Technical Description 5-3
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]).
Other significant developments in adiabatic-expansion CNCs
include the use of electrical photodetectors to measure the light
attenuation from cloud formation (Bradbury and Meuron [1938],
Nolan and Pollak [1946], Rich [1955], Pollak and Metneiks [1959]);
the use of under- and overpressure systems; and automation using
electrically controlled valves and flow systems. The amount of light
attenuated from the droplet cloud is monotonically related to the
concentration of particles and is calibrated either by manual
counting techniques, calculated from theory of particle light-
scattering, or by using an electrical classification and counting
method (Liu and Pui [1974]). A historical review of the expansion
CNCs is given by Nolan [1972], Hogan [1979], and Miller and
Bodhaine [1982].
Two-Flow Mixing CNC
Another cooling method turbulently mixes two vapor-saturated
flows, one hot and one cold, to rapidly cool and supersaturate the
vapor (Kousaka et al. [1982]). The condensation and droplet growth
are fairly rapid and uniform. The flows can be passed continuously
(that is, non-pulsating) through the mixing chamber onto a single-
particle-counting optical detector.
Diffusional Thermal CNC
A continuous-flow, diffusional, thermal-cooling CNC (Bricard et al.
[1976], Sinclair and Hoopes [1975], Agarwal and Sem [1980]) first
saturates the air sample with alcohol vapor as the sample passes
over a heated pool of liquid alcohol. The vapor-saturated airstream
flows into a cold condenser tube where the air is cooled by thermal
diffusion. The alcohol condenses onto the particles and the droplets
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