, dilution
ratios of 4X to 7X can be achieved, depending on impactor and flow rate.
Without an impactor installed, the dilution ratio cannot be greater than ~1.3.
Ideally the flow equalizer should be used in an SMPS configuration with the
CPC set to 1 L/min or above, and the scan time set to 60s or higher. If your
application requires that the flow equalizer be used in an SMPS with CPC
flow rates less than 1 L/min or scan times faster than 60s, it is
recommended that the time delay value be determined empirically for
better size accuracy.
It is recommended that the flow equalizer not be used when the aerosol
particles of interest are smaller than 10 nm due to >10% particle losses
through the flow equalizer assembly at this size.
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