Various studies have been made comparing grain size measurement using the MLI method. Typically, within a group of individuals it is rare to find agreement on the same areas examined of better than ±0.25 µm.

In some cases, a comparative measurement of grain size is preferred over the MLI method, usually for reasons of speed of assessment. The most common of these is the ASTM method (E112) in which charts of a standard grain size are compared with the unknown.

Definition of ASTM number

Grains per square inch at a magnification of 100 times = 2N

Examples of the standard charts

Schematic view of grain size equivalent to ASTM 4    Schematic view of grain size equivalent to ASTM 5
ASTM 4    ASTM 5

If a transparency of these charts is superimposed on the sample to be measured, then its grain size can be easily obtained.

The standard charts are arranged in a series such that each grain size number differs from the next one in the series by a factor of 2 or:

No. of grains mm-2 = 2N

Where N is known as the ASTM grain size index. (NOTE, the original scale was developed in number of grains per square inch!). An alternative form of the above equation is:

log (ASTM number) = N × log 2

Or

log (ASTM number) = 0.3010 × N

The grain size can be quoted in terms of N or converted to the MLI value measured as described previously, using the equation below:

ASTM no. = -6.754 × log (MLI in mm) + 3.036