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  Measurement, Test and Calibration
  Calibration vs Verification

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Author Topic:   Calibration vs Verification
Marc Smith
Cheech Wizard

Posts: 4119
From:West Chester, OH, USA
Registered:

posted 27 July 1998 02:03 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
A 'stolen' Thread:

From: pfrang@nicmad.nicolet.com (Doug Pfrang)
Subject: FYI: Calibration/Pfrang

1. As I said before, you do not necessarily need to CALIBRATE a tool unless you first determine that there are no other cheaper alternatives for VALIDATING that tool in your process. Therefore, to all of the ISO consultants out there who have tried to advise people on whether or not to calibrate a tool: you should never suggest that any tool be calibrated until you first determine whether or not that particular company has any other cheaper alternatives for validating that tool (e.g., brute force trial and error). Virtually none of the ISO consultants who have posted to this list appear to do this; instead, they appear to use some other surrogate (and incorrect) criteria for determining whether or not to calibrate a tool. Often, the suggestion is to calibrate the tool simply because the person making the suggestion can't think of any other way to validate the tool (i.e., because they haven't explored this option with the client), and they know that calibration will be easy to sell to the ISO auditor. While this is certainly the easiest advice to give from their point of view (since it is virtually guaranteed to work, and thus make them look good), it is not necessarily the most cost-effective solution for the client.

2. Keep in mind that validating a tool by calibration does not mean the tool is any more reliable than if it is validated by brute force. In either case, the tool can drift out of accuracy between the time you validate it and the time you use it. Therefore, the time interval that you choose between revalidations is NOT a function of your METHOD of validation, and it is NOT a function of the calibration cycle recommended by the manufacturer of the tool; it is SOLELY a function of how stable that particular tool is in YOUR particular process. In other words, you do not automatically have to revalidate a tool annually just because that is what the manufacturer recommends. You may freely adjust the revalidation cycle longer or shorter based on YOUR OWN experience with that tool. If the manufacturer recommends that you recalibrate a tool annually, but you find it perfectly adequate -- in YOUR process -- to revalidate that tool every decade, then you are perfectly justified in revalidating that tool every decade. Similarly, if the manufacturer recommends that you recalibrate a tool annually, but you find it unacceptable -- in YOUR process -- to wait more than six months, then you must shorten the revalidation cycle to six months. YOUR experience with the tool, not the manufacturer's recommendations, governs the length of the revalidation cycle. Thus, if you have a tool (for example, a tape measure) and your experience shows that -- in YOUR process -- you can go ten years before revalidating that tool, then that duration is perfectly acceptable as long as you have some reasonable basis for setting the revalidation cycle at that length. How do you show you have a reasonable basis? Just keep a record declaring that you have used this type of tool for ten years and -- in YOUR experience -- these tools haven't gone out of spec in that time frame, so YOU are going to set the cycle at ten years for YOUR process.

3. Many people have posted statements to the effect that "the reason we don't need to calibrate this tool is because we don't need to make accurate measurements with it." This analysis is also wrong. The ACCURACY you need for your measurement is IRRELEVANT to the question of whether or not you should CALIBRATE a tool. The ACCURACY you need for your measurement is only relevent to your SELECTION of which tool to use: the tool you select must be capable of the accuracy you require. Once you SELECT a tool that is capable of the accuracy you require, whether or not you choose to CALIBRATE that tool does NOT depend on the accuracy you require; the decision to calibrate it depends SOLELY upon your assessment of whether or not calibration is the best way to VALIDATE that tool for your particular process. Therefore, to say that "I don't need to CALIBRATE this tool because I don't need to make accurate measurements with it" is wrong. The fact that you don't need to make accurate measurements guides your SELECTION of which tool to use, but your decision not to CALIBRATE that tool is based on the fact that you can VALIDATE that tool easily by brute force trial and error. You try the tool in your process and it works for you; therefore, you have VALIDATED it without having to CALIBRATE it, and THAT is the reason you do not have to calibrate it. But then they make the mistake that most people do -- the mistake which leads to most ISO nonconformities related to calibration -- they neglect to document their validation, which is discussed below.

