Biomedical Engineering Reference
In-Depth Information
Fig. 2.11 Collection efficiency curves for the abbreviated ACI at 28.3 L/min, based on the
adjusted calibration data of Vaughan [ 19 ] for stages 2 and 5 of the ACI ( From [ 43 ] , courtesy of D
L Roberts and J P Mitchell )
overlap of the collection efficiency curves for stages 1 and 3 with that from stage 2
is prominent. Hence, a 4.7-
m aerodynamic diameter particle could potentially be
captured on stage 1 or 2 or 3.
However, when only stages 2 and 5 are present, the overlap region between these
two stages is absent, and a 4.7-
μ
m-diameter particle would either be captured or
pass the first stage (Fig. 2.11 ). Consequently, of the particles that would have a
chance of collecting on stages 1 or 2 or 3 with the full-resolution ACI, a larger mass
fraction is available to approach stage 2 of the abbreviated ACI configuration. In
consequence, more particles can penetrate stage 2 of the abbreviated apparatus than
would be the case if stage 1 is present.
It follows that summing up the masses collected on stages 0, 1, and 2 of the full-
resolution system will not give the identical result as the mass collected by stage 2,
in the absence of stages 0 and 1, with the abbreviated CI configuration.
The approach to develop a theoretical interpretation of the behavior above was
similar to that described already for the full-resolution CIs [ 42 ]. On this occasion,
though, an aerosol that is both unimodal and lognormal in terms of its APSD distri-
bution was considered as it enters and passes through the abbreviated impactor.
To calculate how a lognormal aerosol distributes itself throughout an impactor
with some stages removed, the stage efficiency functions, E i ( x ), in (2.15) were set
equal to zero for stages of the full-resolution ACI that are removed and (2.12)
applied to the remaining stages of the abbreviated ACI configuration.
Roberts and Mitchell also considered an abbreviated NGI likewise, by postulat-
ing that the user removes the airflow after stage 3 and reintroduces it just upstream
of stage 6 with special cups for the NGI, as described by Svensson and Berg [ 45 ].
In this arrangement, the abbreviated NGI is much like a full-resolution NGI but with
fewer stages. It is important to note that Mitchell and Roberts did not consider in
μ
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