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  Measurement Uncertainty

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Author Topic:   Measurement Uncertainty
David A
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From:Phila. Pa. USA
Registered: Dec 98

posted 05 December 1998 09:52 AM     Click Here to See the Profile for David A   Click Here to Email David A     Edit/Delete Message   Reply w/Quote
Is there a simple way to determine or express the measurement uncertainty of a device, such as a 1% pressure gauge, 0-100 psi with .5 psi sub-divisions, calibrated against a dead-weight tester with a tolerance of .02% of reading.

I referred to the NIST publication on Measurement Uncertainty and my head hurts. This is more informmation than anyone on this planet needs.

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Timothy
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posted 07 December 1998 01:30 AM     Click Here to See the Profile for Timothy     Edit/Delete Message   Reply w/Quote
I don't know the answer, but your last sentence is well put...

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Don Winton
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posted 30 January 1999 08:33 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
I do not know if this will help, but you may want to try:

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Regards,
Don

[This message has been edited by Don Winton (edited 01-30-99).]

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Don Winton
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posted 01 February 1999 10:25 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
You might want to try this as well.

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Regards,
Don

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Marc Smith
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posted 01 February 1999 05:03 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
Excellent links, Don! Thanks!

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Don Winton
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posted 01 February 1999 05:53 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Thanks, Marc, I do my best. No, seriously, I am still researching my MEA and R&R paper and stumble upon these during the search. If any more pop up, I will advise.

Regards,
Don

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Don Winton
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posted 05 February 1999 09:49 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
quote:
I'm here to learn and you're providing me with good stuff!

Thanks and you to me as well. The R&R paper mentioned above is currently being proof-read (by a very nice IT lady) and should be ready in a couple of days. When I have it finished, I will forward for your review. If you deem it worthy (?), you can post after your review. Anything else I come across, I will be sure to keep the group informed.

Regards,
Don

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David A
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From:Phila. Pa. USA
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posted 06 February 1999 07:18 AM     Click Here to See the Profile for David A   Click Here to Email David A     Edit/Delete Message   Reply w/Quote
Thanks Don, These links are a big help.

David

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Marc Smith
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posted 28 February 2000 07:15 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
From: ISO Standards Discussion
Date: Fri, 25 Feb 2000 16:00:25 -0600
Subject: Re: Q: Measurement Uncertainty /Enop/Martins

From: "Frans J.C. Martins" fjcm

Hi Glover and group,

> From: Pfscott2

> What is meant by "measurement uncertainty" as in 4.11 General 4.11.1
> last sentence?
> Thanks!
> Glover Enop

Very briefly, measurement uncertainty can be explained as follows: In reality no measurement could be absolutely correct. There is always some doubt present regarding the true value of the measurement. This is generally caused by all sorts of factors, for example, drift, temperature, repeatability etc. Example: A digital gauge indicates a fluctuation of 0.999, 1.000 1.001, 0.999, 1.001, 1.000, The "true" value could be calculated by taking the average of all the readings, and then rounded to the correct resolution, in this case, 1.000, but, depending on the bias, the reading could just as well have been either 0.999 or 1.001! Thus the reported indication is 1.000 ▒ 1 LSD (least significant digit) In this example we only looked at one factor, which is resolution. Other factors previously mentioned also need to be taken into account to determine the total uncertainty of measurement. The components contributing to the total uncertainty can be classified as either Type A (determined statistically) or Type B (determined by other means other than statistically) All A and B components are then added through a process of Root Sum Squaring to determine the total uncertainty of the measurement.

The total uncertainty of measurement MUST be reported for any given quantity on a certificate of calibration to in order to satisfy the requirements of ISO Guide 25 (ISO 17025)

Hope this clarifies your query.

Regards in Quality
Frans J.C. Martins

---------snippo--------

From: ISO Standards Discussion
Date: Fri, 25 Feb 2000 16:07:38 -0600
Subject: Re: Q: Measurement Uncertainty /Enop/Pfrang

From: pfrang (Doug Pfrang)

Section 4.11.1, last sentence, states: "Inspection, measurement, and test equipment shall be used in a manner which ensures that the measurement uncertainty is known and is consistent with the required measurement capability."

This means that if your specification requires a measurement to be within a certain tolerance -- say, 0.1 inches -- then your measurement equipment and measurement process must be capable of at least that level of certainty. It does no good to measure something with a 0.1 inch tolerance if your measurement equipment is only good to +/- 0.5 inches.

-- Doug

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Marc Smith
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posted 28 February 2000 07:26 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
See also:
https://elsmar.com/ubb/Forum4/HTML/000047.html
and https://elsmar.com/ubb/Forum4/HTML/000072.html
and https://elsmar.com/ubb/Forum4/HTML/000076.html
and https://elsmar.com/ubb/Forum4/HTML/000120.html

[This message has been edited by Marc Smith (edited 13 November 2000).]

