Number vs. Volume Distribution
The easiest way to understand a number distribution is to consider measuring particles using a microscope. The observer assigns a size value to each particle inspected. This approach builds a number distribution—each particle has equal weighting once the final distribution is calculated. As an example, consider the nine particles shown in Figure 1. Three particles are 1 μm, three are 2 μm, and three are 3 μm in size (diameter). Building a number distribution for these particles will generate the result shown in Figure 2, where each particle size accounts for one third of the total. If this same result were converted to a volume distribution, the result would appear as shown in Figure 2 where 75% of the total volume comes from the 3μm particles, and less than 3% comes from the 1μm particles.