Displaying results
The final step is to display the results. This section explains:
.
Why the results are volume-based.
.
How the distribution parameters are derived.
.
What parameters can be derived from the analysis.
Volume-based results
The first point to remember in interpreting results is that the fundamental size
distribution derived is volume-based. This means, for example, that a result
indicating that 11% of the distribution is in the size category 6.97-7.75mm means
that the volume of all particles with diameters in this range represents 11% of the
total volume of all particles in the distribution.
A simple numerical example illustrates this point. Suppose a spray consists of only
two sizes of particle, 50% by number having a diameter of 10mm and 50% by
number a diameter of 100mm. Assuming that the particles are spherical, the
volume of each larger particle is 1000 times the volume of one of the small ones.
Thus, as a volume distribution, the larger particles represent 99.9% of the total
volume.
Derived distribution parameters
The second point to remember is that the analysed distribution is expressed in a
set of size bands, optimised to match the detector geometry and optical
configuration which gives the best resolution. All parameters are derived from
this fundamental distribution.
Distribution parameters and derived diameters are calculated from the
fundamental distribution using the summation of the contributions from each
size band. In performing this calculation the representative diameter for each
band is taken to be the geometric mean of the size band limits, defined as:
dd
il i-
This number is slightly different from the arithmetic mean, defined as:
dd
il i-
+
2
For example, the size band 404.21 - 492.47mm has a geometric mean of 446.16mm
and an arithmetic mean of 448.34mm. In most cases the difference is small but the
CHAPTER 2
Spraytec Page 2.9
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