4-6 Model 3936 Scanning Mobility Particle Sizer
TM
(SMPS
TM
) Spectrometer
Setting Flow Rates
Use the information in this section to set the flows for each of the
CPC types in under and overpressure modes.
For each of the CPCs there is a preferred operating flow rate for the
polydisperse and monodisperse flows (they are equal—see
Table 4-1). When operated at these flows, the Classifier should be
in underpressure mode. In underpressure mode, the flow is pulled
through the Classifier; in overpressure mode, the flow is pushed
through the Classifier.
Underpressure Mode
When the CPCs are run at the flows recommended in Table 4-1
(flow rates that are not marked with
**
), the valve on the flow
equalizer assembly should be fully closed. This allows all the flow of
the CPC to be drawn through the Classifier. To run the SMPS
™
spectrometer with a lower flow rate than normally used with a
particular CPC (for example 0.6 L/min with a Model 3772 CPC),
adjust the valve on the flow equalizer assembly until the
polydisperse flow (measured with the impactor flow rate) reaches
the set value.
Overpressure Mode
To operate the SMPS™ spectrometer with a flow rate higher than
normally used with a particular CPC (for example, 1.5 L/min with a
Model 3772 CPC), the SMPS
™ spectrometer is in overpressure
mode. This means pushing 1.5 L/min from the aerosol source
through the aerosol inlet and then allowing the filter in the flow
equalizer assembly to vent the excess flow. In this case, the valve
should be fully open. Once again, the actual polydisperse flow can
be monitored with the impactor flowmeter.
SMPS Spectrometer Check
Before operating the SMPS™ spectrometer, check the system to
ensure the system is leak-tight and the flows are laminar. All flow
rates must be set to proper values before checking the system (see
“Setting Flow Rates
” in this chapter).
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