By Paul Webb, Micromeritics
This article begins with a pertinent story from the writers tenure as a graduate student in Physics.
A fellow student and I were working together on a homework set. We had spent the majority of the day on a particularly difficult problem that resulted in a lengthy equation expressed in terms of assorted variables. I turned to the back of the book to compare our result to that of the author's and was astonished by the dissimilarity. I showed the 'correct' solution to my friend, a far better and more confident student than I. After looking at it he asked, "Did we apply the appropriate theorems?" I affirmed that we had. Next, he asked, "Did we make any mathematical errors?" I was confident that we had not. "Then," he proclaimed, "our solution is correct-- we just are expressing it in different terms."
I recall this incident when I encounter a debate over the 'correctness' of the results obtained for particle size measurements by two or more different analytical techniques. Provided that the instruments used are capable of producing high-quality data, the pertinent questions, then, are, "was the sample properly prepared and properly presented to the instrument," and "were the analytical parameters applied correctly." If the answer to both is "yes," then both analytical results probably are equally correct; they are just expressed in different terms.
If what has been stated thus far has not raised any questions, you probably don't need to read the rest of this article. This article is intended to resolve questions users often have concerning comparisons of particle sizing results by different techniques. The techniques referenced are sieving, sedimentation, imaging (including microscopy and machine vision), electrozone sensing, and light scattering. The determination of particle size on the same sample by all of these techniques and others not mentioned will, in the majority of cases, yield different results for mean size, modal size, and quantity distribution by size.
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