Theory of Operation
B-3
Figure B-2
Schematic of EEPS Measurement Column
Charger
The charging of the aerosol is accomplished through two unipolar diffusion
chargers. First, a negative charger puts a negative net charge on the
particles to reduce the number of highly positive charged particles and to
prevent overcharging in the second charger. Then, a positive charger puts
a predictable net positive charge on the particles.
Measurement Column
The electrical analyzer consists of an inner cylinder composed of multiple
electrical sections with different voltages and column diameters. By
stepping the voltage and changing the central column diameter, the
required height of the column is reduced and the particles are more evenly
distributed across 22 electrodes.
The electrode rings are made of a highly insulating plastic with metal rings
molded into the inner diameter of the ring. Features in these parts provide
a large electrical resistance between the electrometer rings while
minimizing the gap between the rings. O-ring seals between the electrode
rings ensure a leak-tight column. Custom electrometers are used to obtain
low noise, fast response and high resolution.
