Statistical Studies to determine Calibration Frequency

R

Ron Dooley

#1
This question is a result of a survey by one of our customers.

The question:

Control of Test & Measuring Equipment.

"Requirement by our customer.
Calibration results to be analyzed using statistical techniques to establish frequency".

I know of no requirement for ISO or QS that has any language concerning statistical studies for calibrations to determine frequency.

Would appreciate any help. Thanks
 
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D

David Mullins

#2
Good Management!
When you open the book that came with the equipment, what is the manufacturers recommendation on calibration frequency. Determining this frequency yourself can be an expensive and time consuming operation. Additionally, I know in Oz there are already numerous standards that provide this sort of guidance.
At the end of the day, the stats to determine frequency relate to the average time taken to exceed the acceptable degree of risk of the accuracy of the equipment.

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Marc

Hunkered Down for the Duration
Staff member
Admin
#3
I don't know that there is a specific requirement for statistical techniques but I would be ready to use cal records as evidence of stability during your cal interval. I would also be ready to say you evaluated the need for statistical analysis of calibration data and found it to not be value added based upon instrument stability. You don't need a graph to tell when the instrument is wearing or such - its stability will fall with use. If heavily used this could be a day or a week. Or it could be a month or a year. Lot's of possibilities.
 

Marc

Hunkered Down for the Duration
Staff member
Admin
#4
By the way, graphing, as I understand it, is a 'statistical technique'. Have any of you run into this?

Because of it being calibration in nature, I'm moving this thread to the Calibration forum.
 

Jerry Eldred

Forum Moderator
Super Moderator
#5
The QS9000 requirement is that calibration is done at an appropriate frequency (4.11(c)). At the end of QS9000 para 4.11 is Note 18, which says that ISO10012 may be used for guidance.

ISO1001201:1992(E), para 4.11 "Intervals of Confirmation" there is further detail. It does state that confirmation needs to be done at intervals which maintain an acceptable level of confidence that the instrument will remain in tolerance for the duration of the interval.

The problem with use of the manufacturers intervals is that it is not always correct, and not al manufacturers document a recommended interval. If you have a small calibration program, and you can provide a manufacturers documented interval for all instruments, you might stay out of trouble.

In my years of experience in calibration I have found that the manufacturer's recommended interval doesn't always work when equipment gets some age on it. A Hewlett-Packard signal generator, brand new out of the box, used in a clean, controlled environment, is very likely to stay within tolerances (as an example). But even with good brand name equipment, as that same unit ages, its ability to remain in tolerance for that same calibration interval will change. Or many other factors such as a less than ideal operating environment, or moving that piece of equipment often (a signal generator that is picked up, put on a cart and transported to another location repeatedly, for example) would not have the same probability of remaining within tolerance.

There is also an upside to adjustment of calibration intervals. A Fluke 77 multimeter, for example may have an interval of 12 months. If you had 100 of those meters and statistically evaluated its confirmation interval, I have seen intervals on that particular model extended up to 3 to 5 years. I seem to even recollect one company that did interval analysis on that model and ended up with no re-calibration required because they never had a single out-of-tolerance.

Interval evaluation in a small program may well be more work than it is worth in some circumstances. But in a large calibration program, many 10's or perhaps 100's of thousands of dollars per year could be saved, as well as the identification of porblem units. I had some DC millivolt calibrators in my program with a 12 month recommended interval. I had two units. Neither one of these, even after a factory overhaul could stay in tolerance for even three months. I eventually quarantined them and told the user to buy something else as a replacement. The process people ended up with test equipment which would do the job right. And risk to customers product was also minimized.

I have managed calibration programs with fixed intervals, but I am also sensitive that statistical interval adjustment is also sometimes a must in some circumstances.

Hope I didn't stir up the pot too much on this. But I am duty bound to tell the truth the best I can.

As for methods, there are many. I grouped similar instruments together annually (for example Fluke 70 series including model 73, 75, 77, then Fluke 77-II series as a separate family), calculated how many of these instruments (as a percentage) remained in tolerance through their calibration interval for any calibrations done that calendar year. I matched that percentage figure against a chart I developed. A given percentage figure correlated to either an increase, remain the same, or decrease in interval. Depending on the percentage figure, the change was incremental (i.e.: a very low percentage figure corresponded to a drastic decrease in interval, etc.).

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Marc

Hunkered Down for the Duration
Staff member
Admin
#6
I agree with you Jerry. My expectations are that whoever is running the show be knowledgable to look at the use of the device and to watch (and understand) the calibration record. In some companies this is a complex task - such as your environment. Some companies are small shops with maybe 5 or 10 devices total.

I believe QS9000's MSA requirement came from the fact that many companies have absolutely no one in-house who really understands the necessary concepts.
 

Marc

Hunkered Down for the Duration
Staff member
Admin
#7
I have a client right now that is doing just that. GE has a cal lab (luckily) just down the street. They keep the database (customer gets a copy of database and software database is in) and the whole shooting match. I do still have to ensure the guy they recently put in charge of the quality program understands the basics, however.
 

Jerry Eldred

Forum Moderator
Super Moderator
#8
Marc,

Your point brought up a thought for any of those reading who may benefit. In my years working as a consultant/contractor, it was sometimes an option at some calibration contractors to offer management of small customers calibration programs. If a company is too small to have an in-house calibration program, they would also be likely not to have qualified personnel on staff to evaluate calibration intervals. One possible solution to this would be that if they use a contractor to do their calibrations, that if it is a reputable, properly qualified calibration contractor, find out if the contractor does interval evaluations.

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