Biomedical Engineering Reference
In-Depth Information
23
/
13
/
DDD
eff
=
(*)(
)
(4.1)
median
can be used to define the in vitro performance of that stage in terms of its
D
50
size
[
22
]. However, errors in mensuration using stop-go pins for the larger mm-sized
nozzles have been reported [
23
], although these pins, if used carefully, could be use-
ful for cleaning impactor jets. Recently, the determination of the flow resistance
(equivalent to pressure drop) across each stage has been proposed by Millhomme
et al
.
, as an “in-use” test and a surrogate for stage mensuration [
24
]. This is because
the pressure drop (Δ
P
stage(
i
)
) across an impactor stage “
i
” with the CI operated at flow
rate,
Q
, can be described well by a Bernoulli-style equation:
2
r
2
=
Q
kA
g
∆
P
(4.2)
stage()
i
t
where
r
g
is the support gas (i.e., air) density and
A
t
is the total area of the nozzle
array. This group also showed that a change in the area-mean jet diameter of a given
CI stage,
D
* from
D
*
0
to
D
*
1
, can be related to a variation in Δ
P
stage(
i
)
, in accordance
with the following relationship:
14
/
12
/
1
/4
=
r
r
D
D
*
*
Q
Q
∆
∆
P
P
gas
()
1
1
1
0
(4.3)
0
0
gas
()
0
1
in which the subscripts “0” and “1” represent the initial and final states, respectively,
for Δ
P
stage(
i
)
. For well-maintained impactors, characteristic of good laboratory phar-
maceutical practices, there is little difference between the area-mean and effective
jet diameters [
19
] so that (4.3) may be rewritten on a stage-by-stage basis in terms
of the effective diameter for that stage,
D
eff
[(4.1)], given by
12
/
14
/
14
/
=
Q
Q
r
r
∆
∆
P
P
D
1
1
0
D
(4.4)
eff
()
1
eff
()
0
0
0
1
In principle, if the discharge coefficient for the nozzle set remains at a constant
value stage pressure drop may be conveniently performed immediately
before
every
APSD measurement, as a system suitability test, thereby guaranteeing that the CI is
fit for purpose. The potential of this approach requires further investigation. This
situation directly contrasts with stage mensuration, which is a comparatively com-
plex task requiring that the stages are individually inspected and is therefore only
likely to be undertaken on an infrequent (i.e., annual) basis. In the event that
D
eff
is
found to be outside acceptable limits (most likely the manufacturer tolerances asso-
ciated with nominal nozzle diameter), the validity of an indefinite number of APSD
determinations that have taken place since the previous inspection could be called
into question. Furthermore, the measurement of the pressure drop across the entire
CI system (Δ
P
CI
) may also be used as an in-use test of leak tightness before each
measurement.
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