When to use k-Factors vs TUR

J

Jack.D

#1
In which cases is it more appropriate to use k-factors and expanded uncertainty over the Test Uncertainty Ratio (TUR)?

My understanding of it is that k-factors are to be used when there is sufficient data to be able to calculate the combined uncertainty, and that the TUR is simply approximating from the accuracy of the reference standard being used.

Does ISO 17025 give preference to using one over the other?

Also if the reference standard is the dominant contribution to the combined uncertainty is giving a k-factor = 2 equivalent to stating a TUR of 2:1?
 
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dgriffith

Quite Involved in Discussions
#2
Re: k-Factors vs TUR

Also if the reference standard is the dominant contribution to the combined uncertainty is giving a k-factor = 2 equivalent to stating a TUR of 2:1?
The TUR and K factor are not related that way. The reference standard is not necessarily the dominant contribution--that may very well be resolution of the UUT.
In general, the TUR is the ratio of the calibration system uncertainties to the UUT specification.
The k factor is a coverage multiplier of the standard uncertainty to give a confidence level. One standard uncertainty x 2 = approximately 95% confidence (1.96 specifically). A k=3 would yield a 99.73% confidence, and so on.

By the way, many think the TUR is easier to do, but you cannot get a TUR without performing an uncertainty budget and analysis at least once. A TAR (test accuracy ratio) only concerns itself with the specifications of the standard and UUT, and is not a good indicator of the quality (risk) of the calibration, in my opinion.
 
Last edited:

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#3
dgriffith is correct.

As for when to use and regarding 17025, in the US, the TUR is considered an "identified metrological specification" and as such is allowed under Clauses 5.6.2.1.1 and 5.10.4.1.b. ILAC P14 was also revised after challenge to continue to allow the use. If you use the TUR, remember it must be backed up by an uncertainty budget.

However, expanded uncertainty is better, even though it is more work initially. Internationally, outside the US, it is typically the requirement, and most countries will not allow the TUR.

Also, the customer can request uncertainty instead of the TUR.

Hope this helps.
 
J

jerryb1426

#4
The use of TUR is for Z540-3 not ISO 17025 which this is not excepted and not allowed. A lot of calibration lab use this in error. If you go to section 5.6 of the standard and ILAC P14 you will have further Guidance.
Lead Assessor
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#5
Actually, in the US, the TUR is allowed, as ILAC revised P14 last year, to conform to ISO/IEC 17025 which allows an "identified metrological specification" in two separate Clauses. In the US, that is codified in the American National Standard ANSI/NCSL Z540.3 as the TUR.

Note, the TUR is based on uncertainty calculation, which inherently means uncertainty is a requirement anyway. To me it may simply be easier to have the uncertainty, rather than the extra work to determine the TUR.

Hope this helps.
 
J

jerryb1426

#6
Note ILAC P14 has nothing to do with TUR ratio and the P14 doc. applies to all ISO17025 Labs world wide TUR in the US are only required for compliance to Z540
If I see a Cal cert for a Standard which is used to calibrate another Instrument with only TUR values and No UNC Values it in violation of P14 and section 5.10 of the ISO17025 standard
Lead Assessor
Jerry Bambach
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#7
It is true that P14 has nothing directly to do with the TUR, except that it is an identified metrological specification, allowed under the revised version of P14. You are correct that one referencing only the TUR cannot be used as a standard to calibrate other items, but is acceptable going to the end user (customer). However, since it is codified in the US under the American National Standard, it is therefore allowed under ISO/IEC 17025 Clauses 5.6.2.1.1 and 5.10.4.1.b as an identified metrological specification. It is of course also up to the AB whether to accept the Z540 series documents within the US.
 
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