Quality Progress - July 2015 - (Page 44)

MEASURE FOR MEASURE BY DILIP SHAH Estimating Uncertainty Metrological traceability to non-SI units IN THE March and May 2015 Measure Measurement process / FIGURE 1 for Measure columns,1, 2 there was a discussion on measurement challenges Uncertainty analysis (traceable to SI units) and how to report measurements with a degree of confidence by reporting measurement uncertainty to support metrological traceability. Input 1 (unit A) Typical contributors: * Repeatability * Reproducibility * Sample variability * Resolution Traceability of unit A Estimating measurement uncertainty for calibration laboratories can still be a significant effort to support metrological traceability. However, most of the calibration laboratory parameters are common and traceable to the International System of Units measurements, Input 2 (unit B) Traceability of unit B Test/ calibration measurement process Single output result (unit D) commonly referred to as SI units, after the concepts for estimation are grasped. Not traceable to SI units For certain test and calibration disciplines, the documented unbroken chain may not be directly traceable to SI units. This may be because there is more than one input to the test method, Participation in a suitable interlaboratory comparison program Input n (unit C) Traceability of unit C and the output is a different unit from that of the inputs, which are expressed end result can be quantified by traditional in units not traceable to SI units. Figure 1 uncertainty estimation means (traceable illustrates this measurement process. to SI Units), the problem may not be as There are more combinations for test laboratories for estimating uncertainty If the traditional estimation methods than for calibration laboratories. There- cannot be deployed, the lab may need to fore, the test laboratories face other chal- participate in a proficiency test or an inter- lenges because they may not have all the laboratory comparison program to claim output test parameters that are directly the metrological traceability (for measure- traceable to an SI unit. ment not directly traceable to SI units). Take, for example, a pharmaceutical organization that mixes the recipe for a medication. It may mix the ingredients 44 QP * www.qualityprogress.com difficult. This requirement is identified in ISO/IEC 17025 Section 5.6.2.1.2.3 When there are many different units for the recipe by weight or volume. While of measurement, the uncertainty con- weight and volume are metrologically tributors may be expressed in a unit-less traceable, the end result may not be. The quantity, such a percentage or parts per end result may be a qualitative or quan- million. The output result is expressed in titative measure in a different unit. If the the same unit-less quantity. http://www.qualityprogress.com

Table of Contents for the Digital Edition of Quality Progress - July 2015

Up Front
LogOn
Expert Answers
Keeping Current
Mr. Pareto Head
Open to Change
Passion Project
Turning on the Light Bulb
What‘s Your Theory?
Measure for Measure
Quality in the First Person
Career Corner
Statistics Roundtable
Standards Outlook
ASQ's Continuing Education and Professional Development Directory
QP Toolbox
QP Reviews
One Good Idea
Back to Basics

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