Determining Calibration Intervals - Extend or change the length of

Can anything be done to extend or change the length of calibration if the calibration record and procedures state it as being an annual interval? I'm thinking I can change the procedure and change the information on the calibration records when they get calibrated this time, so we will be prepared the next time we want to calibrate our IMTE. Is this correct, or can I change the intervals without calibrating some of the IMTE again? We are trying not to send out those IMTE which do not need to be calibrated. This is our first renewal, if you will, for calibration. Now we know that all the IMTE doesn't need to be calibrated annually as it was originally stated.

Thank you in advance for your help.

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[Jerry Eldred]

The definition of a calibration interval (loosely stated) is basically that period of time wihtin which the IMTE (as you call them) can be expected to remain within it's specified tolerance. The interval may be lengthened on any item which you have adequate data to support that the equipment will have an acceptable level of confidence that it will stay in tolerance for the new interval. One such example is the Fluke 77 meter (I am not trying to sell Fluke meters, only an objective use of them as an example). I have gathered data in the past to extend their interval to two or even three years. I am not certain if it was the Fluke 77 or some other similar meter, or who the company was that made the interval adjustment. But I have even heard of cases where a calibration interval on an instrument was extended to no calibration required. That would have to be, of course, based on some rather extremely impressive data over many years (as I would strongly hesitate to do that - matter of fact, I don't recommend that).

I suggest getting a copy of the NCSL (National Conference of Standards Laboratories) recommended practice for the establishment and adjustment of calibration intervals. To adjust them to longer than the original manufacturer's recommended interval, you will need to document your method and data as to ohw you adjust, and how you define your confidence level.

Hope that is of help.

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I forgot one very important piece of information in my first posting. That is: we used an outside calibration house to do our calibration. We are planning on doing some of the micrometers this time, but all the rest will be calibrated on the outside.

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[Rick Goodson]

Just as a follow up to Dan's post. I am sure when he mentioned the Fluke 77 metere he meant a specific meter. You can not extent the calibration cycle of a class or family of instruments based on the results of a single instrument. Each one stands alone.

In your SOP or procedure that covers the elements pertaining to calibration, you should have provisions of some sort to lengthen or shorten a calibration cycle.

Typically, I use the following and as long as things are properly documented, have never had a problem.

"Interval Adjustment:
When using historical data to determine or adjust a calibration interval:
1. The interval can be shortened if two successive calibrations are found to be out of tolerance. The recommended reduction is 20%.
2. The interval may be lengthened if three successive calibrations are found to be in tolerance. The recommended increase is 20%.

This should not be applied to instruments that have a particular accuracy or rating based upon length of time between calibrations.

Calibration cycles may be changed for other reasons as long as there is a written justification for changing the cycle and it is approved. Justification may be expanded capabilities or accuracies; significantly increased/decreased use; change to a more extreme environment where the instrument is used, etc.

All cycle changes will be documented and maintained in the history folder or other permanent file."

All the systems I implement for calibration recall use a permanent history folder for each instrument maintained in the system.

[Marc]

Originally posted by Rick Goodson:

You can not extent the calibration cycle of a class or family of instruments based on the results of a single instrument. Each one stands alone.

I have seen it accepted where a certain instrument type and model was used for a certain measurement. Many stations did the same measurement with the same instrument type and model. The cal cycle for all was determined by the the one with the 'worse' calibration result.

Having been the Quality Manager for a calibration facility in the past and worked with many as a consultant, I cannot say where I have ever seen changing the interval of an entire class of instrument based on the performance of one.

I have seen where the initial calibration interval for a class of instrument was set to the same length when multiple units were put into service at the same time or even an initial interval for a class of instrument. But subsequent use, condition, and historical performance on calibrations would govern individually if the cycle was changed.

What type of instrument was it Marc?

[Jerry Eldred]

I can see where this could become somewhat contentious. I have heard some equally good philosophies on a few different methodologies for interval adjustment. Some of the philosophies are:

1. Use a convenient fixed interval (such as 12 months). This makes work forecasting much easier, but 'over-calibrates' some and 'under-calibrates' others.

2. Family/Class Interval Adjustment. This has good reviews in areas of electronic test equipment (such as the Fluke 77 example (and I am not trying to sell them, only use as a common model). Many electronic test equipment are either unaffected or little affected by level of use, and subsequently, use of a statistically significant sampling of numerous of the same model or model/family may be adequate to determine interval adjustment. This with the implicit exception of 'dogs' or 'gems' (mostly 'dogs'). It is not too difficult to define a dog in terms of reliability. This type of adjustment, however, may not work well at all with mechanical instruments, as they may sometimes tend to go out of tolerance based more on usage.

3. Individual instrument adjustment. This is also quite common, and most readily adapts to the uniqueness of each piece of test equipment. It is higher maintenance, though, and may be a more costly method (in our lab, we are currently using this method, but it is fully automated in our custom database package).

I have seen numerous variations of the above. The bottom line is that your well-defined method be adequate to eliminate calibration interval related risk to product (or service). That is really the bottom line of calibration. If I were a tire manufacturer, and I had temperature controllers to cure an intermediate adhesive layer between my tire tread and the steel belts (fictitious example only; no implication that any tire companies have this problem). The bottom line would be that I calibrated those temperature controllers accurately enough and at adequate intervals to assure that the temperature controller accuracy was continuously maintained to preclude that as a contributing factor to a tire defect.

I only have one more small hand grenade to toss into this discussoin. While visiting with some process engineers in Asia last year, one of them showed me a detailed lengthy report about how important it is in process control NOT to over-control the process. All quantitative process variables have variability (a given). If in controlling a process there occurs a single anomalous variation which throws the process from nominal out toward a control limit, the instinctive reaction would be to adjust the process back to nominal and correct for the error. These engineers showed me the mountain of statistical data that basically says when that happens (process is maintaining at about nominal for a long period, then abruptly shifts away from nominal for one reading), you should adjust nothing. The process statistically will come back to nominal. And the probability is that it did not truly drift away from nominal (I can't, by the way, relate any details of this report for company reasons). When single measurement drifting away from nominal occurs, leave it alone. It was an anomaly.

Now to explain my longwinded rambling. I think we should use some of this philosophy in calibration interval adjustments. If I have a family of Fluke 77's (say 30 of them). My data tells me that they should have (for example) a 2 year interval. If I then calibrate one of the units, and find a gross out of tolerance, in accordance with the above theory I proposed, that gross out of tolerance was an anomaly. And in my lengthy experience with Fluke 77's, that is what it would have been. I have seen corroded function/range switches (somebody left their meter in their car and it got rained on). This is an anomaly, and doesn't truly reflect the propensity of the entire population, and in this case, doesn't even reflect what that specific unit is likely to do next interval. The owner of the meter feels stupid when I give him the compassionate fatherly lecture about leaving such a nice meter out in the rain. Next year, all the units are in tolerance. My point is that I believe in only adjusting intervals based on statistically significant sample. I am doing some short term variability testing on an hp 3458A high accuracy meter at the moment. Short term (smaller data sets) have tendencies to show different patterns of variability than longer term (significant amount of data to be analyzed). We need to be careful about over-adjusting calibration intervals.

Mechanical instruments are a different matter. There is much more of a physical wear and tear issue applicable to those instruments. Micrometers heavily used will perhaps exhibit different wear and tear than those that sit in a tool box all the time, and so must be treated differently. I won't comment on that area, as I do not have much experience in mechanical tools. Individual instrument interval adjustment may well be more appropriate in those cases.

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