Which Size to Measure?

A spherical particle can be described using a single number—the diameter—because every dimension is identical. Non-spherical particles can be described using multiple length and width measures (horizontal and vertical projections are shown here). These descriptions provide greater accuracy, but also greater complexity. Thus, many techniques make the useful and convenient assumption that every particle is a sphere. The reported value is typically an equivalent spherical diameter. This is essentially taking the physical measured value (i.e. scattered light, acoustic attenuation, settling rate) and determining the size of the sphere that could produce the data. Although this approach is simplistic and not perfectly accurate, the shapes of particles generated by most industrial processes are such that the spherical assumption does not cause serious problems. Problems can arise, however, if the individual particles have a verylarge aspect ratio, such as fibers or needles.

Shape factor causes disagreements when particles are measured with different
particle size analyzers. Each measurement technique detects size through the
use of its own physical principle. For example, a sieve will tend to emphasize the
second smallest dimension because of the way particles must orient themselves
to pass through the mesh opening. A sedimentometer measures the rate of
fall of the particle through a viscous medium, with the other particles and/or the
container walls tending to slow their movement. Flaky or plate-like particles will
orient to maximize drag while sedimenting, shifting the reported particle size in
the smaller direction. A light scattering device will average the various dimensions
as the particles flow randomly through the light beam, producing a distribution of
sizes from the smallest to the largest dimensions.

The only techniques that can describe particle size using multiple values are
microscopy or automated image analysis. An image analysis system could
describe a non-spherical using the longest and shortest diameters, perimeter, projected area, or again by equivalent spherical diameter. When reporting a particle size distribution the most common format used even for image analysis systems is equivalent spherical diameter on the x axis and percent on the y axis. It is only for elongated or fibrous particles that the x axis is typically displayed as length rather than equivalent spherical diameter.