Technical Description 5-3
Critical Flow
To achieve the 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.
528.0pressure Critical
U
D
P
P
Values for pressures impacting CPC flow are found on the Status screen
on the instrument User Interface. These pressures are identified as
Ambient, Inlet, Critical Orifice and Aerosol Nozzle Pressure. 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, the Vacuum Pressure is also displayed on the
Status screen.
Pump
An external 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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