Types of Measurement Uncertainty of Different Devices - Is there any Standard?

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L

Lassitude

#22
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: [email protected]
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 [email protected]
 
L

Lassitude

#23
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?
 
B

Bob Hughes

#24
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).]
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#26
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
 
#27
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).]
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#28
From: "Thomas David Nichols" <[email protected]>
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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