7-2 Model 3022A Condensation Particle Counter
The second method is the dilution-bridge technique, which allows
the CPC to be calibrated from 500,000 to 8,000,000 particles/cm
3
(Figure 7-2). It uses three parallel capillary tubes, one with a high-
efficiency filter. The flowrate through the tubes is selected to give an
appreciable pressure drop (on the order of 2.5 cm of water). The
pressure drop is identical across all three legs (A, B, and C) with
identical flowrates in each leg.
A
BC
3-Way
Valve
3-Way
Valve
Figure 7-2
Dilution Bridge Used for High-Concentration Particles
To begin with, the three-way valves are adjusted so that the flow is
through A and B—the tube with a low-pressure-drop HEPA filter.
An aerosol of unknown concentration enters the tubes at one end
and exits at the other end. The concentration of the aerosol
entering the dilution bridge is adjusted so that the output
concentration measured by the CPC is in the range that has been
previously calibrated by the electrical method. The three-way valves
are then switched so that the flow is through tubes A and C. This
doubles the concentration of the aerosol entering the CPC. Using
this procedure, in sequence, the calibration can be carried to any
range. The technique has been used successfully by Pollak [1959].
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