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Author Topic:   Measurement Uncertainty
Marc Smith
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posted 15 January 1999 08:16 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
There has been some chatter about Measurement Uncertainty. I have "Determining and Reporting Measurement Uncertainties", Recommended Practice RP-12, April 1995 from the National Conference of Standards Laboratories.

And I found the below at the ISO site:

Guide to the expression of uncertainty in measurement

When reporting the result of a measurement of a physical quantity, some quantitative indication of the result has to be given to assess its reliability and to allow comparisons to be made. The Guide to the expression of uncertainty in measurement establishes general rules for evaluating and expressing uncertainty in measurement that can be followed at many levels of accuracy and in many fields.

1995, 110 p., price group L, CHF 88,50, ISBN 92-67-10188-9

Can some of you help by citing other publications which address the issue?

Also - any 'Practical' comments on MU?

[This message has been edited by Marc Smith (edited 01-15-99).]

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Don Winton
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posted 15 January 1999 07:20 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
I have been doing some research on a proposed paper I am working on regarding Measurement Error Analysis. In the process, I found many sites, but the following four were helpful. They may help in your quest.

http://www.savtech.demon.co.uk/measurem.htm

http://gunsmoke.ecn.purdue.edu/~nocera/project/tutorial

http://www-uktm.external.hp.com/mikehut/ieepaper/iee2.html

http://www-uktm.external.hp.com/mikehut/one_meas/unc.html

Hope this helps.

Regards,
Don

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Marc Smith
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posted 15 January 1999 07:40 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
Good references! Thanks, Don!

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Don Winton
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posted 30 January 1999 09:57 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Marc,

Is "Determining and Reporting Measurement Uncertainties", Recommended Practice RP-12, April 1995 from the National Conference of Standards Laboratories." you mentioned above available on the web. I would like to see a copy for some research I am doing.

Regards,
Don

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Marc Smith
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posted 30 January 1999 04:10 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
Not that I know about. I bought a paper copy a while ago. But then again, I haven't searched for it on the web.

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Lassitude
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posted 19 July 1999 06:33 AM     Click Here to See the Profile for Lassitude   Click Here to Email Lassitude     Edit/Delete Message   Reply w/Quote
I liked this from:
http://gunsmoke.ecn.purdue.edu/~nocera/project/tutorial

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

Measurement Error and Uncertainty

The difference between the measured value and the true value of an object of measurement is known as the error. The actual value of an error can never be known exactly, only estimated. An uncertainty statement provides an estimate of the possible error in a measurement. The term "error" is almost obsolete in the measurement arena due to it's ambiquity. It has become fashionable in the scientific community to provide uncertainty statements with all measurements due to the useful information it provides. Those still in the habit of providing error statements as opposed to uncertainty statements are looked upon with scorn and ridicule by scientists, engineers, and other technically intellectual types in the scientific community. By utilizing this tutorial, the user shall avoid subjecating himself or herself to terms such as neandrethral-like, doltish, or flat incompetent by the aforementioned persons.

As an example, let's say you designed an experiment to measure the output voltage of a given circuit. The theoretical output of the circuit is 5 volts. You measure 5.02 volts and publish a paper in which you claim the actual value is 5.02 volts with an error of 0.02 volts. You have now opened yourself to flood of spoken and unspoken questions and criticism pertaining to your own personal value and competence in the technologically advanced world.

However, had you published the same paper and claimed that the measured value is 5.02 volts, +/-0.03 volts at a 95% confidence level, and proceeded to list the involved uncertainty factors with estimates of contributions from each as well as thoroughly describing your method for obtaining and combining these factors, you would certainly be praised and hailed as intellectually advanced, equipped with a desirable skill set, and en vogue with all the current trends in the world of measurement science.

Components of an Uncertainty Statement - For purposes of this tutorial, it is assumed that any measurement values used to calculate an uncertainty value have been obtained from a population having a normal distribution. Non-normal distributions require additional attention which is not covered in this material.

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Lassitude
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posted 19 July 1999 06:42 AM     Click Here to See the Profile for Lassitude   Click Here to Email Lassitude     Edit/Delete Message   Reply w/Quote
Have any of you folks reading this thread actually done any uncertainty calculations??

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Lassitude
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posted 19 July 1999 09:14 AM     Click Here to See the Profile for Lassitude   Click Here to Email Lassitude     Edit/Delete Message   Reply w/Quote
Or do you read the manufacturers tolerance like the following:

Subject: Re: Q: Certificate of Calibration/Guerra/Naish
Date: Mon, 26 Oct 1998 10:50:29 -0600
From: ISO Standards Discussion

From: PNaish@aol.com
Subject: Re: Q: Certificate of Calibration/Guerra/Naish

I am surprised that you would get a non conformance when it comes from the manufacturer. I have worked with a number of companies and at least 9 registrars who have not given any of the 75 companies we have worked with a non conformance for this.

