5-1
C h a p t e r 5
Technical Description
The Model 3750 Condensation Particle Counter (CPC) is a continuous-flow
condensation particle counter that detects particles as small as
7 nanometers (50% detection efficiency) in diameter. This section
describes the function of the CPC, its subsystems and its components.
Design o f the CP C
The basic instrument consists of three major subsystems: the sensor, the
microprocessor-based signal-processing electronics, and the flow system.
The sensor and the flow system are described below.
Sensor
The sensor is made up of saturator, condenser, and optical detector,
shown schematically in Figure 5-1. The sensor grows the sampled aerosol
particles into larger droplets and detects them optically. The aerosol flow
enters the saturator section where it passes through a heated, liquid-
soaked cylindrical wick. The liquid evaporates and saturates the air flow
with butanol vapor. Butanol is replenished from a reservoir and a fill bottle.
The flow of combined aerosol and butanol vapor is then cooled in the
condenser. The vapor becomes supersaturated and condenses on the
aerosol particles (condensation nuclei) to form larger droplets. The droplets
pass from the condenser tube through a nozzle into the optical detector.
Liquid that condenses on the walls of the condenser tube runs back down
and is removed by the water removal system into the drain bottle when the
system is ON. Otherwise, the liquid goes back into the saturator and is
absorbed into the wick for reuse.
The sensor’s optical detector is comprised of a laser diode, collimating
lens, cylindrical lens, collection lenses, and photodiode detector. The laser
and collimating lens form a horizontal ribbon of laser light above the
aerosol exit nozzle. The collection lenses and detector incorporate a pair of
aspheric lenses that collect the light scattered by the droplets and focus the
light onto a low-noise photodiode. A reference photodiode is used to
maintain constant laser power output. The surface temperature of the
optics housing is maintained at a higher level than the saturator to avoid
condensation on the lens surfaces.
The Model 3750 CPC operates in single particle count mode up to 10
5
particles/cm
3
. Rather than simply counting individual electrical pulses
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