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Author Topic:   Calibration
edwardp
unregistered
posted 31 October 2000 05:58 PM           Edit/Delete Message   Reply w/Quote
Has anyone attempted to define calibration differently in terms of an ISO 9002 document.
Example: We make an adjustment to a pressure system that is electronically controlled by a board and an adjustible pot. We call this calibration because we are verifying that the specified range of pressure is available.
We would like to define calibration differently for our purposes. In other words, calibration as used by us in our documents means to make adjustments and not to reference to any standard. What are your thoughts? Would a registar buy this argument?

Thanks,

Ed

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Al Dyer
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Posts: 622
From:Lapeer, MI USA
Registered: Oct 2000

posted 31 October 2000 07:19 PM     Click Here to See the Profile for Al Dyer   Click Here to Email Al Dyer     Edit/Delete Message   Reply w/Quote
Ed,

We went through a situation with an auditor with the following outcome.

1: If a gage is truely calibrated it is sent to a cerified lab (or a certified internal lab), calibrated to a traceable standard. I.E. a gage block set, CMM, Surface Plate, Rockwell.

2: If a gage is mastered, set, or tested to one of those "Calibrated Standards" by the use of an internal schedule, we are only verifying the current state of calibration.

This scenario came about by our use of multiple dial indicators that were not truely "Calibrated" but only verified to a calibrated standard on a monthly basis.

I believe that the adjustment of your "gage" is not a true calibration but a verification that your gage is performing within a pre-determined, acceptable range.

As usual there are many opinions on this subject. If the gage calibration and verification standards are precisely documented in a lab manual (or procedure) and the system is followed there would be a good arguement to your registrar for your methods.

One thing to consider when setting frequencies and standards is the Bias, Linearity, and most importantly the ongoing Stability of the gage/tool in question.

If you email me I can send you some excel spreadsheets that might help you determine the ongoing performance of measurement systems.

I like the acronym I have recently seen.
IMHO (In My Humble Opinion)

ASD...

------------------
Al Dyer
Mngt. Rep.
ullysses3@excite.com

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John C
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Posts: 134
From:Cork City, Ireland
Registered: Nov 98

posted 06 November 2000 11:42 AM     Click Here to See the Profile for John C   Click Here to Email John C     Edit/Delete Message   Reply w/Quote
Ed,
Is what you are doing, calibration?
It beats me!
According to my pocket dictionary, calibration is marking the scale of a measuring instrument, so that would seem to say that you are not calibrating, but the word originally meant 'to determine the calibre of a gun' and I'd bet there would be a few older definitions available which might well fit.
I can imagine that it will cause more confusion to change it than to go on as you are, and, if you think this is so I'd say, use calibration and change to 'verify range' if some auditor gives you a problem. Either way, it has no impact on your calibration program as long as you are not confused between the ISO meaning and your specific use. If is causes a problem, then change. If it doesn't, then don't worry.
rgds, John C

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Graeme
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Posts: 30
From:Lilburn, GA, USA
Registered: Sep 2000

posted 13 November 2000 04:38 PM     Click Here to See the Profile for Graeme   Click Here to Email Graeme     Edit/Delete Message   Reply w/Quote
Ed,

Calibration is specifically defined in several international standards as, essentially, verifying the performance of a measuring instrument to a specified standard. In other words, calibration is always a documented method of verifying the performance of a measuring instrument, usually under controlled conditions, and always by comparing it in some way to a specified standard.

The result of a calibration is data that allows you to make a quality decision about the performance of the instrument. One of many decisions you might make is to adjust the instrument -- and after the adjustment you will need to calibrate it again. However, all calibration procedures start with the asumption that the instument being calibrated is in proper working order. A purpose of calibration is to determine the degree of truth of that assumption.

There are several common but incorrect usages of the word "calibration".


  • Adjustment of a measuring instrument. (Calibration provides the data that tells you something is not correct. What you do in resonse to the data is a quality decision. Adjustment is one possible decision, and is a repair action. Adjustment when it is not needed falls under Deming's definition of "tampering".)
  • Alignment of new instruments at a factory. (That is part of the factory production process for the item, not a performance verification test.)
  • Standardizing a measuring instrument before using it each day. (That is part of operator setup. Calibrated tools or standard methods are probably used, but the standardization itself is not calibration.)

Repeated use of "calibration" in these incorrect ways only perpetuates the confusion among all of us. In a quality system in particular, it is my opinion that words defined in national and international standards should be used only in the ways they are defined. If nothing else, it will reduce the amount of time you have to spend educating your assessors each time they visit!

