The Fineness of Grind Gage is a flat steel block in the surface of which are two flat-bottomed grooves varying uniformly in depth from a maximum at one end of the block to zero near the other end. Groove depth is graduated on the block according to one or more scales used for measuring particle size.
The Hegman scale or National Standard scale may be abbreviated "NS" on the gage. The scale ranges from 0 to 8 with numbers increasing as the particle size decreases.
NPIRI : National Printing Ink Research Institute. The scale begins with "0" at the infinite point and extends to "10" at a depth of .001 inches. While it is an arbitrary scale, it is a logical one in that these divisions are equivalent of tenths of mils.