it was actually krumbein who did the roundness chart and power did sand grain.
I did a piece of work on it in my first year and the book Geomorpholgical Techniques was a good help in not only giving method but spotting flaws in that method as well.
“Krumbein (1941) supplied a roundness chart for pebbles, while Power’s chart is supposed to apply to sand grains only”
The chart is the first thing that comes up when you search for Krumbein on google image search.
Hope that it is useful
Source: Geomorpholgical Techniques 2nd Edition, Andrew Goudie et al, Routledge, Visual comparison (126.96.36.199)