5-2 Model 3022A Condensation Particle Counter
where
γ
= surface tension of the condensing fluid
M = molecular weight of the condensing fluid
ρ
= density of the condensing fluid
R = universal gas constant
T = absolute temperature
d = Kelvin diameter.
The higher the saturation ratio, the smaller the Kelvin diameter.
The saturation vapor pressure P
s
is defined for a flat liquid surface.
For a round liquid surface, such as the surface of a droplet, the
actual saturation vapor pressure is greater. In other words, the
smaller the droplet, the easier it is for the vapor molecules to
escape the liquid surface. The Kelvin diameter defines the critical
equilibrium diameter at which a pure droplet is stable—there is
neither condensation nor evaporation. Smaller liquid particles will
evaporate and larger particles grow even larger by condensation.
The larger particle will grow until the vapor is depleted, causing the
saturation ratio to fall until it is in equilibrium with the particle
droplet.
If the saturation ratio is controlled to a level below the critical
saturation ratio—the point at which homogeneous nucleation takes
place—condensation will not take place in a particle-free
environment.
The lower size sensitivity of the CPC is determined by the operating
saturation ratio. For the CPC this ratio is several hundred percent,
whereas in the atmosphere, this ratio is only a few percent for
water.
Historical Notes
Historically, the CPC has been called a condensation nucleus
counter (CNC). CNC technology uses three techniques to cool and
supersaturate the condensing vapor: adiabatic expansion, two-flow
mixing, and diffusional thermal cooling.
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