What do those terms mean?
Well, they actually mean the same thing, but are expressed in opposing orders. In a strainer or filter, they measure the size of the openings that materials pass through and consequently what is stopped.
A micron, short for “micrometer", is a millionth of a meter in size. That works out to about 0.00004 inches. So, calling it “tiny" would be an understatement, but tiny things add up. Bacteria can be as small as 0.2 microns or as big as 20 microns, although they are usually on the smaller size. A human red blood cell ranges between 8-12 microns. White blood cells are larger at about 25 microns. A human hair is 75 microns in diameter, although that varies depending on the person. And on it goes. Beach sand is about 100 microns, which is the smallest size screen that Direct Material keeps in stock for our y-strainer. We also stock 20 mesh (840 micron-wide openings), by-the-by.
Mesh is a bit more straightforward compared to microns. Take one square inch of filtering material (screen) and count the holes in it. If you have 80 openings in that area, then you have an 80-mesh screen. If you have 200, then it is 200-mesh and so forth. Therefore, the higher the mesh number, the smaller the particles that are filtered. That means the two measures run in reverse order, so a #10 mesh has openings of 2,000 microns and a #400 mesh has openings of about 37 microns wide. It's also worth noting that #400 mesh is about the largest (or smallest, depending on your perspective) that mesh size is generally used. Screens finer than that are usually expressed in microns because the holes are so small that “mesh" becomes imprecise, since different gauges of wire are used to make the screen. Everything as clear as mud? Wonder what size of screen you should use to filter mud? Take a look at the chart to the right. It breaks everything down in mesh, microns, millimeters and inches.