Equivalent Spheres - Malvern Mastersizer Series Getting Started

C H A P T E R 7
P a g e 7 . 2
G e t t i n g S t a r t e d
30
20
10
0
0.1
The Malvern software allows the result to be converted to other distribution
forms such as a number distribution for example, but it should be remembered
that the initial measurement is volume based and any subsequent conversions are
liable to introduce systematic errors.

Equivalent spheres

The Mie theory presumes that the particles you are measuring are perfect spheres.
In practice they are very rarely so. This causes a problem in the definition of the
term "measure the particles size" - if the particle is an irregular shape which
particular dimension do you measure?
As an example, imagine that I give you a matchbox and a ruler and ask you to tell
me the size of it. You may reply by saying that the matchbox is 50mm x 25mm x
10mm. You cannot say that "the matchbox is 25mm" as this is only one aspect of
its size. It is not possible to describe the three dimensional matchbox with one
unique dimension. Obviously the situation is even more complex for irregular
shaped particles such as grains of sand or the pigment particles in paint.
Most people want a single measurement to describe their sample i.e. they wish to
say that their sample is made up of 50 micron particles for example. What is
required is a unique number that describes the particle. There is only one shape
that can be described by one unique number and that is a sphere. If we say we
have a sphere of 50 microns, this describes it exactly. We cannot do the same even
for a cube as 50 microns can refer to its edge or to a diagonal.
One way to get a single unique number to describe your irregular shaped particle
is to compare some feature of the actual particle to an imaginary spherical particle.
Some typical methods of doing this are:
%
2
1.0 10.0 100.0 1000.010000.0
Particle Diameter (µm.)
1
M A N 0 1 0 1