Technical Description 5-7
Critical Flow
To achieve the 1.0 L/min critical aerosol flow through the sensor, an orifice
is used, operated at the critical pressure ratio to provide a critical flow.
Critical flow is very stable and is a constant volumetric flow, assuring
accurate concentration measurements despite varied inlet pressure.
The critical pressure ratio is found by dividing the absolute pressure
downstream of the orifice P
D
, by the absolute pressure upstream of the
orifice P
U
. This ratio must be below 0.528 for air.
528.0
P
P
U
D
pressure Critical
Values for pressures impacting CPC flow can be obtained using firmware
commands for both 3772 and 3771 CPCs. They are also found on the
Status menu on the front panel display for 3772. These pressures are
identified as the Ambient pressure, the Orifice pressure, and the Nozzle
pressure. The ambient pressure is typically the barometric pressure at the
inlet. The orifice pressure is the differential pressure across the aerosol
flow orifice. The nozzle pressure is the differential pressure across the
nozzle. Figure 5-1 identifies the location of the pressure transducer sample
ports.
To verify that critical pressure (therefore critical flow) is achieved under
extremes in inlet resistance, determine the orifice upstream pressure from
(A – N). The downstream pressure is the upstream pressure minus the
orifice differential pressure (A – N – O).
Flow is critical if the following is true:
(5-1)
Vacuum source, either a central building vacuum or a stand-alone vacuum
source (e.g., TSI Model 3032 Vacuum Pump), should provide at least 60
kPa (18 in. Hg) vacuum and 1.0 L/min critical volumetric flow at the inlet of
each CPC supported. The flow in the CPC is regulated by a critical orifice.
Changes in the inlet pressure will not affect the flow rate through the
instrument. A vacuum source that can provide a higher volumetric flow
(e.g., TSI Model 3033 Vacuum Pump) is needed when running multiple
CPCs.
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