------------------
Graeme C. Payne
ASQ Certified Quality Engineer
Graeme@asqnet.org

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Rick Goodson
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From:Wuakesha, Wisconsin, USA
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posted 14 November 2000 09:41 AM     Click Here to See the Profile for Rick Goodson   Click Here to Email Rick Goodson     Edit/Delete Message   Reply w/Quote
Ed

I have to agree with the rest of the comments. What you are doing appears to be verification of the equipment, not calibration.

During a recent pre-audit I was involved in, the auditor mentioned that a check of the range or operating point prior to use is verification not calibration. Within the ISO 9000 family calibration has a specific definition that requires a comparison, under specific conditions, against a known standard with a unbroken path back to a internationally or nationaly recognized standard.

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msmaha
unregistered
posted 24 November 2000 08:40 AM           Edit/Delete Message   Reply w/Quote
Ok, we are in the same boat. We make, rework and calibrate/certify Spline Gages and Gears. When we use a master say on our M&M gear checker. Are we calibrating the machine or verifing. Oh, I have a better one. This uncertainty thing is a joke. I have sent out a indicator for calibration. .0001 grad. It comes back with .00008 uncertainty. Give me a break.

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Roger V
unregistered
posted 30 November 2000 05:57 AM           Edit/Delete Message   Reply w/Quote
Edward, When you make adjustments to ensure the specified pressure range is available, presumably you need to measure that pressure somehow? If the measuring gauge (gage in US?)is itself calibrated (i.e. verified to conform to specified limits of uncertainty, and traceable to a standard) then you are indeed calibrating. If your measuring gauge is not calibrated, then you cannot claim your product meets its specified pressure range.

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John C
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From:Cork City, Ireland
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posted 22 December 2000 11:01 AM     Click Here to See the Profile for John C   Click Here to Email John C     Edit/Delete Message   Reply w/Quote
I had second thoughts about this so I've edited it out. In fact, I tried to do this yesterday, shortly after I sent it, but had problems with my system.
John C

[This message has been edited by John C (edited 24 December 2000).]

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John C
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From:Cork City, Ireland
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posted 22 December 2000 04:24 PM     Click Here to See the Profile for John C   Click Here to Email John C     Edit/Delete Message   Reply w/Quote
Graeme C. Payne,
You quoted a definition of calibration and prove it by saying that certain international standards use the same definition. I did some web surfing and looked in three dictionaries and have to report that they come down heavily on the side of the 'check and adjust' definition. One dictionary gave both; the 'mark a scale' and a version similar to your definition.
Regards International Standards; My ISO 9001:2000 agrees with the 'adjust to standard'; It says, in 7.6; 'calibrated or verified', note, not 'calibrated and verified'.
So who am I to believe?
It seems to me that, as is common, both definitions can be acceptable though yours may be strictly more correct.
But, when it comes to 'confusion'; Who is confused? Until we read your letter, no-one was confused. I send my out-of-tolerance scales away to be 'calibrated', to my 'calibration' lab service and they come back checked and adjusted. I send my cheque and thank the guys for calibrating it. They say; 'That's no problem, that's what we do. We like doing it'.
I'm happy, the calibration boys are happy and everyone else does the same and they're all happy.
Now, after your posting, the thing will come back with a note saying it's out of tolerance and an invoice for $30 for 'calibration'. Well you can bet they won't get the $30. So they're unhappy, I'm unhappy and everyone else is unhappy.
Except you. I suppose, then, you'll be happy.
rgds, John C

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Graeme
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From:Lilburn, GA, USA
Registered: Sep 2000

posted 26 December 2000 09:24 PM     Click Here to See the Profile for Graeme   Click Here to Email Graeme     Edit/Delete Message   Reply w/Quote
As John C mentioned, different dictionaries have different definitions of "calibration". So do different international standards, which is what a quality systems auditor is more likely to be referring to. As I said earlier, "calibration" is a word where there may be significant differences between the technical definition and the common usages.

Going back to Edward's original question - is it permissible for a company to redefine "calibration" in a way that leaves out any reference to a standard. Again, my opinion is a strong "NO". The one thing that is constant in technical definitions of calibration in the international standards (norms) is that the essential part of calibration is comparing a measuring instrument of unknown accuracy to a measurement standard (etalon) of known value and uncertainty.

Reference:
International vocabulary of basic and general terms in metrology (ISO, 1993):

quote:
"A set of operations that establish under specified conditions the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material, and the corresponding value realized by a standard.
Notes: 1. The result of a calibration permits either the assignment of values of measurands to the indications or the determination of corrections with respect to indication.
2. A calibration may determine other metrological properties, such as the effect of influence quantities.
3. The result of a calibration may be recorded in a document, sometimes called a calibration certificate or a calibration report."

