Know Your Numbers

March 1, 2003
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Investigate how labs come up with their uncertainty numbers.

A manufacturer using outside, contracted calibration services needs to pay close attention to the pretty certificates he receives from the laboratory.

The problem is measurement uncertainty statements on certificates that often show the same values as the Best Measurement Capability on the Scope of Accreditation issued by the laboratory’s accrediting agency.

Simply put, uncertainty numbers on the Scope refer to a measurement in the laboratory on an item in pristine condition. The laboratory will use the highest reference standards and most accurate instruments they have for such tests, not necessarily the ones used for everyday work.

If the gage you send for calibration is not mangled or rusty, and the laboratory uses the same masters and equipment to calibrate it that were used in tests to support their uncertainty claims, then the uncertainty numbers could be the same as those on the lab-oratory’s Scope of Accreditation. The odds are they won’t be the same as the numbers that are listed, because if the laboratory calibrated your gage the way they did for the test they submitted for accreditation, you’d be paying much more for their everyday service.

When it comes to on-site calibration, laboratory conditions are rarely duplicated. Thus, there may be a big difference between the uncertainty on the report given to you as compared to the value shown on the Scope of Accreditation. Don’t blame the calibration laboratory if the uncertainty numbers for such work end up in the twilight zone.

If you have concerns about the uncertainty numbers reported to you, talk to the laboratory that did the work. When on-site work is involved and the calibration laboratory is using the same values as shown on their Scope of Accreditation, ask them for the temperature range for which the uncertainties are valid. If the response you receive is something such as, “One-half a degree,” or some other value that seems to be only obtainable in a laboratory setting, and you know the temperature in your facility fluctuates by several degrees, it’s time to take a close look at the calibration laboratory’s uncertainty budgets that they report to you and determine what else may not be quite right.

My comments are based on the assumption that the uncertainty values on laboratory Scopes reflect reality of one kind or another. Unfortunately, many don’t. You knew that was coming, didn’t you? Some organizations accredit laboratories whose Scopes are deserving of awards for creative writing but bear little resemblance to real-life calibration. Such laboratories are able to accomplish this because organizations that offer cheap and fast accreditation are able to do so by focusing on quality systems using submitted paperwork only. Such accreditation agencies rarely go beyond what is written on paper and verify the laboratory’s results independently. The technical data for the Scope of Accreditation is often ignored, or the assessor has little or no knowledge or experience and accepts anything that looks reasonable as an uncertainty claim.

What’s the big deal? It may be no big deal if you’re only interested in paperwork for your files. But if you need to prepare uncertainty budgets for your everyday measurements—and every manufacturer should, while others must for the sake of fulfilling standards requirements—those must include the uncertainties for the measuring equipment. If those uncertainties don’t reflect reality, your measurement uncertainty budgets won’t either and everything goes downhill from there.

Reputable accrediting agencies scrutinize reports issued by laboratories they have accredited. This ensures that correct practices are being followed in reporting measurement uncertainties. Unfortunately, some of those who accredit or register calibration laboratories may not be as diligent. If you want to ensure the numbers you receive reflect the reality you’ll have to live with, carefully review the reports you receive and the Scopes behind them. And, if necessary, evaluate the accrediting agency endorsing them.

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