Mesh is a unit of particle size. Very often, the effect obtained with a specific pyrotechnic mixture depends not only on its chemical composition but also on the size and shape of the particles. In those cases, the particle size is specified in ‘mesh’ units, referring to the screen that is used to sleeve the powder. For example, 80 mesh refers to a screen that has 80 holes per linear inch. The smaller the mesh number, the larger the particles. The larger the number, the finer the particle size. (A 10 mesh screen has 100 holes per square inch. 80 mesh screen has 1600 holes per square inch.) When a narrow range o particle size is required, it may be specified as in the following example: -100 mesh + 40 mesh, meaning that the material is too large to fit through a 100 mesh screen, but small enough to pass through a 40 mesh screen.
The minus (-) and plus (+) signs mean that the particles of -200 mesh aluminium would pass through a 200 mesh screen and +200 mesh aluminium means that all the particles are retained on a 200 mesh screen. Generally anything smaller than 400 mesh is reffered to as micron.
A micron is another measurement used for measuring particle size. A micron is on-millionth of a meter or one twenty-five thousandth of an inch.
Screen mesh numbers and relative opening size