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Yochio Ito
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posted 28 February 2000 11:09 AM           Edit/Delete Message   Reply w/Quote
When I receive new calibration certificates, I do something like a validation of the results: total uncertainty + max. error (data from the certificate) must be smaler than a criteria stabilished for each kind of instruments.
Sometime ago, an auditor charged me if I was adding to this total uncertainty, the uncertainties of standards used for calibration. But I'm not, because I believe this uncertainties won't cause influence in my measures at production.
Can anybody tell me if that is the correct procedure?
Thanks,
Yochio

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Marc Smith
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posted 01 March 2000 03:33 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
As I understand it uncertainties are additive. But if you have one uncertainty item that is a factor of say 10 or 100 of the others I would say what you did - it's not significant to the total.

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Ryan Wilde
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posted 19 May 2000 09:55 PM           Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Marc Smith:
As I understand it uncertainties are additive. But if you have one uncertainty item that is a factor of say 10 or 100 of the others I would say what you did - it's not significant to the total.

Uncertainties are a variation of additive. They are combined in a mind-reeling Root-Sum-of-the-Squares sort of fashion.

The general rule for uncertainties is the factor of 10. If the particular contributor is less than 10% of the total uncertainty, then it is considered unimportant.

Ryan Wilde

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Ryan Wilde
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posted 19 May 2000 10:03 PM           Edit/Delete Message   Reply w/Quote
I've read a few posts in this forum regarding measurement uncertainty. The sad fact is, there is no easy way to do uncertainty, unless you are a statistician moonlighting as a metrologist. The process is very statistics laden, with interpretive distributions, degrees of freedom, yada yada yada.

I suggest taking a course. Not one of those week-long courses, just an introductory course. I took one, and like most things, it is relatively simple after you learn how. The first step is a big one, but after you actually undertand the concepts, it makes sense and isn't difficult. It is, however, time-consuming.

Best of luck!
Ryan Wilde

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Atul Khandekar
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From:Pune, India
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posted 22 May 2000 09:11 AM     Click Here to See the Profile for Atul Khandekar   Click Here to Email Atul Khandekar     Edit/Delete Message   Reply w/Quote
Hello,

While calculating Uncertainty of Measurement, we take into account the various sources of uncertainty. For example, Uncertainty due to resolution, due to repeatability & reproducibility , due to Standards used, due to temperature, and so on.

I wonder if there exists a standard (if not exhaustive) list of the uncertainty sources and their frequency distributions (Rectangular/Triagular...) that must be considered for various types of instruments and gages.

Any pointers?

Thanx,

-Atul.

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Ryan Wilde
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posted 23 May 2000 10:29 PM           Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Atul Khandekar:
Hello,

While calculating Uncertainty of Measurement, we take into account the various sources of uncertainty. For example, Uncertainty due to resolution, due to repeatability & reproducibility , due to Standards used, due to temperature, and so on.

I wonder if there exists a standard (if not exhaustive) list of the uncertainty sources and their frequency distributions (Rectangular/Triagular...) that must be considered for various types of instruments and gages.

Any pointers?

Thanx,

-Atul.


Here you go Atul, here is a tad of what is generally accepted. These are dimensional examples, because that is what I generally deal with.

- Repeatability and Reproduceability is a normal distribution. They are statistically derived, therefore they are calculated to a normal distribution.

-Temperature is sinusoidal in nature, therefore it is a "u-shaped" distribution.

Standard uncertainty - This is also a normal distribution. Actually, if your standards calibration was properly reported, it is two standard deviations.

Standard / M&TE accuracy - If all you have to go on is a tolerance, then you have one of two options. Usually, you would use a rectangular distribution, because there is an equal probability that the actual accuracy is anywhere in that range. The exception is VERY STABLE GAGES from extremely reliable manufacturers. If you know beyond a shadow of a doubt that your equipment was adjusted to nominal, and it has little to no drift, then you would use a triangular distribution. I have never had the opportunity to use this distribution.

Thermal expansion uncertainty - This is added because although material expansion is characterized, it is not perfect. Example: The accepted formula for steel growth is APPROXIMATELY 11.5Ám/m/íC. Due to metallurgical imperfection, there is uncertainty involved in this number. I use 10%, because I tend towards the conservative side. This 10% would be expressed in a rectangular distribution, because you really have no idea where in that 10% the actual growth is.

I hope this gives you just a bit more insight. Deciding on a distribution is somewhat subjective. You have to visualize how the error is likely to occur. Once you can do that, it gets to be quite easy to choose an appropriate distribution.

Ryan Wilde

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mbruner
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Posts: 11
From:Greenwood,IN,USA
Registered: Feb 2000

posted 05 June 2000 08:55 AM     Click Here to See the Profile for mbruner   Click Here to Email mbruner     Edit/Delete Message   Reply w/Quote
Here is a website metrology forum (HP equip)that provides some down to earth explanations on measurement uncertainty (even for blondes as user Dawn has requested). In addition they give some info on how these concepts are generally implemented at their facility. DEAD LINK REMOVED

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