Re: Determining Measurement Device Calibration Frequency
It all depends on how much risk you want to take.
Exactly! You have summed it up nicely.
In my experience measuring equipment manufacturers recommendations cannot always be relied on. Bizarrely, some even have no clue about true calibration!
Their ideas are just that, ideas, since they have no data about the use of the equipment. You could have a device which gets hammered for three sifts gauging threads in stainless steel, and it wouldn't last a week. Some things could go years before they need to be rechecked.
Spot on!!

Man, don't get me started

, Andy. I think it's getting a little better with mfg. specifications and all, but it is still pretty sad. Bottom line: Mfg. companies are in the business of making equipment and money... not necessarily interested in good metrology practices (Fluke is one exception, IMHO). Hence the reason you can rely on competent metrology sources for assistance. If nothing else is available, start with mfg. recommendations, but manage the process after that.
If you read the ISO 9001:2000 standard carefully enough you begin to realize that part of the answer is there.
In element 7.6a is says " . . . calibrated or verified at specified intervals . . ." Who gets to set the interval? You do. Obviously, you don't want it to be so frequent that it is a burden, but what sets the other limit? The answer is in the paragraph after the lettered list: "In addition, the organization shall assess and record the validity of the previous measuring results when the equipment is found not to conform to requirements. The organization shall take appropriate action on the equipment and any product affected."
So you need to not set the interval so frequently that it is an unnecessary burden, but frequently enough that you don't run into problems that could make it necessary to go through the exercise of figuring out what if any bad product was shipped and how to resolve it.
As someone else said, a good place to start is with the equipment manufactures recommendations. Another good source is your local calibration lab. Then adjust from there pending the results of follow-up calibrations. Mix a little common sense into the equation as well.
Good points, Jim. Taking up on your common sense suggestion...
Regardless of your interval, don't turn your brain off with your process. If your process is going along with decent stability and all of a sudden you get some noise, it might not be a bad idea to check the instruments, use a spare, etc. Also, if you are nervous about your interval, start short, and then stretch as you are able. Send your instruments to a good calibration source that provides as found/ as left data, so you can make informed decisions regarding calibration frequency.
The main thing is manage the process. Adjust the frequencies when the situation calls for it. When (or if) you get it right, you be having your instruments calibrated (and probably adjusted) at the right time frame, maximizing benefit and minimizing cost.