B-4
Model 3090/3090AK Engine Exhaust Particle Sizer
TM
Spectrometer
Flow Path
After passing through a 1µm cut cyclone, the aerosol enters the charger at
10 L/min and close to atmospheric pressure (see
Figure B-3). It then passes through the charging region where it receives a
predictable charge. In the charger, 0.6 L/min of clean air is added to the
flow to keep the charger electrode clean. After the charger, 2 L/min is
removed from the center of the charging region, where charging is less
uniform, and the rest of the aerosol passes near the center electrode. A
recirculating flow of particle-free, laminar sheath air at 39.4 L/min joins the
particle flow at the top of the center electrode for a total of 48 L/min through
the column. Particles are then separated by electrical mobility as flow
moves from the top to the bottom of the column. Particles with high
electrical mobility (small particles) are deflected to the electrode rings near
the top of the column, and those with low electrical mobility (large particles)
are deflected further downstream. Air flow that is not filtered and
recirculated is passed out of the exit of the instrument at 10 L/min.
Data Inversion
There are a number of parameters affecting the electrometer currents that
need to be compensated if high time resolution is to be obtained. Particles
that flow past the detection sta
ges but don’t contact the electrode rings can
create image charges. In addition, there are time delays between when a
small particle from an aerosol packet is detected on an upper stage of the
column and when a large particle from the same aerosol packet is detected
on a lower stage in the column.
An inversion algorithm is used to deconvolute the data and make
corrections for the image charges and the time delays in the column. The
algorithm also converts currents from the 22 electrometers into 32 size
channels of output. This allows the maximum resolution of the instrument
to be represented by output channels that are equally spaced on a log
scale between 5.6 nm and 560 nm.
Concentration Limits
Unlike many instruments which have a single value as the lower detection
limit, the EEPS spectrometer has a range of values depending on particle
size and averaging time. This is due to the inherent noise in each
electrometer as well as the particle charging probability vs. particle size.
Figure B-4 gives a range of lower limits for several averaging intervals
based on particle size. The EEPS software calculates a different detection
limit based on each of the possible averaging intervals that can be selected
in the software.
The upper limit of concentration is based on the fixed upper limit of current
that can be detected by each electrometer channel. This limit is also
plotted in Figure B-4. However, this limit does not change with averaging
time. The EEPS software uses this limit to show maximum limit values (red
