On 8/12/2013 2:23 PM, John Phillips wrote:
A calibration indicates that the unit under test is withing manufactures specification. The equipment and procedure used has to be "good enough" (bad words in a cal lab) to have a high probability (nothing is 100%) of insuring the calibration documentation is valid. Things can can be a little looser if you are calibrating a 1% meter with a 10 ppm meter but it does not work the other way around.
You can calibrate either way. You can't however, calibrate the 10 ppm meter so it's in spec using a 1% meter. That's different. Calibration merely means that it's documented how close it is to a reference, such as NIST, not that it's within the manufacturer's spec. The 10 ppm meter would end up with a 1%+ calibration - precise but not accurate. Not particularly useful, but valid. A good cal lab would do a calibration to specification, where the uncertainties place the 10 ppm meter within spec.
As I cited and someone else already quoted, calibration is the "property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty." Nothing to do with making a device meet its specifications.
That's why an eBay seller can claim they'll do a "calibration traceable to NIST," because they're not claiming any particular accuracy. It's really not worth anything, unless they give specific uncertainties or claim calibration to manufacturer's specification.
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