I often take shots at those companies with great quality systems that get their calibration reports, scan them for red flags and then file them away if none are present. They rarely get into the details on those reports until something goes wrong and they start looking for answers in reports that may be out of date. Those simple yes/no answers they insisted be put on the reports won’t help if they don’t know the meaning behind what they’re reading.
To be fair, some of the verbiage on calibration reports can leave anyone guessing so I thought it might be helpful to those who actually read these reports to understand the meaning and/or implications of the terms used for dimensional calibration work.
Report Validity: This subject comes up when the report is a year or more old and the answer to this “’How long is it good for?” comes as a surprise. It’s good until the gage or instrument leaves the lab. I know, it’s not a very comforting statement but it reflects reality. Once the item leaves the control of the lab many things can happen to it that could render the lab findings obsolete. So, if you have concerns in this regard, get it calibrated again.
Uncertainty: This is how close the reported value is to the true value or size of the item calibrated is likely to be. It doesn’t mean the reported value is ‘out’ by that amount, only that it could be. Every measurement has an element of uncertainty—even those made by NIST.
Recalibration Date: This is an indication of when the item in the report is supposed to be calibrated again. Some labs have software that automatically generates this number which can be based on a variety of factors—known and unknown. If the lab is properly accredited to ISO 17025, the customer must specify this information so labs don’t use it to drum up business. ISO 17025 does not require this info on a report.
Feature/Element Measured: This is pretty straightforward most of the time. But when it’s about pitch diameter of a thread plug gage, not so much. If you sent the gage to NIST for calibration after you decided your regular calibration source didn’t do the job right, you’ll end up with a different set of numbers. So, which one is the right one? Both could be right. The reason for this is NIST will measure the pitch diameter in much the same way as your lab but they will correct the measured size for variations in linear pitch etc. while your regular lab will provide ‘simple’ pitch diameter where these corrections are not provided unless requested. Why is that? Because few companies will pay the cost of doing it like NIST and a large percentage of labs calibrating thread gages do not have the proper hardware to measure linear pitch to the level of precision required.
Shouldn’t traceability be shown? This is a common question since most quality standards require measurements to be traceable to one thing or another. It is not a reporting requirement of ISO 17025 accredited labs because the original document(s) from NIST, NRCC, NPL etc. are part of their on-site assessment of labs. The accrediting agency assessor will review traceability documentation to determine if it is there and relevant. Without seeing such original documentation, there is no way of knowing whether the traceability is directly to NIST or through one or more other labs in the food chain. The more times the NIST numbers are transferred, the greater their contribution to uncertainty in the lab using them at the end of the chain. In some cases, this is not a big problem, but when your numbers are being reported in millionths of an inch or microns, it can become a very big deal.
Before and After Values: These are important to the user of calibrated equipment since a significant difference between them could call into question measurements or checks made by them before they were calibrated. The difference will be due to the lab making adjustments to improve performance. In the dimensional field it may simply mean re-setting a zero point. If any adjusting is done by the lab, both numbers must be reported to comply with 17025.
Send other issues to me and I’ll try to answer them in a future column.
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