Jim,
Thanks for your input. I am concerned when we begin adding additional features like 2-sigma warning limits onto a run chart of, in this case, assumed normally distributed measurement data we could elevate the Type I error. However, your recommendation to perform a recheck will usually minimize the built-in Type I error resulting from this exercise. Therefore, I’m not too concerned with 2-sigma warning limits as long as a recheck is done per your recommendations.
Per you last question on the calibration timing, my thoughts are as follows:
1. My goal on new measurement systems is to capture as close to the actual time as possible when the measurement system would naturally drift out of calibration under normal use.
2. I do not actually know the actual drift timing, because my check period sets the sensitivity for detecting this drift. Ideally, for a new measurement system, I would like to evaluate it on much shorter periods than that set for the other systems.
3. I want to build in some limited assurance of maintaining a continuously calibrated system for continuous manfacturing operations while minimizing the chance of sending out product that is outside the spec.
With the above as a ground rules in place, in order to establish the calibration interval for a new measurement system I would evaluate the system routinely throughout the day. If possible, up to 3 times per day over a period of one to two weeks, or until a drift is observed using a run chart with dynamically adjusted control limits. After observing an apparent drift, as indicated on the control chart, I would note the number of days I operated without a system calibration. I subtract one day from that number to provide a cushion against the timing confidence, as I’m sure there will be some error in estimating the drift time. For greater calibration interval assurance one may use up to 3 days difference from the estimated time determined from the run chart. What we don’t want to do is set a calibration interval that is sure to find the system out of calibration for the better part of a day’s manufacturing.
Jim, let me know if you think this logic makes sense. I’ve found in past work when using this approach that I can achieve what I call “Just in Time Calibration” while minimizing the chance of sending our product that is out of specification. Usually, this method establishes the most economic interval possible.
One last note, all system wear down, including measurement systems. Therefore, one should consider re-evaluating a given measurement system using the prescribed methods more than once in the lifetime of the measurement system. For instance, I would advocate a full capability study to be done on a pair of calipers once ever 1 to 1 1/2 years. Laser micrometers may go out to 2-3 years. The idea here is that you are trusting these systems to give you accurate information each time you use them. A period check will minimize the chance of receiving unwelcomed surprises.
Ken
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