Determining Uncertainty from Gage R&R

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RichardJDrexler

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Folks,

Let me preface this with the fact that I am not six sigma certified.

Process:
1. Weigh tin (Pre-weight)
2. Add solvent w/ contaminate to tin
3. Evaporate solvent; leaving contaminate in tin
4. Weigh tin again (Post-weight)
5. Delta of weights is weight of contaminate

I am trying to determine the uncertainty of a measurement taken with a balance. Given the nature of the work it is extremely important to over estimate the weight of the contaminate rather than under estimate it.

Results from Gage R&R:
Equipment Variation: 0.00003g
Appraiser Variation: 0.00001g
Interaction Variation: 0.00001g

Given my variation results, is it safe to call the sum of my variations the "error" of a possible measurement?

Right now the assumption is that the balance is accurate down to 0.00001g since that is the resolution displayed on the screen. So if the pre-weight measured 2.00000g and the post-weight measured 2.00001g we would say there is 0.00001g of contaminate. I don't think this is accurate given how much a reading on the balance fluctuates after it decides the reading is acceptable.

My thought is that we need an error around the measurement to be conservative. So if the pre-weight measured 2.00000 +/- 0.00005g and the post-weight measured 2.00001g +/- 0.00005g then the amount of contaminate I could have in the worst case is actually: 2.00006g - 1.99995g = 0.00011g. Therefore to be conservative I should use 0.0011g as my result. Additionally, this means I would never be able to claim that I have a contaminate with a weight less than 0.00010g.

I have attached the spreadsheet with my work.

Cheers.
Rich
 

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Jen Kirley

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Good day Rich, I am sorry for the delay in this response.

There is a great deal of information about GR&R and uncertainty out there, ranging from NIST to Universities to bloggers. I am not affiliated with ISO Budgets. I selected this page because it made the most sense to me.

Another Elsmar thread (see a list of related threads at the bottom of this page) describes GR&R as an incomplete tool for measurement uncertainty because of the uncertainty in calibration.

I hope other Covers will also comment. Statistics is not my strongest skill.
 
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