What is the least acceptable TAR value? Mettler UMX2 micro balance - Class E2 weights

#1
Hi gang,
I've run into a pickle with performing calibrations on some precision Mettler UMX2 micro balances. I have class E2 weights that were recommended by Mettler to calibrate the balance on a regular schedule. We calibrate the balances using the differential method. If you look at the specs for the UMX2 micro balance, it's enough to make your hair stand on end:
Linearity: ±0.004mg
Repeatability: 0.0009mg
Resolution: 0.001mg

Using class E2 weights, my cal ratio is 2.82:1 based on the uncertainties of the weight and resolution of the balance. Going to class E1 is not that much better. What are minimum TAR values that you can accept? I've read that 1.5:1 is OK if you use guardbandding, but in my case, I'm using the entire range of the balance. Thoughts?
 
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Hershal

Metrologist-Auditor
Staff member
Super Moderator
#2
Actually, there is no absolute minimum for TAR, or for TUR either. ANSI/NCSL Z540-1-1994 specifies a 4:1, but less than a 4:1 will require stated uncertainties.

Hope this helps.

Hershal
 
#3
Hershal said:
Actually, there is no absolute minimum for TAR, or for TUR either. ANSI/NCSL Z540-1-1994 specifies a 4:1, but less than a 4:1 will require stated uncertainties.

Hope this helps.

Hershal
Hi Hershal. Thanks for your input. When you say "stated uncertainties", does this mean stating the uncertainty of the weight we use along with the TAR (TUR) also?

I'm developing a method to use in our procedures, but it looks like we will have to caclutate this on the calibration certificate, as putting it on a procedure might not be the correct way to handle this, since the uncertainty might change depending on the calibration facility.
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#4
Yes, the weight (sometimes also called a Mass standard) is a Type B uncertainty.

You need to also take into account your Type A (the readings) both for repeatability and reproducibility.

Other Type B influences include temp (variation from 20C), eccentricity (where the weight sits on the balance), bouyancy, local acceration of gravity, altitude (this should be minimal effect in Temecula), resolution of the balance, and if the weights are picked up with hands (EVEN WITH THE FINGER COTS OR WHITE GLOVES) there is the thermal coefficient of expansion for the weights.

All these go into developing the stated uncertainties for the calibration of the balance.

Hope this helps.

Hershal
 

Charles Wathen

Involved - Posts
#5
In case anyone is intrested, I received a reply from Robert Brown:
http://www.blogger.com/profile/6367980

Here is his answer:
========================================================
QUESTION: "I'm in the process of developing some type of minimum TAR for Micro Balances. Since the best weight will not guarantee a 4:1 or even a 3:1 ratio, I need to come up with an acceptable level TUR value for these balances."

ANSWER:
TUR is an issue when testing to see if the measured value complies with specifications (a failure could result in an "Out of Tolerance" report).

CASE 1: No Specification Compliance:

If there are no specs for the weights and/or...
1) The weights are only used within your quality system
2) Correction factors are on the calibration report and applied when the wieghts are used.
3) No one depends upon the weights being within a secification of nominal.
Then Out of Tolerance issues do NOT apply. Be sure that the UNCERTAINTY of the Balance measurement (is calculated by squaring the repeatability of the balance, and squaring the uncertainty of the weight we use to check the balance) is DECLARED, and USED when the weight is used to make measurements with declared uncertainties.

CASE 2: Specification of Compliance:

If on the other hand you are verifying that a weight has not drifted too far from a nominal value, and that would result in an out of tolerance condition, then TUR applies. The key problem is that the user of your weight will not be applying correction factors and will be depending upon your assurance and the width of the specification.

In this case,
ISO-17025 p5.10.4.2 "The calibration certificate shall relate only to quantities and the results of functional tests. If a statement of compliance with a specification is made, this shall identify which clauses of the specification are met or not met."

This requirement of ISO-17025 is often interpreted as "guard-banding". In a recent (Aug 17, 2003) paper by David Deaver, "Good, Bad or Indeterminate" there is the only statement that I have seen that refers to least acceptable TUR = 1.5, BE CAREFUL! If you allow a TUR of 1.5 then guard banding will KILL you!! Many false failures will be the result. Dave's recommendation is part of a larger guardbanding strategy (that I support)
1) No guardband if TUR > 4
2) Guard band of 20% for all 1.5 < TUR < 4
3) Calibration NOT ACCEPTABLE if TUR < 1.5

Definition: Guardband
A test limit that is tighter than the actual specification, in order to avoid false pass errors.
========================================================
I also had a good conversation with Robert on the phone (he called me) to get a better clarification of the subject matter. Nice to see people like Robert helping out others in the field of Metrology. Robert L. Brown was presented at the NCSL International 2003 Symposium GUM Uncertainty Analysis With No Greek Letters
http://metrologyforum.tm.agilent.com/uncalc.shtml
In there is a link to his PDF file.
 
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