Other Dimensions: Calibration Report Details

January 2, 2008
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In my last column I noted how some calibration laboratories offer what appear to be cheap calibration ‘reports.’ I showed how this is done-usually by doing less work and reporting little data. While ISO 17025 lists the basic information a calibration report should contain, there are many laboratories that play fast and loose with these requirements, particularly those not accredited to the standard.

I also urged folks who use outside sources for their calibration to get a list of the elements that will be inspected and reported in order to make a proper comparison between competing facilities. Or ask for copies of reports a lab has issued for the types of items you’re thinking of sending them with the recipient’s name blocked out to maintain confidentiality.

This will give you a better picture of what you will get for your money. Here are some of the elements that are often misrepresented or omitted:
  • Accreditation. Is it meaningful? Anyone can go around accrediting laboratories-in fact, there’s one company I know of that will take care of the whole thing by mail. If mail-order accreditation gives you a warm fuzzy feeling, go for it. On second thought, would you let someone drive your car if they got their driver’s license that way?

  • Traceability. This does not have to be shown on the report if the laboratory has been properly accredited to comply with ISO 17025. But if the laboratory isn’t, I would want to see the actual report from whoever calibrated their primary or working standards.

  • Data. Are you being short-changed? On plug and ring gages there should be a minimum of three readings on go gages and two on no-go gages.

  • Is the report a product of measurements actually carried out by the company that issued it or did they subcontract the work? If they did, check out the subcontractor.

  • Are recalibration dates shown? If the answer is yes, find out who assigned them. If it’s the laboratory, they don’t comply with ISO 17025. Only the owner of the items being calibrated has adequate information to make such a call.

  • Uncertainty. The report should show the uncertainty for each measurement made. And that uncertainty should be shown in the units of measurement shown on the report. Motherhood statements claiming it to be within a certain ratio is not of much use for dimensional work. Check the uncertainty against the tolerances involved to ensure the lab has at least some chance providing a meaningful measurement.

  • Tolerances. If tolerances are shown, somewhere in the report it should be noted whether they apply to new or used gages or numbers pulled out of the air. New product tolerances may be shown for comparison purposes but should be noted as such. Only the gage owner can determine what tolerance is acceptable for their applications.

  • Accept/Reject. If the report has declarations regarding acceptability of the item calibrated, I would want to know what criteria was used or where it came from if it is shown on the report. If the answer is the new gage specifications, you could be discarding perfectly useable gages. I also would ask the laboratory how they treat their measurement uncertainty when making such statements. Do they reject an item based on their reading of size and ignore their uncertainty?
Under ISO 17025 a calibration report that does not show actual values but simply lists a description of the gage and indicates it is within a stated specification is acceptable. But the issuing laboratory must keep all the data from its measurements of the gage on file. And they must indicate if they did not check all of the elements called for in the specification by noting what they did or didn’t measure. If this was not done, the customer would assume that all elements were measured-often this is not the case. If you accept such statements in lieu of data, you should confirm what was measured to be safe.

These are some of the basics you should know about. If you don’t, a sharp quality auditor may discover details on reports that are suspect, or details that should be in reports but are not.

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