Table of contents 1 Why is particle size important?

WHICH SIZE TO MEASURE?
A spherical particle can be described using a single number—the diameter—
because every dimension is identical. As seen in Figure 1, 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, 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 very large 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 

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