Sensors Mag

Calibration Confusion

October 25, 2006 By: G. Raymond Peacock, Temperatures.com Inc.


E-mail Ray Peacock

There are a lot of questionable calibration certificates and services out there. Not all deliver the traceability required to meet ISO and AIAG standards. Let me help you take the guesswork out of the selection process and establish a sound quality assurance program.

Choosing a Vendor

Recently, a major instrumentation provider began offering upgraded calibration services for temperature, pressure, and other sensors that includes five different types of calibration certificates. Some of these are what we used to call supplier verification, or certification of authenticity. I am not sure what the others are. But none of the certificates use National Institute of Standards and Technologies (NIST) or ISO terminology to express measurement uncertainty and traceability. So if a supposedly capable vendor doesn't appear to meet serious measurement needs, who does? How do you choose a vendor? Is your decision based on price or calibration thoroughness? Do you have a quality assurance (QA) program or just prefer to do things right? What must your calibration certifications contain to meet the ISO 17025 measurement quality assurance standards and (more importantly, if you're in the U.S., Canada, or Mexico) the Automotive Industry Action Group (AIAG) Measurement System Analysis Guidelines published by the "Big 3" automakers?

It seems reasonable to expect that one complete and conforming certificate would be sufficient. It only needs to meet your QA program requirements, be it ISO-9000 or QS-9000.

Calibration and Traceability

As a minimum, your calibration should be directly traceable to national standards, and your certificate must provide the full details. Anything less is not traceable. It should also state the uncertainty of the measurements recorded and the serial numbers of the traceable devices used in the calibration. But there's more than just calibration services to consider.

Why not buy your instruments from a vendor that offers calibration accredited by a major third party. By doing so, you ensure traceability from the first time you use the instrument, and you have a logical place to get the devices recertified.

This doesn't seem too hard; just make sure your purchasing guys are on board. Better yet, only solicit bids from vendors with accreditation.

Accreditation

Who provides accreditation for calibration labs in the U.S.? The American Association for Laboratory Accreditation (A2LA) and the National Voluntary Laboratory Accredited Program (NAVLAP) serve the general public and many military and scientific organizations within the U.S. government. You can view lists of approved calibration laboratories on the NAVLAP and A2LA Web sites. But that usually isn't necessary because most vendors are proud of their accreditation. For example, Fluke Corporation publishes their NAVLAP Certificate on their Web site.

It's interesting to see that only 29 organizations in the U.S. have been approved by NAVLAP for calibration of temperature-measuring devices, and several of those are government agencies and private, in-house operations. Needless to say, the outfit offering the five different types of calibration certificates isn't on the list yet. Note that NAVLAP updates its list during the last week of each month.

If your equipment vendor or calibration service is accredited by one of these two organizations, you can expect to receive a traceable, properly documented certificate without making special requests or having to wade through extra verbiage.

Other Resources

There are other organizations and groups involved in measurement quality that provide in-depth training, materials, and education programs. Organizations such as the American Society for Quality (ASQ) and NCSL International have been around for many years, helping organizations and individuals better understand quality and measurements.

Then, too, there are private trainers such as Measurement.com Associates. They've been offering a range of in-house and public seminars and courses on quality, measurement, calibration, and measurement systems.

If you want to learn more about the background and science of measurement uncertainty and metrology, look no further than the NIST Web site, where you can not only get an introduction to the subject but also a complete handbook on statistics in measurement that will blow your socks off. Did I mention that's all free?

Start with the NIST page on the term uncertainty, NIST Policy on Reporting Measurement Uncertainty, and take it from there. Soon you'll find the page describing the terminology used and the difference between accuracy and uncertainty. Bottom line: calibration and measurement systems' performance are quantitatively described in terms of the measurement uncertainty, not accuracy. The term used to describe a calibration result is in the traceable uncertainty of the results. Same thing with process measurements, as any careful process engineer will tell you. But the terminology is only the beginning.

That's it for this month. Good measuring!

References

National Voluntary Laboratory Accreditation Program (NAVLAP)

Standards Services Division, NIST

100 Bureau Drive, Stop 2140

Gaithersburg, MD 20899-2140

Phone: 301-975-4016

Fax: 301-926-2884

Email: NVLAP@nist.gov

Web:

NAVLAP currently provides accreditation services for calibration laboratories in the following areas of measurements: dimensional, electromagnetics - DC/low frequency, electromagnetics - RF/microwave, ionizing radiation, mechanical, optical radiation, thermodynamics, time and frequency.

The American Association for Laboratory Accreditation (A2LA)

5301 Buckeystown Pike, Ste. 350

Frederick, MD 21704

Phone: 301-644-3248

Fax: 301-662-2974

Web: http://www.a2la.org


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