Calibration grace period after due date

Jerry Eldred

Forum Moderator
Super Moderator
#11
I don't like the idea of even associateing the word "Grace Period" with a calibration interval. As a metrologist, it strikes me that its introduction may bely a less than adequate understanding of really what a calibration interval is. Some people, I think, may perhaps think the interval is just some magic amount of time between the dates you are supposed to put a new sticker on an instrument. Among those who may believe that, are some that don't believe or understand the purpose of calibration. So it isn't a far stretch to think that some may think it perfectly reasonable to add a grace period.

But when you understand the meaning of the interval, which is essentially the period of time to a defined confidence under which an instrument is likely to yield in specification results. And if you go beyond that interval, it isn't "grace" but risk. So if a company wants something written into a quality manual about some added days allowable, it should be more properly called "increased risk period." Because when you extend the interval of a calibrated instrument, you are by definition increasing the absolute uncertainty of every measurement it makes, and increasing the risk of an out of tolerance measurement. This is why the FDA regulates it so tightly. Those who wrote those instructions understood I think, well, that to go beyond the due date is to incur an unaccep0table statistical risk. Who are we to second guess the OEM of a good instrument (who determined the interval through experimentation).

The only acceptable way to extend an instrument's calibration intercal is through historical data demonstrating that it still meets the defined confidence level of in tolerance measurements (which many instruments do). But it is not a good quality practice to arbitrarily extend intervals with no appropriate data (which is what it appears is the case with grace periods).

A little of a vent, but I believe this is correct.
 
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Jim Wynne

Staff member
Admin
#12
I don't like the idea of even associateing the word "Grace Period" with a calibration interval. As a metrologist, it strikes me that its introduction may bely a less than adequate understanding of really what a calibration interval is. Some people, I think, may perhaps think the interval is just some magic amount of time between the dates you are supposed to put a new sticker on an instrument. Among those who may believe that, are some that don't believe or understand the purpose of calibration. So it isn't a far stretch to think that some may think it perfectly reasonable to add a grace period.

But when you understand the meaning of the interval, which is essentially the period of time to a defined confidence under which an instrument is likely to yield in specification results. And if you go beyond that interval, it isn't "grace" but risk. So if a company wants something written into a quality manual about some added days allowable, it should be more properly called "increased risk period." Because when you extend the interval of a calibrated instrument, you are by definition increasing the absolute uncertainty of every measurement it makes, and increasing the risk of an out of tolerance measurement. This is why the FDA regulates it so tightly. Those who wrote those instructions understood I think, well, that to go beyond the due date is to incur an unaccep0table statistical risk. Who are we to second guess the OEM of a good instrument (who determined the interval through experimentation).

The only acceptable way to extend an instrument's calibration intercal is through historical data demonstrating that it still meets the defined confidence level of in tolerance measurements (which many instruments do). But it is not a good quality practice to arbitrarily extend intervals with no appropriate data (which is what it appears is the case with grace periods).

A little of a vent, but I believe this is correct.
All calibration intervals are, at some level, arbitrary. This is of necessity because a line has to be drawn somewhere. In most instances it probably makes no difference whatsoever whether a thing is calibrated on June 1 or June 3. That's why I always specify a month in which a device must be recalled, rather than a specific day. A device is calibrated on June 1, 2013 which means that it must be calibrated in June of 2014.

There is rarely any evidence, in my experience at least, to indicate that several days one way or another will make any difference in a consistent way, and there's no point in painting oneself into a corner with intervals that aren't likely to be met.
 

Jerry Eldred

Forum Moderator
Super Moderator
#13
And I honestly wasn't trying to stir the pot. In practicality, I agree that a few days is not normally critical. I just wanted to go on record from the metrologist perspective that the intervals truly are a specification, just as a tolerance limit for a range, etc. would be. All instruments drift (statistically) away from their accurate specifications. And given time (statistically) they will all drift out of spec. Many instruments are robust enough that even though the interval may be a year (for example), it may three to five years (sometimes more) before they will drift out of tolerance. I remember aone man lab I ran a number of years ago, that had a couple of thousand instruments. When I was hired, the particular site had not been getting anything calibrated for quite a few years (long story). So I started working through the entire inventory - calibrating all of these instruments that had been unmaintained for I think at least five years (but still in use).

I accumulated percent in tolerance data (95%, K=2 being the standard). First year in tolerance percentage was about 65%. Second year percentage jumped to around 95%, and third year (if I recall correctly) went to maybe 96% or 97%.

This made a believer out of me. So again, I don't disagree the generally (practically speaking) some grace period will likely make very little difference. Most companies don't dedicate enough resources to fully and properly evaluate intervals. Many intervals may very well justify being extended, as mfr interval - which is very often a year - is based on sometimes 99% confidence, as well as testing during design of the instrument. So with the normal 95% confidence, along with the fact that most people don't evaluate their intervals fully, it is very likely that with robust equipment, some grace period will have little actual impact.

So in theory, if the interval is set correctly, a grace period would incur risk. But in practice, often, the risk is so small that it isn't statistically significant. There are items where it makes very little difference (good Fluke handheld multimeters, for example), and others where it could be important (Vacuum transducers that I calibrate are almost always right at their tolerance limit, some process critical torque wrenches, time bases of some RF equipment that sometimes drift across much of their range during the interval), etc.

Okay, enough rambling for the day. Again, I hope I am not stirring up the pot too much; just trying to get some metrological viewpoint expressed on the topic.
 

BradM

Staff member
Admin
#14
All calibration intervals are, at some level, arbitrary. This is of necessity because a line has to be drawn somewhere. In most instances it probably makes no difference whatsoever whether a thing is calibrated on June 1 or June 3. That's why I always specify a month in which a device must be recalled, rather than a specific day. A device is calibrated on June 1, 2013 which means that it must be calibrated in June of 2014.

There is rarely any evidence, in my experience at least, to indicate that several days one way or another will make any difference in a consistent way, and there's no point in painting oneself into a corner with intervals that aren't likely to be met.
:agree1:

In general, there are enough issues going to challenge the program, without having to address the additional burden (in my opinion) of having to calibrate something down to an exact day.
 
#15
Re: Calibration grace period

Unless there is a customer requirement or one from the OEM of the gage you can add a tolerance window to your cal due date, +/- 30 days depending on the gage type and as long as you?ve made provision in your QMS. I never won a ?Grace Period? argument.
A grace period without justification based on data is all that's required. Days mean nothing, in actual fact. A gauge may have zero use in 30 days, or demand for production can increase usage 300%, so could lead to an item wearing outside of acceptable limits.

As has been stated, past trends are the only real way to determine if a grace period means anything.
 
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