In fact all three dimensional objects require three numbers to describe them.
Considering this, it is difficult to see how we can arrive at a single number which
uniquely describes the matchbox’s particle size. Obviously the situation is even
more complex for irregularly shaped particles.
In particle size analysis, most people want to have a single measurement to
describe their spray, for example, they wish to say that their spray is made up of
droplets with a 50mm diameter. What is required is a unique number that
describes the droplet. If we say we have a sphere with a 50mm diameter, this
describes it exactly.
One way to get a single unique number to describe an irregular shaped droplet is
to compare some feature of the actual droplet to an imaginary spherical droplet.
This technique is known as equivalent spheres.
Some typical methods of doing this are:
.
Equivalent surface area – calculation of the diameter of a theoretical sphere
that has the same surface area as the original droplet.
.
Equivalent maximum length – calculation of the diameter of a theoretical
sphere that has the same maximum dimension as the original droplet.
.
Equivalent minimum length – calculation of the diameter of a theoretical
sphere that has the same minimum dimension as the original droplet.
Many other methods are also available.
The Spraytec uses the volume of the droplet to measure its size. In the example
above, the matchbox has a volume of 50 x 25 x 10mm = 12500mm
3
.Ifthe
Spraytec was able to measure this size of “particle” it would take this volume and
calculate the diameter of an imaginary sphere that is equivalent in volume – in
this case it is a sphere of about 30mm diameter.
Obviously, a different answer is obtained if the surface area or maximum
dimension of the matchbox is used 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 made using
the same technique, or measurements of spherical particles.
CHAPTER 2
Spraytec
Page 2.8 MAN 0368
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