I thought this was a topic I understood well but hav begun to doubt myself when I look at some other calibration certificates.

Take the calibration of a 1" micrometer at 1" that reads by 0.001"

The uncertainty of this calibration using gage blocks is roughly 0.0006". 1/2 the resolution of the micromter is the dominant factor.

The accuracy of a 1" mictometer is 0.001". I would say that I do not meet a 4:1 TUR because 0.001"/0.0006" is 1.67. My TUR is 1.67.

When I read the definition from the Metrology Handbook it says that TUR = (UUT Tolerance / STD Uncertainty).

This might lead me to believe that the TUR would be 0.001"/0.000010" = 100 where 0.000010" is roughly the accuracy of a 1" gage block.

This question comes up when I read calibration certificates from accredited labs that says " uness otherwise noted, we meet a 4:1 TUR....".

By my definition, any calibration certificate with this wording would have to report the uncertainty on every certficate where the accuract of the unit under test is equal to 1 division. (ie most weighing scales, micrometers, etc..)

For certificatges I issue, I have used a more stingent definition of the term which would lead me to provide a good calibration certificate to my cusromer either way.

I would apprecaite any thoughs on the topic.

TUR
"Uncertainty of the measurement standards shall not exceed 25% of the
acceptable tolerance." To elaborate, this statement is somewhat misleading, as the actual MU of the calibration process should not exceed 25% of the acceptable tolerance, not just the unc of the master standard. The idea is to take into consideration the major sources of uncertainty, including environmental and operator.

So you would need to know the accuracy of your instrument...so in the case of your micrometer lets say you have an accuracy of +/- .0001" in this case you would need a calibration process which has an uncertainty less than .000025". At this point you would need to know the calibration uncertainty of your calibration process and ensure it is less than the allowable .000025" if you want to meet the 4:1 TUR.
The two items that come into play are your instruments' accuracy, and the uncertainty of your calibration process.

Please discuss further if this doesn't sound correct to you, I don't deal with TUR on any significant basis so I may not be completely on par when it comes to it's practice....

http://www.transcat.com/PDF/TUR.pdf

Seems a handy description of the TUR idea. Easily confused with TAR...accuracy and uncertainty are two different things though.

Generally, anytime there is a question of 4:1, you should ask your accredited cal lab for the calculations.....NOT their BMC.....BMC is after all often unrealistic.....

If in fact they can support the 4:1 then OK.....although there is a trap.....your default uncertainty now becomes 25% of the rated accuracy of your instrument for that measurement because you don't have any other number.....and often that will be higher than the actual calibration uncertainty.....a good thing actually as that is generally a more conservative number.....

However, if they can't support the 4:1 they should report the specific uncertainty.....