Product Overview
1-3
H o w i t W o r k s
The Model 3786 UWCPC is a water-based Condensation Particle
Counter designed to measure the concentration of airborne
ultrafine particles. In operation the UWCPC draws in an air sample
and counts the number of particles in that sample to provide a
particle concentration value that is displayed as the number of
particles detected per cubic centimeter of sampled air.
The UWCPC uses a laser and optical detector to detect the
particles, passing the sampled flow through a viewing volume
illuminated by a laser. The instrument relies on a condensation
technique to deposit a working fluid on the particle to grow or
“amplify” their size to a value that can be detected readily with a
conventional optical system. This instrument utilizes a new
patented technology (Aerosol Dynamics Inc. Berkeley CA) that
allows water to be used as the working fluid without the
requirement of mixing or adiabatic expansion techniques. The
aerosol sample is uninterrupted and follows a laminar-flow path
from the sample inlet to the optical detector. Refer to Figure 1-2 for
a diagram of the UWCPC flow system.
The aerosol enters the sample inlet and immediately half of the inlet
flow is extracted, filtered and then combined with the remaining
sample flow as clean sheath air. This combined flow enters a region
surrounded with wetted media. The aerosol stream is saturated
with water vapor and is temperature equilibrated. The sample then
passes to a growth section where the wetted walls are heated to
produce an elevated vapor pressure. The high diffusivity of water
vapor allows the vapor to reach the center of the sample stream at a
faster rate than the thermal diffusivity of the vapor can equilibrate
to the higher temperatures near the walls. This results in a
supersaturated condition along the radius of the flow stream.
Particles in the flow stream act as nuclei for condensation. Water
continues to condense on the particles as it passes up the growth
tube and the enlarged particles are then detected by the optical
detector. The clean sheath air is used in this system to keep the
aerosol sample flow in the center of the growth tube where super-
saturation is the highest.
