Biomedical Engineering Reference
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Fig. 10.62
FPF
<5.0μm
(reported as fine droplet fraction,
FDF
<5.0μm
) by Tservistas et al
.
for an aque-
ous aerosol generated by a vibrating membrane nebulizer using FSI, NGI, and laser diffractometry
(LD) as measurement methods (
From
[
52
]—
courtesy of M. Tservistas
)
used, promotes this undesirable effect. The precise mechanism of heat transfer to
the aerosol is uncertain; however, in one explanation, water evaporation takes place
by heat transfer from the mainly metallic CI as the aerosol passes through the equip-
ment, resulting in undersizing of the formulation [
57
]. Evaporative changes origi-
nating from this cause are routinely avoided by cooling the CI before and/or during
measurement [
54
].
In a feasibility study designed to assess the use of the FSI for nebulizers,
Tservistas et al
.
[
52
] took a similar approach, comparing fine droplet fraction
(
FDF
<5.0μm
) measured with a cooled FSI (down to 18 °C) from an aqueous formula-
tion delivered by an investigational e-Flow
®
vibrating mesh nebulizer (PARI GmbH,
Starnberg, Germany) with those recorded under ambient conditions at 22 °C
(Fig.
10.62
). Interestingly, they extended the lower flow rate range of their FSI to
15 L/min by blocking three of the six nozzles on the FSI insert to retain the stage
cut-off diameter of 5
m aerodynamic diameter at 50% of the design flow rate for
this AIM apparatus (Fig.
10.63
).
Under ambient conditions, the FSI-generated
FDF
<5.0μm
values at 15 L/min were
substantially equivalent to those produced using a cooled NGI. The cooled FSI pro-
duced a lower
FDF
<5.0μm
, although the difference was relatively small, less than 5
per cent. The highest
FDF
<5.0μm
was obtained by laser diffractometry (LD).
These results suggest that the lower thermal capacity of the FSI, a function of its
much reduced mass relative to the NGI, is advantageous in terms of accuracy
μ
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