Things aren't always what they seem to be. This is certainly true of calibration certificates and services. To identify the real stuff and achieve the traceability required to meet ISO and AIAG standards, try the following roadmap.
Separating Wheat from Chaff
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 17045 measurement quality assurance standards and (more importantly, if you're in the U.S., Canada, or Mexico) the Automotive Industry Action Group 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.
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, solicit bids only 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.
More
There's more to be said about calibration, traceability, and measurement uncertainty; check the October issue of Sensors' Industrial Automation newsletter for more.