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. Equivalent surface area. You can calculate the diameter of a theoretical
sphere that has the same surface area of the original particle.
. Equivalent maximum length. This is where the diameter of a theoretical
sphere is the same as the maximum dimension of the original particle.
. Equivalent minimum length. This is where the diameter of a theoretical
sphere is the same as the minimum dimension of the original particle.
There are many other methods available to do this. This technique is known as
"equivalent spheres".
The Mastersizer uses the volume of the particle to measure its size. In the
example above the matchbox has a volume of 50x25x10mm = 13750mm
Mastersizer was able to measure this size of "particle" it will take this volume and
calculate the diameter of an imaginary particle that is equivalent in volume - in
this case it will be a sphere of 30mm diameter.
Obviously you will get a different answer if you where using the surface area or
maximum dimension of the matchbox to calculate an equivalent sphere. All of
these answers are correct but each is measuring a different aspect of the matchbox.
We can therefore only seriously compare measurements that have been measured
using the same technique.
Derived distribution parameters
The third point is that the analysed distribution is expressed in a set of size classes
which are optimised to match the detector geometry and optical configuration
giving 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:
d di
i l −
This number will be slightly different to the arithmetic mean:
+
d
d
−
i l
i
2
For example the size band 404.21 - 492.47 microns has a geometric mean of
446.16 microns and arithmetic mean of 448.34 microns. In most cases the
difference is small but the geometric mean is chosen in these calculations as more
appropriate to the logarithmic spacing of the fundamental size classes.
C H A P T E R 7
G E T T I N G
S T A R T E D
!
. If the
P a g e 7 . 3
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