What is Particle Size?

The goal of all particle-sizing techniques is to provide a single number that is indicative of the particle size. However, particles are three-dimensional objects for which at least three parameters (length, breadth and height) are required in order to provide a complete description. Most sizing techniques therefore assume that the material being measured is spherical and report the particle size as the diameter of the “equivalent sphere” which would give the same response as the particle being measured.

The way the equivalent sphere approximation works is shown above for an irregularly-shaped particle. The diameter reported for this particle will be dependent on the physical property measured by the chosen technique. In the case of laser diffraction the diameter of the sphere that yields an equivalent light scattering pattern to the particle being measured is reported. To a good approximation this corresponds to the sphere of equivalent average cross-sectional area.

Once the particle size has been calculated for the measured sample, the distribution of particle sizes within the sample needs to be displayed. Laser diffraction systems are configured to ensure equal volumes of particles of different sizes yield an equivalent scattering response. A volume distribution, showing the volume percentage of particles that have given size, is therefore reported. This corresponds to a mass distribution in the case where the particle density is the same for all sizes.