4. The root cause of almost every ISO nonconformity related to calibration is NOT that the company has failed to CALIBRATE a particular tool; the root cause of the nonconformity is that the company has failed to provide an appropriate QUALITY RECORD showing that a tool has been VALIDATED for the given process. ISO auditors do not looking for calibration records per se; they look for QUALITY RECORDS showing that tools have been VALIDATED for the processes in which they are used. The reason that companies often (wrongly) interpret such nonconformances as requiring the tool to be calibrated is that calibration provides a handy QUALITY RECORD for the company's files, which resolves the nonconformity because it provides adequate evidence of VALIDATION of the tool. However, in virtually every instance, the company could also resolve the nonconformity without calibrating the tool, if it would simply VALIDATE the tool using some other appropriate method and produce a QUALITY RECORD showing that the tool has been VALIDATED and the method by which validation was done.

-- Doug Pfrang

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Marc Smith
Cheech Wizard

Posts: 4119
From:West Chester, OH, USA
Registered:

posted 27 July 1998 03:15 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
And an interesting follow-up:

From: pfrang@nicmad.nicolet.com (Doug Pfrang)
Subject: Re: Calibration/Pfrang/Volker/Pfrang

> Your example is not very illuminating for real-world problems. You
use
>a wrench and it turns the nut without damaging it-- fine. Your wrench
>is not an instrument, it's a tool. People don't calibrate "5 mm
>wrenches". You would probably want to calibrate a torque wrench,
>however. How do you "brute force validate" that?? Say you're
>installing motor cylinder heads. You do 100 motors and everything seems
>fine. "Brute force validated"? What happens if in a year some of them
>start warpping because they were unequally stressed? Would anyone ever
>even know that the problem goes back to your torque wrenches? Does it
>make any difference if no one figures out the root cause?
>
> Is it possible to "brute force validate" anything, if your process
>produces occasional non-conforming products?
>
> In the real world we often don't know why a product fails-- there
are
>too many variables in production to pinpoint the exact cause of
>failure. That's why quality systems such as ISO 9000 lay such great
>importance on constancy of process and calibrated measurements.

OK, maybe my example was a bit too simple, so let's back up and see if we can clear things up. First of all, let me repeat that the main point of my posting was that calibration is a SUBSET of validation. This is a really key point, because many people have been posting questions about calibrating things that obviously could be validated more easily by brute force; namely, things like tape measures and injection molding machines. My point is that IF you can validate a device more easily by brute force trial and error, then you do not HAVE to calibrate a device just because it has numbers on it or just because it makes a measurement. You CHOOSE whether or not to calibrate a device based on whichever method of validation makes the most sense in your particular process.

Second, when it comes to CHOOSING whether or not to calibrate a particular device for a given task, it makes no difference whether we label that device a "tool" or an "instrument." ANY device (even an "instrument") CAN be validated by brute force, because the only thing you care about is whether that device does what you want it to do in your process. A 5 mm wrench, a torque wrench, a voltmeter, a soldering iron, whatever. They are ALL "tools." If you want to label some of them "instruments," that's fine, but I think that tends to confuse people into believing that the decision process is different for "instruments" than it is for "tools." In fact, it's not, and that's another point I'm trying to make. Whether we call something a "tool" or an "instrument" doesn't change the analysis at all.

Let's take the torque wrench example. In your example, the engines fall apart a year later because of UNEQUALLY stressed parts. This problem is obviously NOT a result of the tool being UNCALIBRATED; it is a result of the tool being DEFECTIVE: the bolts are UNEQUALLY stressed because the tool's readings are not REPEATABLE. You cannot resolve this problem by CALIBRATING the tool; you must resolve this problem by REPLACING the tool, because, in your example, the tool is broken. It can't make repeatable readings.

