Biomedical Engineering Reference
In-Depth Information
Table 2.1
Values of
d
50
(
m) and GSD
stage
for the NGI size-fractionating stages at flow rates
associated with its archival calibration
Q
= 15 L/min 30 L/min 60 L/min 100 L/min
Stage
d
50
GSD
stage
d
50
GSD
stage
d
50
GSD
stage
d
50
GSD
stage
1
a
14.1 1.39 11.7 1.34 8.06 1.33 6.12 1.35
2 8.61 1.16 6.40 1.19 4.46 1.21 3.42 1.26
3 5.39 1.15 3.99 1.21 2.82 1.24 2.18 1.27
4 3.30 1.12 2.30 1.11 1.66 1.17 1.31 1.22
5 2.08 1.14 1.36 1.11 0.94 1.17 0.72 1.20
6 1.36 1.13 0.83 1.14 0.55 1.15 0.40 1.28
7 0.98 1.12 0.54 1.17 0.34 1.20 0.24 1.38
a
Preceded by USP/Ph.Eur. induction port—values are slightly different if a preseparator is also used
μ
deemed to have penetrated the stage. The sharpness of a given CI stage efficiency
curve is defined in terms of the geometric standard deviation (
GSD
stage
) by analogy
with the properties of the lognormal distribution function:
d
d
(2.9)
GSD
stage
=
84 1
.
15 9
.
A stage having its
GSD
stage
at unity corresponds to the ideal size separator. In
practice, however,
GSD
stage
is ideally <1.2.
GSD
stage
values in excess of this limit do
occur with compendial CIs, particularly for those stages that size fractionate
particles larger than about 5
m aerodynamic diameter where gravitational settling
contributes significantly to the size-separation process. Values of
d
50
and GSD
stage
can be determined by calibration of the impactor with uniform-sized particles of
known mean values of
d
ae
; however, the process is both time-consuming and highly
exacting [
18
-
20
]. A further refinement of the calibration process is to undertake a
so-called archival calibration of a single CI whose stage dimensions (principally
nozzle diameters) have been intentionally manufactured to be as close as possible to
their nominal values. This procedure avoids the need to calibrate each and every CI
at the time of manufacture and also after use for validation purposes. Such a calibra-
tion was undertaken for the first time early in the development for the NGI at volu-
metric flow rates of 100, 60, and 30 L/min [
21
] and later extended to 15 L/min to
enable this CI to be used for the measurement of nebulizing systems [
22
]. Values of
d
50
and GSD
stage
from these archival calibrations are summarized in Table
2.1
.
Equivalent information is given in Table
2.2
for the ACI at 28.3 L/min that is
widely used for MDI-based evaluations. However, to date, an archival calibration of
the ACI has not been undertaken, so the actual values of
d
50
and GSD
stage
are pre-
sented based on the historic data provided by Vaughan in 1989 [
19
]. These
calibration- produced
d
50
values are compared with the corresponding nominal val-
ues that have been adopted by manufacturers of this impactor since that time. It is
important to note that it is common practice to work with the nominal values of
d
50
for the stages of ACI. However, Stein and Olson, in an extensive investigation of
impactors manufactured before the year 1995, reported that significant deviations in
μ
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