3-12 Model 3772/3771 Condensation Particle Counter
Flow Rate Control
The Model 3772/3771 CPC uses a critical orifice to accurately control the
air flow in the instrument. The critical orifice operates at or below a critical
pressure to control the 1.0 L/min volumetric aerosol flow. More is found in
Chapter 5 “Technical Description.”
Problems with the aerosol flow can be detected by monitoring the pressure
drop across the nozzle, and verifying that the critical orifice pressure is
maintained.
Temperature Control
The temperatures of the condenser, saturator, and optics are maintained at
22 °C, 39 °C, and 40 °C, respectively, with specified ambient temperatures
in the operating range of 10 to 35 °C. The temperatures are controlled
through feedback circuits on the main electronics board and are viewable
via firmware commands. For the 3772, the temperatures are also viewable
with the Status display screen. If the temperatures are out of range on
either CPC, the status indicator LED on the front panel will be off. For
ambient temperatures outside the instrument operating range, the
instrument temperature performance may not be maintained. Moderate
increases in saturator temperature and optics are tolerated in some
instances, depending on measurement requirements.
Inlet Pressure Measurement
With adequate external vacuum, the instrument is capable of operating at
inlet pressures in the range of 75 to 105 kPa. The inlet pressure is
measured by an absolute pressure sensor, and is essentially the
barometric pressure if no inlet restriction is present. Inlet Pressure is
accessible through firmware commands on both 3772 and 3771 CPCs and
it is also viewable via the Status display screen for the 3772. Refer to
Chapter 4 for more details.
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