Now, let's use the same tool, but let's use a torque wrench that isn't broken. You're building an engine, you want to torque the head bolts to a torque that will hold the engine together, and you have a torque wrench that is not calibrated. The question is: do you need to calibrate that torque wrench or not? Well, as I said before, the answer to this question depends totally on YOUR PROCESS, NOT ON THE TOOL. Let's say YOUR PROCESS is to build engines where SOMEONE ELSE has already determined what the torque should be. Ford Motor Company TELLS YOU that the torque on those head bolts must be 85 ft-lbs. OK, fine, FORD has VALIDATED that torque value by some method (possibly their own trial and error), so YOU don't need to VALIDATE whether 85 ft-lbs is right or wrong. All YOU need to do is ensure that you torque the part to 85 ft-lbs, and you know ahead of time that this value will work. How will you do that? Well, probably by calibrating your torque wrench so you can torque the head bolts to 85 ft-lbs.

But now, let's change the process. Let's say YOUR PROCESS is to build engines where someone else has NOT already determined what the torque should be. You're building a custom race engine, using your own head casting, and you don't know what the torque should be because no one else has VALIDATED the value for you. So, does it matter whether your torque wrench is calibrated or not? No, it doesn't, because you don't know if the torque should be 25 ft-lbs, or 90 ft-lbs, or 115 ft-lbs. Sure, you might use the numbers on your torque wrench as a REFERENCE, because in your experience you believe that the torque should be somewhere between 50 and 100 ft-lbs, but you don't know what the torque really needs to be. So, calibrating your torque wrench does NOT answer the question of what the torque needs to be. HOW do you determine what the torque needs to be? By brute force trial and error. You torque the part to some value and give it a try. In this example, it really doesn't matter if the torque wrench is calibrated or not, because you're not trying to hit a specific torque value that someone else as validated for you; you're just trying to hit a value that holds your custom engine together. In fact, you could just as easily use a torque wrench that has no numbers on it at all -- just a blank dial -- and you could simply put a mark on the dial that corresponds to how far you torqued the head bolts each time. Then, if you discover that you torqued the bolts too tight or too loose, then you just move the mark up or down a little until you get it right. Once you get the torque right, you just use your little mark on the dial as your personal indicator for your custom engines; and this method works just fine in YOUR PROCESS, because you're not trying to match some factory torque spec that someone else validated for you. Now, do you need to run out and calibrate that torque wrench? No, you don't, because you have VALIDATED that torque wrench in YOUR PROCESS by brute force trial and error.

And the same analysis applies to ANY device in ANY process, and it makes no difference whether the device is a "tool" or an "instrument."

> Once you start covering yourself a lot of documentation, then you're
>really just "calibrating" with your own home-made procedure; the
>distinction between calibration and brute force validation seems lost to
>me, then.

That gets back to the main point I was trying to make: that calibration and brute force validation are just two sides of the same coin. The mistake that I see people making all the time is that they see the record-keeping requirement in the ISO-9000 standard and they think they need to run out and hire a third-party to calibrate every piece of equipment in their facility. As a result, they talk about calibrating things like tape measures and injection molding machines, and some of them spend a ton of money calibrating things that don't need to be calibrated. That's wrong. What I'm trying to do is get readers to see that:
(1) they have an ALTERNATIVE to calibration -- namely, brute force validation -- which they may use as an OPTION in appropriate situations; and
(2) the decision-making process that people should go through to DECIDE which validation method is best depends ENTIRELY on each facility's SPECIFIC PROCESS and NOT on the TYPE OF DEVICE being used.

True, no matter which validation method you chose, the ISO-9000 standard says you need to have records showing that your equipment is validated, but HOW you VALIDATE your equipment is up to you. In some situations, it makes more practical sense to VALIDATE a device by CALIBRATION, but, in other situations, it makes more practical sense to VALIATE a device by BRUTE FORCE. Whether you call your equipment "tools" or "instruments" DOES NOT MATTER. What matters is that you validate your equipment, and that you have records to prove it.

To sum up: the ISO-9000 standard mandates that you keep validation records for every device in your facility that impacts product quality, but it does NOT mandate HOW you validate every device in your facility that impacts product quality.

-- Doug Pfrang

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