Surface roughness can be measured as either an arithmetic average (aa) or a root-mean-square (rms) average. which value is almost always greater for most surfaces:

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Chemistry

Madelynn Huffman

1 June, 22:17

Surface roughness can be measured as either an arithmetic average (aa) or a root-mean-square (rms) average. which value is almost always greater for most surfaces:

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Liliana Ayers · Answer 1

Rms

The rms average amplifies the effect of isolated large deviations from the mean, while the arithmetic average evens out the larger deviations. For instance, let's assume you have a nice surface finish that is about + / - 3 microns from the desired surface and have one large defect of + 10 microns. Let's see what happens with the math:

Surface roughness measurements:

-2.49, 2.5, - 1.67, - 1.62, - 1.72, 0.4, - 1.75, - 1.63, 2.07, 10

The AA value for the 1st 9 values in the above list is 1.761111

The RMS value for the 1st 9 values is 1.855505

Notice that they're both fairly close to each other.

Now let's recalculate using all 10 values.

AA value is 2.585. A jump of 0.8238.

RMS value is 3.6192. A jump of 1.7637. The increase in the RMS value is over twice that of the AA value.