Re: Vetting a potential calibration vendor (supplier)
TMJ05, no sweat, we are here to help.
Joe222 is right in that it is a ratio. This used to be known as Test Accuracy Ratio or TAR.
The definitions have changed as well. Joe is also right about Z540, which has three versions.
ANSI/NCSL Z540-1-1994 (now retired) in Paragraph 10.2.b describes, allows, and for some applications requires use of the 4:1 TAR. This Standard is still in common use and is likely to remain so for the foreseeable future. This Standard is literally the combination of two previous Standards. ISO Guide 25 takes care of Part 1 and MIL-STD-45662A takes care of part 2, both long since retired.
ANSI/NCSL Z540-2-1997 is also known as the "U.S. GUM" or Guide to the Expression of Uncertainty in Measurement, and while exceptionally informative and technically correct, it is also the most boring document in the history on Mankind in my opinion. But for Metrology professionals, Z540-2 or the ISO GUM is indispensable.
ANSI/NCSL Z540.3-2006 is the current replacement for Z540-1 and Clause 5.3 (and sub-Clauses) describe the Test Uncertainty Ratio known as TUR.
Joe is perfectly correct for TAR, but the TUR is now defined as "The ratio of the accuracy tolerance of the unit under calibration to the uncertainty of the calibration standard used." (where: Uncertainty is expressed as expanded uncertainty at approximately 95% confidence) And this is a two-sided approach to account for various ways of expressing tolerances.
Now, if this is confusing, don't stress, you are not even remotely close to being alone, many are.
If your organization is open to assuring ou have the training that you will find useful, on a short notice, there is training in uncertainty at NCSLI in about 12 days in San Antonio, website:
www.ncsli.org
If like most they prefer to plan a bit, then other organizations also provide training in ANS/ISO/IEC 17025:2005 and uncertainty among other topics. There are calendar entries and notes describing upcoming training.
Hope this helps.