The Tracks of Traceability

A calibrated $2 thermometer is still a $2 thermometer

Everyone in the quality profession has heard the term “NIST traceable.” Having calibration traceability to the National Institute of Standards and Technology (NIST) is desirable for most measurement devices. It is also enshrined as a requirement in some regulatory documents. Unfortunately, NIST traceability does not ensure measurement quality. Here’s why.

NIST, a U.S. government organization based in Maryland, is the official keeper of the flame for the highest level of measurement accuracy. If you want to know how good your thermometer is, you can send it to NIST, and it will compare it to the very best temperature standards (“calibration” is simply the comparison of one device to another) and send it back with a report. Of course this isn’t practical; NIST cannot calibrate the millions of thermometers that are in daily use, and the service is expensive.

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Traceability

I agree with most of what the author has stated, with some add-ons as follows:

1. There are several more Accreditation Bodies that accredit Test and Calibration labs to ISO/IEC/ANSI Std. 17025-2005. A few are listed here: International Accreditation Service (IAS; www.iasonline.org); ACLASS, one of three brands of the ANSI-ASQ National Accreditation Board (www.aclasscorp.com); Standards Council of Canada(http://www.scc.ca/programs-services/laboratories/calibration); Perry Johnson laboratory Accreditation Inc (http://www.pjlabs.com/) ansd so on.

2. The important point is that to provide traceability, the Accreditation Body (AB) that provides the accreditation to ISO/IEC/ANSI Std. 17025-2005 has to be a "full signatory member" of the APLAC MRA and ILAC MRA. (APLAC=Asia Pacific laboratory Accreditation Cooperative; MRA= Mutual recognition Agreement; ILAC = International Laboratory Accreditation). By being a "full signatory memeber", the AB participates in a "Peer Review Audit Process" i.e. it agrres to have its own process of accreditation to be audited by a randomly chosen peer AB, which is difference for each such audit. This assures a better level of Quality and traceability.

3. Another missing point in the article is the fact that all calibrations are a snapshot of the UUT (unit Under Test-i.e. unit being calibrated,here the proverbial $2 thermometer) performance at one point in time. Nothing in the calibration process ensures that the calibrated unit will make it perform consistently over a long period of time, such one year, which often the interval between two consecutive calibrations. To ensure the documentation of consistent performance with the traceability and accuracy inherent in a NIST traceable calibration, frequent "quick checks" between two consecutive calibrations; and their documentation is required. This also enables the users to minimize the corrective actions needed when an instrument indicates that it is no longer performing within its defined tolerance limits. As an example, if the $2 thermometer on an annual calibration cycle starting on 1/1/2011 was subjected to a monthly "quick check for four equally spaced temperature points within its range, and it showsa performance within tolerance for Sept 30, 2011 check but shows an out-of-tolerance performance on Oct. 31, 2011 check, it means that it went bad sometime in that period, and the corrective traceback and checking of readings applies only to the readings in that period. If such intermediate checks are not done, the corrective action must apply to all readings, going back to Jan. 1, 2011.