Technical Description 5-5
Design o f the C P 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 laminar
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 using a
thermoelectric device (TED) 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 3772/3771 CPC operates in single particle count mode up to
10
4
particles/cm
3
. Rather than simply counting individual electrical pulses
generated by light scattered from individual droplets, the CPC uses a
continuous, live-time coincidence correction to improve counting accuracy
at high particle concentrations. Coincidence occurs when the presence of
one particle obscures the presence of another particle creating an
undercounting error. “Live-Time Counting” is discussed later in Chapter 6.
This option can be turned OFF by firmware command “SCC,0”.
To Next Page
Loading...
