2019

MEASURE FOR MEASURE

In No Uncertain Terms

Find the meaning behind key words to get most out of guide

by Dilip Shah

In December 2007, ISO/IEC Guide 99:2007—International vocabulary of metrology—Basic and general concepts and associated terms (VIM) was released. It replaces what is known in the metrology community as the second edition of the VIM, making it the equivalent to the third edition.

ISO/IEC Guide 99:2007 was developed by a joint committee that was comprised of representatives of: the International Bureau of Weights and Measures, the International Engineering Consortium, the International Organization for Standardization (ISO), the International Federation of Clinical Chemistry and Laboratory Medicine, the International Union of Pure and Applied Chemistry, the International Union of Pure and Applied Physics, the International Organization of Legal Metrology and the International Laboratory Accreditation Cooperation (ILAC).

It is worthwhile for those involved in the test and calibration business to obtain a copy of this guide.

Defining harmony

Because the guide harmonizes terms for many different industries, it is important to take a closer look at all the terms. In this column, I will examine measurement uncertainty and metrological (measurement) traceability. The notes after the definitions clarify several measurement scenarios for the user and provide guidance:1

  1. Measurement uncertainty, uncertainty of measurement, uncertainty: Non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used.

    Note 1: Measurement uncertainty includes components arising from systematic effects, such as components associated with corrections and the assigned quantity values of measurement standards, as well as the definitional uncertainty. Sometimes, estimated systematic effects are not corrected for. Instead, associated measurement uncertainty components are incorporated.

    Note 2: The parameter may be, for example, a standard deviation called standard measurement uncertainty (or a specified multiple of it), or the half-width of an interval, having a stated coverage probability.

    Note 3: Measurement uncertainty comprises, in general, many components. Some of these may be evaluated by type-A evaluation of measurement uncertainty from the statistical distribution of the quantity values from a series of measurements and can be characterized by standard deviations. The other components, which may be evaluated by type-B evaluation of measurement uncertainty, can also be characterized by standard deviations and evaluated from probability density functions based on experience or other information.

    Note 4: In general, for a given set of information, it is understood the measurement uncertainty is associated with a stated quantity value attributed to the measurand. A modification of this value results in a modification of the associated uncertainty.

  2. Metrological traceability: Property of a measurement result whereby the result can be related to a reference through a documented, unbroken chain of calibrations, each contributing to the measurement uncertainty.

    Note 1: For this definition, a "reference" can be a definition of a measurement unit through its practical realization, or a measurement procedure that includes the measurement unit for a nonordinal quantity or a measurement standard.

    Note 2: Metrological traceability requires an established calibration hierarchy.

    Note 3: Specification of the reference must include the time at which the reference was used in establishing the calibration hierarchy, along with any other relevant metrological information about the reference, such as when the first calibration in the calibration hierarchy was performed.

    Note 4: For measurements with more than one input quantity in the measurement model, each of the input quantity values should be metrologically traceable, and the calibration hierarchy involved may form a branched structure or a network. The effort involved in establishing metrological traceability for each input quantity value should be commensurate with its relative contribution to the measurement result.

    Note 5: Metrological traceability of a measurement result does not ensure the measurement uncertainty is adequate for a given purpose or that there is an absence of mistakes.

    Note 6: A comparison between two measurement standards may be viewed as a calibration if the comparison is used to check and, if necessary, correct the quantity value and measurement uncertainty attributed to one of the measurement standards.

    Note 7: ILAC considers the elements for confirming metrological traceability to be an unbroken metrological traceability chain to an international measurement standard or a national measurement standard, a documented measurement uncertainty, a documented measurement procedure, an accredited technical competence, metrological traceability to the International System of Units and calibration intervals.2

    Note 8: The abbreviated term "traceability" is sometimes used to mean metrological traceability, as well as other concepts, such as sample traceability, document traceability, instrument traceability or material traceability, where the history (trace) of an item is meant. Therefore, the full term, metrological traceability, is preferred if there is any risk of confusion.

Perfect pair

These two definitions are important because of how intertwined they are. Without documented measurement uncertainty for a measurement parameter, we do not have metrological traceability. To estimate measurement uncertainty, documented calibration and measurement uncertainty data is required from the laboratories that calibrated the equipment to fulfill the "unbroken chain of calibrations" requirement.

Laboratories claiming metrological traceability must have documented, unbroken chains of calibrations, each contributing to the measurement uncertainty. If calibration or test laboratories have doubts about having documented measurement uncertainty budgets, these definitions remove those doubts. For verifying metrological (measurement) traceability, you must have measurement uncertainty budgets.

We can illustrate these two definitions graphically in Figure 1 and Table 1. In Figure 1, each successive level of metrological hierarchy’s combined measurement uncertainty includes the previous level measurement uncertainty. In Table 1, the measurement uncertainties are combined using the root sum square method, as outlined in the guide to uncertainty of measurement.3

Table 1

Figure 1

Laboratories accredited to ISO 170254 must have documented measurement uncertainty budgets for each parameter under their scope of accreditation. Because ISO 17025-accredited laboratories are assessed by third-party accrediting bodies, their claims of metrological traceability are thoroughly verified and validated.

Unaccredited laboratories need to have measurement uncertainty budgets, along with documentation available to prove claims of metrological traceability, along with measurement uncertainty budgets. Traditionally, estimating and calculating measurement uncertainty is one of the more difficult tasks for laboratories preparing for ISO 17025 accreditation.

Many tools and techniques exist for estimating measurement uncertainty. A future column will outline a generic process to estimate, calculate and develop a measurement uncertainty budget.


References

  1. ISO/IEC Guide 99:2007—International vocabulary of metrology—Basic and general concepts and associated terms, International Organization for Standardization, 2007.
  2. ILAC P10:2002 ILAC Policy on Traceability of Measurement Results, International Laboratory Accreditation Cooperation, www.ilac.org/documents/ILAC_P10_2002_ILAC_Policy_on_Traceability_of_
    Measurement_Result.pdf (case sensitive).
  3. ANSI/NCSL Z540.2-1997 U.S. Guide to Expression of Uncertainty in Measurement, American National Standards Institute and National Conference of Standards Laboratories, 2002.
  4. ISO/IEC 17025:2005—General requirements for the competence of testing and calibration laboratories, International Organization for Standardization, 2005.

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