Biomedical Engineering Reference
In-Depth Information
100
80
60
40
20
0
0.001
0.01
0.1
1
10
100
Aerodynamic diameter (µm)
FIGURE 2.10
Collection efficiencies of a MOUDI-II with 14 impaction stages. (Courtesy
of MSP Corp., Shoreview, MN.)
The originally designed thermal precipitators used a heated wire in the center between
two parallel plates. The aerosol flow was perpendicular to the wires and parallel to the
plates, such that the particles were deposited onto the plates (Roach 1959). The down-
side of this design was that particles were deposited spatially inhomogeneously, due to
the inhomogeneous thermal gradient. The inhomogeneity of the deposit required high
efforts to evaluate the particle size distribution. More recent developments (Azong-
Wara et al. 2009, 2013; Thayer et al. 2011) use a homogenous thermal gradient between
two parallel plates, which are maintained at different temperature levels to establish
the required gradient. Since up to approximately 300 nm the thermophoretic velocity
is nearly independent of particle size, these samplers deposit particles <300 nm with
very high spatial homogeneity, which allows for a much simplified electron micro-
scopic analysis of the particle size distribution, because only a low number of electron
microscopic images need to be evaluated.
2.5 SIZE-INTEGRATED AND TIME-INTEGRATED MEASUREMENTS
Size-integrated and time-integrated measurements include particle sampling over
the averaging time and subsequent analysis. Sampling and analysis do not include
particle size discrimination. The sampling may, however, include a step to remove
particles above a specified size limit, for example, an impactor as preseparator to
sample only the alveolar fraction <4 µm. Due to its simplicity, this type of measure-
ment can hence be considered as the simplest type of measurement.
2.5.1 f
filtration
m
ethods
Filters can be employed to collect all particles from an aerosol that is drawn through
the filter without particle size segregation. Filter media can generally be divided
into surface filtration and depth filtration media. Membrane filters are often used as
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