As far as an accredited lab versus a manufacturer being better, there are good and bad in both. But some manufacturers such as Starrett are accredited by Navlab here in the USA.

When approaching the amount of information that is required, if you get a copy of guide 25 it is easy to understand and helps with knowing what is needed for calibration. However, another approach is to obtain a copy of the ISO certificate from the manufacturer as well as any other certification they have. If you have an internal supplier qualification form have them complete it as well. The using the supplier qualification process and history from previous performance (as demonstrated the first time you sent the equipment out for calibration or the hard data sent by them) you can place them on a dock to stock plan with their certificate of conformance along with their catalogue or data sheet as meeting the requirement until you send it out the first time for calibration.

The uncertainty of the instrument is based upon the catalogue or data sheet which will tell you it is within a tolerance. As long as that tolerance provides you with the accuracy you need, and they have provided a certificate of assurance that they have met it and that it meets international standards you should be able to pass your audit. If you ask for them you can sometimes get a copy of the procedures used by the manufacturer so as to show your registrar the method the manufacturer is using.

I can also send you a sheet with the information that you should be seeing on the certificate after you send it out. You can email me direct if you would like a copy.

Phyllis Naish at Pnaish@aol.com

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Lassitude
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posted 19 July 1999 12:42 PM     Click Here to See the Profile for Lassitude   Click Here to Email Lassitude     Edit/Delete Message   Reply w/Quote
quote:
Mrc,
Is "Determining and Reporting Measurement Uncertainties", Recommended Practice RP-12, April 1995 from the National Conference of Standards Laboratories." you mentioned above available on the web. I would like to see a copy for some research I am doing.

Regards,
Don


Don:

Do you still need a copy of this?

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Bob Hughes
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posted 23 September 1999 04:03 PM     Click Here to See the Profile for Bob Hughes   Click Here to Email Bob Hughes     Edit/Delete Message   Reply w/Quote
I have found an uncertainty calculator (software type) at the WEB site of a local magazine published here in San Diego. The title is Uncertainty Calculator V2.5 from Compaq Computer Corporation's, Corporate Metrology Department. It is freeware!! I have not had the chance to try it yet but I figure that almost anything would be better than trying to figure out what the NIST 1297 or the RP are attempting to teach me. The magazine is CAL LAB and they are on the WEB at www.callabmag.com The programs are under the downloads tab. Look the site over as there is a lot of good info. I heard a rumor that some company used the program during an audit and the results were accepted by the registar. Only rumor...can not confirm.

[This message has been edited by Bob Hughes (edited 24 September 1999).]

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Bob Hughes
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posted 23 September 1999 04:05 PM     Click Here to See the Profile for Bob Hughes   Click Here to Email Bob Hughes     Edit/Delete Message   Reply w/Quote
Sorry that site is www.callabmag.com

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Marc Smith
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posted 20 October 1999 12:07 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
I coped this off the listserve. Any takers here?

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

From: Charley Scalies
Subject: Q: Calibration Issues for Small Firms /Scalies

Measurement Uncertainty
How do/can very small firms, with no in-house technical expertise in metrology, comply with the ISO90001, 4.11.1 requirement to be sure inspection, test and measuring equipment is "..used in a manner which ensures that the measurement uncertaintly is known..." Alternatively, how would you describe "measurement uncertainty" to a lay person?

Accuracy vs. Precision
How would you describe the difference to a lay person?

Charles J. Scalies
scalies@pobox.com

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Sam
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posted 20 October 1999 09:00 AM     Click Here to See the Profile for Sam   Click Here to Email Sam     Edit/Delete Message   Reply w/Quote
Refer to "Fundamentals of Dimensional Metrology" 2nd edition - Ted Busch, Wilke Brothers Foundation, Delmar Publishers.
This is a very good book for the lay person.

Uses a very good example comparing accuracy,precision and reliability to a target.
Example; you fire 5 shots at a target:

1- Accuracy - did you hit the target?
2- Precision - How close are the five shots grouped together?
3- Reliability - Can you do this consistently.

Note also that there can be varying combinations of accuracy,precision and reliability.

Measurement uncertainty Maximum error that may be added (algebraically) to a reading.
i.e., systematic errors, random errors, human errors . . . .

[This message has been edited by Sam (edited 27 October 1999).]

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Marc Smith
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posted 22 October 1999 01:44 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
From: "Thomas David Nichols"
Subject: RE: Q: Calibration Issues for Small Firms
/Scalies/Nichols

Suppose you have a digital thermometer that displays two decimal places, and the last digit is stable. That is, it doesn't flicker unpredictably 3-7-6-2-9-4, etc. Then the precision of the thermometer is 0.01 degree, or plus/minus 0.005 degree.

