How The Fast Cpc Works - Kanomax Fast CPC 3650 User Manual

Fast Condensation Particle Counter Model 3650: How It Works

How the Fast CPC Works

The Fast CPC is a thermally diffusive, laminar-flow condensation particle
counter. The Fast CPC measures the concentration of particles larger than a
known threshold size within a gas sample. The instrument also detects
particles that are too small to be detected optically (using light scattering
techniques) by condensing a vapor onto the particles so that they grow by
several orders of magnitude in size.
At vapor pressures below the saturation vapor pressure at the sample
temperature, condensation is energetically unfavorable because of the
significant energy required to increase the liquid surface area upon
condensation. Super-saturated vapors are able to overcome the energetic
barrier where the super-saturation required to initiate condensation onto a
particle is inversely proportional to the particle size.
Super-saturation is achieved by changing the wall conditions of a sample
conduit containing an aerosol saturated with the working fluid. In the Fast
CPC, which uses n-butyl alcohol as the working fluid, the conduit walls in
the super-saturation region are maintained at a temperature below the
saturated sample temperature. Transitioning to a conduit where the wall
temperature is below that of the saturated sample leads to heat and mass
transfer towards the wall. Because the mass diffusivity of n-butyl alcohol
vapor is lower than the thermal diffusivity of air, the sample cools (lowering
the saturation vapor pressure) at a faster rate than the vapor can diffuse
towards the walls. The ratio of the actual vapor pressure (Pv) to the
saturation vapor pressure (Psat) is the saturation ratio (S).
The Fast CPC has a short response time to capture time-variant aerosol
particle concentrations. The instrument uses short sample path lengths and a
patent-pending round flow conduit to shorten the diffusion length and thus
facilitates shorter saturation and super-saturation conduits.
Figure 1 is a schematic diagram of the Fast CPC.
Fast CPC User Manual 4 Version 1.2