TSI Instruments 3775 User Manual

5-2 Model 3775 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 counter is determined by the
operating saturation ratio. For the counter this ratio is several
hundred percent, whereas in the atmosphere, this ratio is only a
few percent for water.
History
Historically, the counter 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. The CPC uses the latter.
Adiabatic Expansion CNC
The first CNC was developed over a century ago by John Aitken
[1888]. His simple and completely mechanical device cooled water-
saturated air by adiabatic expansion using a pump. The droplets
were counted as they fell onto a counting grid and a calculation was
made to determine the concentration of dust particles in the sample
volume. He made several improvements to his invention and his
portable dust counter was used for many years (Aitken [1890–91]).

TSI Instruments 3775 Specifications

General

Brand	TSI Instruments
Model	3775
Category	Laboratory Equipment
Language	English