CHAPTER 5
Technical Description
5-1
The Model 3022A is a continuous-flow, condensation particle
counter (CPC) capable of detecting particles as small as 0.01
micrometer in diameter. This section describes the function of the
CPC, its subsystems, and components. A discussion of its theory
and history is given first.
Theory
The CPC acts very much like an optical particle counter, except
that the particles are first enlarged by a condensing vapor to form
easily detected droplets. The science behind the CPC, therefore, is
focused on how to condense the vapor onto the particles. Portions
of the following discussion are taken from a paper by Keady et al.
[1986].
When the vapor surrounding particles reaches a certain degree of
supersaturation, the vapor begins to condense onto the particles.
This is called heterogeneous condensation. If supersaturation
becomes high enough, condensation can take place even if no
particles are present. This is called homogeneous nucleation or self-
nucleation, whereby molecules of the vapor form clusters due to the
natural motion of the gas and attractive van deer Waals forces to
form nucleation sites.
The degree of supersaturation is measured as a saturation ratio,
which is defined as the actual vapor partial-pressure divided by the
saturation vapor pressure for a given temperature:
P
supersaturation =
__
P
s
For a given saturation ratio, the vapor can condense onto particles
only if they are large enough. The minimum particle size capable of
acting as a condensation nucleus is called the Kelvin diameter and
is evaluated from the following relationship:
P (4
γ
M)
saturation ratio =
__
= exp
_____
P
s
ρRTd
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