The Elsmar Cove Discussion Forums Propagation of Errors in Calculations - Thickness of a coating material inside a tube
 Forum User Name Keep Me Logged In Password
 Register Photo Albums Blogs FAQ Registered Visitors Social Groups Calendar Search Today's Posts Mark Forums Read

 Elsmar Cove Forum Visitor Notice(s) It is the Memorial Day holiday weekend in the US so activity in the forum will be slow through the weekend. Since Memorial Day is observed on Monday 25 May this year, I expect activity here to be slow through the weekend and on Memorial Day. Please see the forum Calendar for more information on the holiday. Logged In Registered Members can click the red X to close this notice box.

# Propagation of Errors in Calculations - Thickness of a coating material inside a tube

 Search the Elsmar Cove @import url(https://www.google.com/cse/api/branding.css); Search Elsmar Monitor the Elsmar Forum Monitor New Forum Posts Follow Marc & Elsmar Elsmar Cove Groups Sponsor Links Donate and \$ Contributor Forum Access Courtesy Quick Links Links that Elsmar Cove visitors will find useful in your quest for knowledge: Howard'sInternational Quality Services Marcelo Antunes'SQR Consulting Bob Doering'sCorrect SPC - Precision Machining NIST's Engineering Statistics Handbook IRCA - International Register of Certified Auditors SAE - Society of Automotive Engineers Quality Digest Portal IEST - Institute of Environmental Sciences and Technology ASQ - American Society for Quality

 Related Topic Tags measurement (general)
Post Number #1
23rd April 2012, 05:09 PM
Propagation of Errors in Calculations - Thickness of a coating material inside a tube

Hi All, I have a problem in calculating the thickness of a coating material inside a tube. The tubes inside diameter is measured before and after coating. The coating thickness is calculated by substracting the diameters and dividing by two to get the thickness of the coated material. The before coat diameter is 0.3949". The after coat diameter is 0.3893". The Total Gage R&R standard deviation is 0.000127". I need to calculate the coating thickness value and its associated error.

I am thinking the thickness without error = (0.3949 - 0.3893)/2 = 0.0028"
The associated error from the resulting calculation would be

± Error = [5.15 * (0.000127”2 + 0.000127” 2).5]/2 = ± 0.000462”
I would record the thickness as 0.0028 ± 0.000462”?

I saw some examples where they standard deviation was used in place of the 5.15 multiplier value i.e., 0.0028 ± 0.00018. I would think this would only capture some 66% of the uncertainty.
P.S. I used Big 3 MSA value of 5.15 instead of 6

Post Number #2
24th April 2012, 03:25 AM
 Stijloor Total Posts: 14,919
Re: Propagation of Errors in Calculations - Thickness of a coating material inside a

A Quick Bump!

Thank you very much!!

Stijloor.
Post Number #3
25th April 2012, 07:46 AM
 Stijloor Total Posts: 14,919
Re: Propagation of Errors in Calculations - Thickness of a coating material inside a

Again A Quick Bump!

Can someone help?

Thank you very much!!

Stijloor.
Post Number #4
25th April 2012, 10:26 AM
Re: Propagation of Errors in Calculations - Thickness of a coating material inside a

Quote:
 In Reply to Parent Post by [COLOR=black I need to calculate the coating thickness value and its associated error.[/COLOR] I am thinking the thickness without error = (0.3949 - 0.3893)/2 = 0.0028" The associated error from the resulting calculation would be ± Error = [5.15 * (0.000127”2 + 0.000127” 2).5]/2 = ± 0.000462” I would record the thickness as 0.0028 ± 0.000462”?
HI - A couple of questions. If you're stating the margin of error around the average coating thickness I'm not sure why you've included the GRR StDEV twice. Also, the CI formula (margin of error) has us taking the Z value (here 5.15 or 6) * StDEv/SQRT of sample size - I don't see the SQRT of n in your calculation - CAn you share the actual formula you're using to express the margin of error?

Also - yeah folks have moved to using 6 rather than 5.15 as it shows a confidence level of 99.73 rather than 99.

Cheers,

Post Number #5
25th April 2012, 11:52 AM
Re: Propagation of Errors in Calculations - Thickness of a coating material inside a

Well, I am thinking the reason I am using the standard deviation twice is we measure the ID twice and calculate the difference between the two measurements to get the thickness of the coating. i.e.,
Machined ID = 0.3955"
Coated ID = 0.3925"
Coating thickness = (Machined ID - Coating ID)/2 = (0.3955" - 0.3925")/2 = 0.003".
Each measurement is made by the same instrument and each individual measurement would induce error. In this case we made two measurements to get the coating thickness. (If I only made one measurement I think I would only use the error Std Dev once.) The first measurement error is compounded with the second measurement error when both are used in a calculation. (See attachment if I am lucky enough to have it uploaded for an explanation.) The standard deviation comes from using Minitab's Gage RR via a two factor cross random ANOVA array. We performed this using three operators and three trials with 10 parts. Thanks for the help!
Attached Files: 1. Scan for viruses before using, 2. Please report any 'bad' files by Reporting this post, 3. Use at your Own Risk.
 Propagation of Errors.pdf (326.8 KB, 11 views)
Post Number #6
30th April 2012, 12:03 PM
 Allattar Total Posts: 234
Re: Propagation of Errors in Calculations - Thickness of a coating material inside a

In propagating errors you add variances, not standard deviations.

You would square both standard deviations, and then add them. Then square root to get the standard deviation of the system. In which case your error, if assuming the same standard deviation on measurements before and after becomes Std Dev * root 2.

The problem can be broken down into

Difference + Error(overall) = (Before + error(Measure) - (After + error(measure)

but my heads to frazzled today to work it further than that
 Thank You to Allattar for your informative Post and/or Attachment!
Post Number #7
30th April 2012, 12:28 PM
 Bev D Total Posts: 3,248
Re: Propagation of Errors in Calculations - Thickness of a coating material inside a

Allatar is on the right track. unfortunately AIAG has the wrong formulas for truly extracting measurement error from total observed variation. As Allater said - you do not use standard deviations you use variances.

But befroe commenting furhter, I need to ask why you want to know the measurement error? what will you do with the info? how will you use it?
the answers will provide us wiht the means for giving you a useful answer.

for example, if you aren't doing anything with the data, but are simply reporting it to your customer, then we woudl give one answer. If you were going to use it to guardband yoru measurements or develop a better measurement sytem, we would give a very different answer...

 The Elsmar Cove Discussion Forums Propagation of Errors in Calculations - Thickness of a coating material inside a tube

 Bookmarks

 Visitors Currently Viewing this Thread: 1 (0 Registered Visitors (Members) and 1 Unregistered Guest Visitors)

 Forum Posting Settings You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules

 Similar Discussion Threads Discussion Thread Title Thread Starter Forum Replies Last Post or Poll Vote WilliamM ISO 17025 and related Metrology Topics - Measurement Devices, Calibration and Test Laboratories 5 20th September 2013 09:40 AM Nicco Reliability Analysis - Predictions, Testing and Standards 9 11th December 2011 08:38 PM ARM129 Other ISO and International Standards and European Regulations 3 17th February 2011 06:28 AM raji_rajesh Manufacturing and Related Processes 3 17th October 2010 04:45 AM RGohil Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 6 14th February 2006 11:33 AM

The time now is 10:42 PM. All times are GMT -4.