Most mixing processes, up to 50%, have a solid component in the fluid. Sometimes the solids are integral to the process, and in others they are an unwanted contaminate. Specifying a mixer is a very difficult job in many cases.

The solids may be a uniform size, or more commonly will be a mixture of sizes. Defining a practical size distribution can be difficult and may be based on estimates of the particle size. The use of the terms 80% suspension point, 95% or 100% can be either accurate or estimates.

If the process involves the reaction of the solids, the actual suspension may or may not be a critical factor. Definitions such as, on bottom motion, off bottom suspension, or complete uniformity are very helpful, even if they are qualitative.