Now suppose you use this thermometer to measure a calibration standard that is known to be within 0.1 degree of 50 degrees. The thermometer's readings vary between 46 and 52 degrees, with a mean of 49 degrees and a standard deviation of 1.5 degrees. This tells you the accuracy of the thermometer.

To summarize, precision is how well you can read the instrument, while accuracy is how close the reading is to the true value.

Measurement uncertainty is a little harder to pin down. Basically, it is the size of the band around a measured value that is known to contain the actual value with some probability, often 90% or 95%. Expressed a different way, it is the band around the true value that will contain the stated percentage of a large number of measurements of the value. It combines all the sources of uncertainty: instrument accuracy, operator errors, the relationship between what is available to measure and what is desirable to measure, the time required to take the measurement compared to the rate of change of the thing measured, the effect of the measurement on the thing measured, and possibly many other factors.

Measurement uncertainty, in turn, has an uncertainty of its own. Your process can tolerate a large error in some measurements, as when room temperature can range from 40 to 120 degrees F, and it never goes below 60 or above 80. In this case, a very simple test can show that your thermometer is good enough, and you really don't need to know exactly how accurate it is. On the other hand, if you are polishing a mirror to a flatness of .05 wavelength of red light, your whole business may depend on how well you can measure flatness.

Either way, though, if the uncertainty in a measurement doesn't matter, the measurement itself doesn't matter.

Thomas David Nichols

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Marc Smith
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posted 22 October 1999 01:49 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
Ah! The Hubble space telescope mirror problem?

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David Guffey
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posted 22 October 1999 07:55 AM     Click Here to See the Profile for David Guffey   Click Here to Email David Guffey     Edit/Delete Message   Reply w/Quote
Reminds me of the metric conversion problem with the Mars probe.

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Marc Smith
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posted 23 October 1999 01:33 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
Subject: Re: Calibration Issues for Small Firms /Scalies/Everson/Kozenko
Date: Fri, 22 Oct 1999 15:01:05 -0600
From: Moderator

Subject: RE: Calibration Issues for Small Firms /Scalies/Everson/Kozenko
From: Write9000@aol.com

> Shooting arrows at the target and getting them close the
> bullseye is the accuracy.
>
> Precision is if I am able to have the arrows hit the same small area
> everytime, but they made still be away from the
> bullseye....I was precise, but not accurate..

Mine's similar, but I believe the explanation called for is "degree of precision."

Picture a car tire swinging laterally on a rope, over a 30 foot arc, ten

yards in front of you.

Accuracy would be throwing a football at the swinging tire and at least hitting the darn thing. Anything is accurate if it hits the target. Remember the expression: "Close doesn't count, except in horse shoes and hand grenades."

Diagonally from tire rim to tire rim it's usually about 15 inches (... and my contract with NASA expired so I won't convert that to cm's ). The girth of a spirally-thrown football is about, oh, eight inches, so the degree of precision of a throw that goes through the moving tire could be expressed as the required degree of precision of plus or minus one girth. (In other words, in order for the football to clear the center of the tire, it needs to be precisely on center, or no more than one girth-thickness in any direction away from center, along the same plane created by the tire.)

Degree of precision is usually expressed as a set value with a plus-or-minus dimension, but there are cases when it may be expressed as Plus 0 minus X, or Plus X minus 0, or variants in between.

For example, when reaching for a description of the degree of precision in throwing a water balloon, one would rather have a tolerance of Plus 0 so that the splash still would hit the target, which usually happens even at Plus 0 minus 5 feet. Hitting the "20" on a dart board, however, would call for a vertical Plus 9 inches Minus 0 if the bulls eye were considered the "anchor" for the measurement. This one's a complicated example too, because the "20" on a dart board is pie shaped, so the horizontal (side to side) tolerance of the shot increases as the vertical goes from 0 to 9 inches.

Clear as mud?

David Kozenko

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

Also see:
https://elsmar.com/ubb/Forum4/HTML/000036.html
and 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 28 February 2000).]

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Weyli
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posted 28 February 2000 06:20 AM     Click Here to See the Profile for Weyli     Edit/Delete Message   Reply w/Quote
I wonder how to actually set the acceptance criteria for an IMTE.

Let say a thickness displacement meter used to measure 0.80+/-0.0125mm.
(Measurement uncertainty for the IMTE is +/-0.0004mm)

Question:
Is it correct if I set the acceptance criteria (Acceptable uncertainty) for the IMTE as 30% of 0.0125mm?

Thanks in advance.

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