Uncertainty calculation - Estimation for a Mettler Toledo XP 56 balance at 20 grams

BradM

Staff member
Admin
This is an estimation for a Mettler Toledo XP 56 at 20 grams.

Can you folks take a look at see what you think? Any comments, suggestions, etc. would be greatly appreciated.

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Daniel Walker - 2011

Re: Uncertainty calculation - Estimation for a Mettler Toledo XP 56 balance at 20 gra

Component #2: This should be the tolerance of the standard, distributed rectangularly.

#'s 3 and 7 are redundant. Since you are determining the standard deviation of your balance with your operator in your environment, component #3 is not needed.

#'s 4 and 5 are also covered since you have determined the balances standard deviation. Calculating the Sp (standard deviation of process) incorporates lines 3, 4 and 5.

Check out NIST publication 6919 http://ts.nist.gov/WeightsAndMeasures/upload/NISTIR6919.pdf...... you may be thinking about it a little too much.

BradM

Staff member
Admin
Re: Uncertainty calculation - Estimation for a Mettler Toledo XP 56 balance at 20 gra

Thank you, Daniel.

BradM

Staff member
Admin
Re: Uncertainty calculation - Estimation for a Mettler Toledo XP 56 balance at 20 gra

BTW... Daniel I do have that standard. Thank you for the mention.

I had an associate point out that my #7 should have been divided by 9, to allow for degrees of freedom.

They also suggested I should account for balance linearity, off-center loading, and thermal/vibration considerations. Any thoughts on these, or how to estimate them?

D

Daniel Walker - 2011

Re: Uncertainty calculation - Estimation for a Mettler Toledo XP 56 balance at 20 gra

It seems as though you may be trying to replace calibration data with an uncertainty number. Linearity and off-center loading errors should be identified during routine calibrations. All scale calibrations should include multiple points as well as placing a weight in the four "quadrants" of the weighing platter to ensure off-center accuracies. The results for the off-center loading (section test) should be accompanied by an associated uncertainty. It will probably be the same uncertainty as the linearity test at the same load.

Thermal/vibration is a tough one. One could argue that since the standard deviation test was performed in the environment where the balance is being used, the effects are accounted for in the results.

BradM

Staff member
Admin
Re: Uncertainty calculation - Estimation for a Mettler Toledo XP 56 balance at 20 gra

Again, thank you Daniel. When we perform the calibration test, the balance is verified at 10, 50, and 90% of the range. Also, the corners of the balance are verified to the calibration tolerance. We have a fairly experienced balance technician; he knows when something is not right.

BradM

Staff member
Admin
Re: Uncertainty calculation - Estimation for a Mettler Toledo XP 56 balance at 20 gra

..The results for the off-center loading (section test) should be accompanied by an associated uncertainty. It will probably be the same uncertainty as the linearity test at the same load.
I got confused on this one. What component of the uncertainty calculation would change within the off-center loading, as compared to the uncertainty that I calculated already? Saying, if I am checking off-center or linearity at 20 grams, wouldn't the uncertainty be the same?

Thermal/vibration is a tough one. One could argue that since the standard deviation test was performed in the environment where the balance is being used, the effects are accounted for in the results.
Good point. I'm not interested in making this more difficult than it needs to be, so I will probably adhere to this logic, unless I have evidence that additional consideration should be made.

D

Daniel Walker - 2011

Re: Uncertainty calculation - Estimation for a Mettler Toledo XP 56 balance at 20 gra

No, your right. My original statement was a bit confusing. my point was the uncertainty would be the same if you were testing the corners or the center.

BradM

Staff member
Admin
Re: Uncertainty calculation - Estimation for a Mettler Toledo XP 56 balance at 20 gra

Here is the final, revised uncertainty estimation for the balance.

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Thaumaturge
Super Moderator
Re: Uncertainty calculation - Estimation for a Mettler Toledo XP 56 balance at 20 gra

Here is the final, revised uncertainty estimation for the balance.
Thanks for the update.

If available, should component 2 be calculated from historic data between calibrations of the mass standard? Would it be the standard deviation of the drift values at each calibration?

I assume that the M3003 referenced in component 4 is the one published by UKAS. How was the air buoyancy uncertainty calculated?

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