5-1
CHAPTER 5
Technical Description
The Model 3775 CPC is a continuous-flow condensation particle
counter that detects particles as small as 4 nanometers (50%
detection efficiency) in diameter. This section describes the function
of the CPC, its subsystems, and its components. A discussion of
operation theory and history is given first.
Theory
The CPC acts very much like an optical particle counter. However,
the particles are first enlarged by a condensing vapor to form easily
detectable droplets. The science behind the counter, 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 is too
high, condensation can take place even if no particles are present.
This is referred to as homogeneous nucleation or self-nucleation,
whereby molecules of the vapor form clusters due to the natural
motion of the gas and attractive van der Waals forces to form
nucleation sites. This condition is avoided by accurately controlling
operating temperatures. The CPC operates below the
supersaturation ratio to avoid homogenous nucleation.
The degree of supersaturation is measured as a saturation ratio
(P/P
s
), 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
ρ
RTd
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