Technical Description 5-5
Sensor
The sensor itself is made up of a saturator, condenser, and optical
detector, as shown in Figure 5-1. The aerosol enters the saturator
section and passes through a heated, liquid-soaked wick. The wick
dips into the liquid reservoir and continually draws liquid up
through the inclined tube. The liquid evaporates and saturates the
aerosol stream with vapor.
The vapor-saturated aerosol then passes into a vertical condenser
tube, which is cooled using a thermoelectric device. The vapor
cools, becomes supersaturated, and begins to condense on the
particle 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 tube runs back down into the
saturator and is absorbed into the wick for reuse.
The sensor’s focusing optics consist of a laser diode, collimating
lens, and cylindrical lens. This combination forms a horizontal
ribbon of laser light above the aerosol exit nozzle measuring 10
micrometers by 2 millimeters. The collecting optics incorporate a
pair of aspheric lenses that collect the light scattered by the
droplets at 90° side-scatter and focus this light onto a low-noise
photodiode. The main beam is blocked by a light-stop in the rear of
the sensing chamber. A reference photodiode is mounted at the
back of the light-stop; it monitors the intensity of the incident
beam. The surface temperature of the optics housing is maintained
at a higher level than the saturator to avoid condensation on the
lens surfaces.
The CPC uses three modes of particle counting: single-particle
realtime counting; single-particle “live-time” counting; and
photometric calibration.
At concentrations below 1000 particles/cm
3
, the individual
electrical pulses generated by light scattered from individual
droplets are all counted over the actual time, that is, in “real” time.
At concentrations between 1000 and 10,000 particles/cm
3
, the
individual electrical pulses are counted, but only when the signal-
processing electronics is available for counting, that is, in “live-
time.” The length of time during which a particle is actually being
detected is called the “dead-time” and is not included in the live-
time. This arrangement automatically corrects for particle
coincidence. At concentrations above 10,000 particles/cm
3
, the
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