Confidence Limits in (Infrared )Temperature Measurements

Repeatability and Reproducibility

One way the overall effects of instrument calibration uncertainties and other variances due to operators can be determined is through a set of controlled R & R tests, or Repeatability and Reproducibility tests. The basics of R&R testing lie in statistical results from controlled tests.

There is a well-defined formalism used, for example, by The Automotive Industry Action Group, AIAG. They are one of the biggest driving forces (no pun intended) in improving production quality in North America and have published a series of booklets and practices recommended for measurements and measurement devices.

Any company expecting to do business with a major auto manufacturer or their suppliers in the USA, Canada or Mexico must follow these practices in order to be a minimally acceptable supplier.

Included in AIAG’s basic measurement quality assurance are R&R measurement procedures for testing equipment and operators. Although written primarily for dimensional gauging (a significant portion of automobile production quality requirements), the practices are applicable to any measuring device.

Within the automotive industry, this type of testing is often called GRR, standing for Gauge R&R. The handbooks and sample data sheets are available at modest fees from the AIAG and some of the software vendors to the industry.

Within the semiconductor industry, a very similar need was evident. They worked with their own research and production resources and NIST to develop a measurement practices policy that follows the same methodology as the AIAG’s. The resulting measurement practices handbook is freely available on the Web and can be viewed and downloaded from the NIST web site. The version of the Handbook on the NIST web pages is integrated with the Dataplot statistical software.

In order to use Dataplot from the Handbook, it must be downloaded and installed on your computer.

The basic procedure for R&R testing is also straightforward. One starts with a calibrated measurement device and has an operator measure a variety of objects, usually about three to five, each having a different value, that are different but not necessarily known. The only requirement is that they do not change during the tests.

Several operators using the same objects perform the same set of measurements, usually with only one instrument shared among them. That corresponds to one testing round. Then the round is repeated, usually two to five times.

If different environmental conditions are likely to affect the results, then one or more of the objects can be in different real or simulated environments. The key is to have each operator or “appraiser” repeat the same measurement more than once, usually a minimum of two or three times.

Each of the operators measures the same objects. This enables one to statistically evaluate and separate the effects of the operator, the effects of the equipment and the effects of the environment. It also enables one to determine the statistics related to the combined effects of all the major influencing factors.

There are numerous software packages on the market as well as a complex, but free package, Dataplot, from NIST that will not only help one set up R&R tests, but can also guide one through the test steps and calculate the resulting statistics from the measurement data.

Conclusions

If you don’t follow good measurement practices you will lose to the supplier that does.

References/Resources

ASTM Standard E1256, Standard Test Methods for Radiation Thermometers (Single Waveband Type). W. Conshohocken, PA: American Society for Testing and Materials, 1995. (On the web at http://www.astm.org – downloadable  for a small fee)

Nicholas, J.V. and D.R. White, Traceable Temperatures Second Ed., John Wiley & Sons, Ltd., 2001.

Taylor, B. N. and C. E. Kuyatt. NIST Technical Note 1297:Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, National Institute of Standards and Technology, US Dept. of Commerce, Gaithersburg, MD, 1994. (On the web at: http://physics.nist.gov/cuu/Uncertainty/index.html).

AIAG-Automotive Industry Action Group, MSA-3 Measurement Systems Analysis (MSA) Third Edition for Automotive QS-9000 Suppliers,  2002,  (Can be purchased by telephone from AIAG Customer Service department at (248) 358-3003 or on the Web at: http://www.aiag.org/publications/quality/iatfquality.html) Automotive Industry Action Group (AIAG), 26200 Lahser Road, Suite 200, Southfield, MI 48034

Engineered Software, Inc. Measurement Assurance (Software supporting the analytical techniques detailed in the Automotive Industry Action Group (AIAG) Measurement System Analysis Manual), Engineered Software, Inc., 43737 Timberview Drive, Belleville, MI 48111 (On the Web at: http://www.engineeredsoftware.com)

Croarkin, C., Editor, Measurement Process Characterization, Chapter 2 in the NIST/SEMATECH e-Handbook of Statistical Methods, National Institute of Standards and Technology, US Dept. of Commerce, Gaithersburg, MD (On the web at http://www.itl.nist.gov/div898/handbook/index.htm)

Filliben, J. J. and A. Heckert, Dataplot (A free, public domain, multi-platform {Unix, VMS, Linux, Windows 95/98/ME/XP/NT/2000, etc.} software system for scientific visualization, statistical analysis, and non-linear modeling with GUI interface by R. R. Lipman), National Institute of Standards and Technology, US Dept. of Commerce, Gaithersburg, MD 1978-2002 (On the web at http://www.itl.nist.gov/div898/software/dataplot/)

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Author: G Raymond Peacock, Temperatures.com, Inc. Southampton PA, USA

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Tagged as: confidence limit, gage r&r, grr, irinfo, irinfo2003, nist, r&r, traceable calibration, uncertainty