Weight Calibration Uncertainty

[Debbie]

Our calibration lab has initiated uncertainty measurements of weight standards.
They are responsible for calibrating around 60 different weight sets (around 25 weights per weight set) and around 200 individual weights.

They just showed me what they were doing to calculate uncertainty -- it looked close and might be OK -- but I thought I'd check to see what more experienced people thought.

They calculated uncertainty based on the uncertainty of the lab standard weight and the standard deviation of three reps of ABBA sequences (A= standard ; B= unknown). THey calculated a standard deviation based on six differences (A's - the adjacent B's).

Is this OK -- or would it be better to pool the standard deviation from like weight calibrations? THey have no historical data.

 

[DICKIE]

Debbie,
How did they "calculate" the uncertainty? How are the measurements being performed? What is the resolution of any instruments? Are instruments analog or digital? What are the uncertainties of all of the components of the measuring system?

 

[Debbie]

Thank you very much for your response. The balance that is used is digital. It is considered to be the "Standard" balance for weight calibrations. For a given weight calibration, the same balance is always used, i.e., for all 1 gram weights, a particular Mettler or Sartorius balance is used.

Calculations at this point are based on three repeats of six comparisons of working standard to lab standard. Here's an example of the data


1g
A 0.99997
B 1.00015
B 1.00021
A 0.99998
A 0.99995
B 1.00020
B 1.00019
A 0.99987
A 0.99993
B 1.00019
B 1.00014
A 0.99991
A&B DIFF 0.00024


MEAN A 0.99994
MEAN B 1.00018

Uncertainty was calculated by determining the standard deviation of the lab standard's weight (not the uncertainty of the differences between the lab standard and the reference) and combining that with the standard deviation of the reference.

I see referenced in EA-4/02 that the difference within each ABBA grouping should be considered a single observation. If I directed the lab in that direction, there would only be three observations. Since they calibrate many similar sets of weights to the same reference, can I pool the standard deviations from similar comparisons (e.g., all similar 1 gram weight references)?

Sorry this is so long, I really appreciate comments and responses that I have gotten.

 

[DICKIE]

Debbie,
I'll give you some of the details of a 1 gram analysis I just did.

Subject unit is a class F 1 gram mass.

Measurement parameter is 1 gram reference mass calibrated with a stated uncertainty of 0.01832 mg from the calibration source.

five readings on the class F mass yield standard deviation of .005 mg at 95.0 confidence level.

next I use uncertainty=(xmass*(1-(air density/ref mass density)))/(1-(air density/mass density\))-.001

combining these values gives uncertainty of .018mg with infinite degrees of freedom.

root sum square of the component uncertainties give combined uncertainty of .0259mg at 95% confidience level and 348 degrees of freedom.

Hope this helps

Greg

 

[tomvehoski]

Try downloading the freeware proficiency calculator from http://www.proficiency.org. It is in the downloads section. There are examples for mass and scale calibration, along with many others. It was very useful on a recent 17025 audit for a scale calibration company I consulted for.

Hope this helps,

Tom

 

[Debbie]

Tom,

I downloaded the software -- looks real good.
I have a few things to figure out -- (I'm referring to the UC mass 10 kg nominal.unc file. For example 'comparator linearity'.

This example and the replay from Greg have helped me a lot. We were initially going to ignore boyancy -- but I can see that I need to take that into account.

I'm sure we will be using this software. I like that examples are included.

Debbie

 

[Debbie]

Greg,

Thanks for your help. I've looked up a lot of information on boyancy -- I see that I can't ignore it. I'll have to go down to the lab and check out conditions to figure out the air density. I already know temperature and humidity.

I really appreciate you taking the time to review the information in my post and reply.

Thanks again.

Debbie

 

[tomvehoski]

You might be able to get more help from your state department of weights and measures. In Michigan, the Dept. of Agriculture certifies most of the calibration standards in use by scale companies. They might be more willing to help than a commercial lab. I found a link to all of the different state departments here:

*** Dead Link Removed ***

You can try http://consumeraction.gov/

 

[howste]

Try downloading the freeware proficiency calculator from http://www.proficiency.org. It is in the downloads section. There are examples for mass and scale calibration, along with many others. It was very useful on a recent 17025 audit for a scale calibration company I consulted for.

Hope this helps,

Tom

Tom, is there any chance you still have some of the freeware downloads from the site? They no longer have a download section on their website.