Grade 3 is double the tolerance of Grade 2, but keep in mind that we are still talking +8/-4 microinches for Grade 3 up to an inch, which is generally still overkill for most people.
Test Uncertainty Ratio (TUR) - The uncertainty of the standards and conditions vs. the uncertainty that you need to prove conformance. Say you have a 1 inch, Grade 3 test block, and you are in a room that is 75°F. To be on the really safe side, you take the worst case for the block (8 microinches) and add in the uncertainty of the calibration for the block (should be around 4 microinches). So your blocks have a basic uncertainty of 12 microinches. The temperature effect on the blocks will be 6.4 microinches per inch per degree, or in this example, 6.4 x 7°F (75-68), or 44.8 (we'll round to 45) microinches. We'll play it really safe again, and just add our uncertainties, so 12 + 45 = 57 microinches.
Now, say you are testing something to a tolerance of 0.0005", and your test uncertainty is 57 microinches. 0.0005 / 0.000057 = 8.77, so your TUR would be 8.8:1.
GUM - Guide to the Uncertainty of Measurement. This is a book of pretty deep statistics that most people cannot even fathom, mainly because it seems it was written with the idea that you are already well-versed in statistics, and just need to know how to implement it.
Ryan
P.S. I know, I left a lot of stuff out, and should have used RSS instead of addition, and taken a bunch of other things into account, but this was to be really basic.
