A set of test blocks came back marked scratched and out of calibration. I called and was told they could be re-graded from grade 2 to a grade 3. Was also told that this was ok to do. Had something to do with the test to uncertainty ratio--GUM. Anyone have any info as to where I can find out what they are talking about?

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.

Thank you very much for responding. Where can I find the information the which you are describing?

I guess I can anwer my own question. FYI I found a web site www.agilent.com/metrology. Answered all my questions.

I guess I can anwer my own question. FYI I found a web site www.agilent.com/metrology. Answered all my questions.

As Marc says A search is a wonderful thing

But if you had not asked we wouldn't have the link