TSI Instruments 3775 User Manual

Technical Description 5-7
Flow System
Refer to Figure 5-1 while reviewing information on instrument flow
provided in this section.
The CPC relies on an on-board high-vacuum pump to maintain
constant critical flows through a critical orifice and a variable
orifice. The CPC has two inlet flow options: high flow mode,
nominally 1.5 L/min (1500 cm
3
/min) and low flow mode, nominally
0.3 L/min (300 cm
3
/min). The flow rate through the sensor is
always 0.3 L/min, independent of the inlet flow rate setting. More
information on instrument flows is provided below.
Critical Flow
To achieve the 0.3 L/min nominal 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.
Critical pressure =
528.0≤
U
D
P
P
Values for pressures impacting CPC flow are found on the Status
screen shown in
Figure 4-9. These pressures are identified as A, O
and N. Pressure A is the inlet pressure, typically the ambient
barometric pressure. Pressure O is the differential pressure across
the aerosol flow orifice. Pressure N 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:
528.0≤
−
−−
NA
ONA
(5-1)
Control of the aerosol inlet flow requires a variable orifice for
bypass and makeup air flow. This orifice is also operated at a
critical pressure ratio.

TSI Instruments 3775 Specifications

General

Brand	TSI Instruments
Model	3775
Category	Laboratory Equipment
Language	English