The definition above is included by reference in the new ISO/DIS 9000 and ISO/IEC 17025:1999, and is included verbatim -- but without the notes -- in ISO 10012-1:1992 and ISO 10012-2:1997. An essentially identical definition in the United States is in ANSI/NCSL Z540-1:1994; the only substantial difference is the addition of one note relating to calibration factors and curves. Essentially identical definitions are also in other sources, notably the NCSL Recommended Practice 1 on calibration intervals.

In addition to the essential core of calibration (above) most technical definitions in the United States do include the concept of adjusting an instrument, usually as one of several possible actions. Some international standards also include this. Almost all calibration laboratories will, as part of the added value of the calibration process, adjust an out-of-tolerance instrument. (Personally, I would not use one that does not - and all of the labs I have worked with do adjust instruments.) However desirable, though, adjustment is not an essential part of "calibration" as defined by ISO.

The most common additional definition in the United States is:

quote:
"the comparison of measuring and test equipment or measurement standard of unknown accuracy to a measurement standard of known accuracy in order to detect, correlate, report, or eliminate by adjustment any variation in the accuracy of the instrument being calibrated."

This or essentially similar definitions appear in a number of US references: NCSL Recommended Practices 2 and 3; MIL-STD-1309D; MIL-STD-45662A (canceled in 1994); NIST Special publication 260-100; ANSI/ASQC M1-1987. A similar definition appears in ISO 10012-1:1992 and ISO 10012-2:1997 under the label "metrological confirmation". Note, though, that the core concept is still comparison of a measuring instrument to a standard, just as in the ISO definition. "Adjustment to remove variation" is simply one of several actions that may be taken. Therefore, my earlier message was incorrect to this extent, because in that I was referring only to the ISO definition which does not include any of the actions that result from the comparison.

In the actual practice of calibration laboratories I have worked in and with for may years, the generalized calibration work process flow looks something like this --

code:
Start
Compare the instrument under test to a measurement standard. ("calibrate" it.)
If the performance is within specifications,
Report the results and return the instrument to the customer.
Otherwise,
Report the out-of-tolerance condition to the customer.
If the instrument can be adjusted to meet specification,
Adjust the instrument in conformance to lab policy.
Go back to start.
Otherwise
Consult the customer and lab policy for appropriate action.
If the instrument is repaired, go back to start.
End

To return to Edward's original question yet again: every technical definition of "calibration" that an auditor is likely to refer to states the core concept of comparing an instrument to a measurement standard. If you are not doing that, then you are not calibrating. And if you are adjusting your instruments without reference to any measurement standard, you are making the indications on the dials (or whatever) pretty much meaningless. If you are calibrating (as defined by ISO) then say so and do so. If you are adjusting your instruments for some other reason and without using a measurment standard (why?), then say so but don't use the word "calibration". Remember that the calibrated measuring instruments, when properly used, are your eyes into the process - they are supposed to be showing you what the process is doing. If they are telling you something you don't like, adjust the process, not the instruments. The situation described by Edward sounds like an operator's performance check of a process, and is meaningful only if there is a calibrated and correctly used method of measuring the actual pressure. But a performance check is not a calibration. It is a repeatable method, ususally using calibrated measuring instruments, of verifying that the process is producing acceptable product.

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Marc Smith
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From:West Chester, OH, USA
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posted 26 December 2000 10:48 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
> ...calibration is always a documented method of verifying the performance of a
> measuring instrument, usually under controlled conditions, and always by
> comparing it in some way to a specified standard.

It's a comparison to a 'defined' standard - I agree 100%.

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John C
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Posts: 134
From:Cork City, Ireland
Registered: Nov 98

posted 27 December 2000 08:50 AM     Click Here to See the Profile for John C   Click Here to Email John C     Edit/Delete Message   Reply w/Quote
Graeme,
I take all that, and agree with it all. Also, with Marc's statement that calibration is to an accepted standard. But I'm coming at this from a different point of view altogether, and it is, I believe, a point of view which should be accepted as a basic principle, ie; "We set out to become compliant to a 'Standard'. We can only do that when we know what that 'Standard' is. The 'Standard' is described in a limited document, limited, that is, to the words written within it's pages". If that principle is not accepted, then we are open the attentions of unscrupulous people whose interest is only in using the standard as an opportunity to generate cash. I can fall back on the dictionary meaning and uphold my case in any court but if it comes to pitting my definition against your definition, then I'm never sure where I stand and a guy can walk in any day of the week and tell me that I've got it wrong.
That's no good to me. I need to know where I stand. I need to know when I'm right. It's my job. It's what I get paid for. My clients need it as well.
Having a standard that is open to interpretation is a contradiction of terms. It also helps to bring the standard into disrepute and make it an expensive and valueless burdon. And that really riles me. I care about this standard.
rgds, John C

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