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geometric mean is chosen in these calculations as more appropriate to the
logarithmic spacing of the fundamental size classes.
The same principle of calculation applies to the standard deviation.
For mono-size distributions the distribution mean is reported as the geometric
mean of the size class and the standard deviation is reported as zero.
The procedure used for other parameters of the distribution is to create a
piecewise-linear fit to the fundamental result. Intermediate values are then read
from this curve, allowing interpolation of percentile points, which do not
coincide with the measurement size band boundaries.
What is measured?
The software always calculates a number of standard values , described below. In
addition to these, the user can select or define a set of derived parameters
according to need. These are described in Chapter 10.
The
standard values
are the following:
.
Trans – the transmission, a measure of the amount of laser light reaching
the beam power detector. When no spray is present in the measurement
zone, the transmission is 100%. A certain amount of light is blocked out
when particles are introduced into the measurement area.
The drop in transmission that occurs when a spray is introduced into the
measurement zone can be used to trigger the start of a measurement.
.
Cv – the volume concentration. This is calculated from the Beer-Lambert
law and is expressed as a percentage.
.
SSA – the Specific Surface Area, the total area of the particles divided by
their total weight. If using this value, the density of the spray must be de-
fined (this is one of its optical properties). Note that this figure is a mathe-
matical calculation based on the assumption that the particles are both
spherical and non-porous.
.
Percentiles – Dv(10), Dv(50) and Dv(90). Dv(10), for example, is the par-
ticle size below which 10% of the spray lies. This may be termed the “ten
percent cut-off point”.
.
Span – this measures the width of the distribution. The narrower the dis-
tribution, the smaller the span becomes. The span is calculated as:
Dv(90)
Dv(50)
-
Dv ( ) 10
.
Moments – the volume mean diameter D[4,3] and the surface area mean
diameter D[3,2]. D[3,2] is also called the Sauter Mean Diameter.
C H A P T E R 2
S p r a y t e c
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