What is Particle Size (Particle Size Distribution)?

• Particle size refers to the size of a particle, and particle size analyzers are equipment that measure this. It expresses particles of various shapes as a single number using the theory of equivalent spheres, which is used for quality control and product specification verification.

This is the first session explaining the basic principles of particle size analysis.

Measurement of Particle Size (Particle Size Distribution)

What would be the size of a matchbox with dimensions 20*10*5mm? It is probably not possible to answer accurately as being 20mm. This is because a 3-dimensional matchbox cannot be described with a single unique number.

It becomes even more difficult to describe particle size (particle size distribution) in cases of complex shapes like sand particles or pigment particles in a can of paint.
However, someone in charge of quality control would like to have a single number to describe the particles. For instance, they would want to verify whether the average particle size (particle size distribution) of the last batch of products increased, decreased, or met the specifications upon shipping.

How can we represent a 3-dimensional object with just one number?

Equivalent spheres, Particle Size Distribution in Equivalent Spherical Form

There is a sphere that can be represented by a unique number. When we refer to a sphere with a diameter of 50μm, it can be precisely expressed. However, when referring to the edges and diagonals of a 50μm cube, expression becomes challenging.
A matchbox with many characteristics can be expressed with a single number, such as its weight related to volume and surface area. If there is a technique for measuring the weight of a matchbox, we can convert its weight to that of a sphere.

Remember that weight = 4⁄3 πr³ • P

This is the theory of equivalent spheres. We measure the characteristics of a particle and assume it to be a sphere. Thus, we obtain particle size distribution as a single specific number (the diameter of the sphere) to describe the particle. However, if the shape or size of a cylinder changes, the volume/weight will vary. At least, with the equivalent spherical model, we can say that it has become smaller or larger.

For example, what is the equivalent spherical particle size (particle size distribution) of a 100 x 20μm cylinder?

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