Practical Interpretation of Metrological Traceability on Gauge Block as to VIM 3

Transcription

1 MAPAN - Journal Practical of Metrology Interpretation Society of Metrological India, Vol. 26, Traceability No. 1, 2011; on pp. Gauge 79-86Block as to VIM 3 ORIGINAL ARTICLE Practical Interpretation of Metrological Traceability on Gauge Block as to VIM 3 LUNG-HEN CHOW*, YI-TING CHEN, LIANG-HSING CHEN and GWO-SHENG PENG Center for Measurement Standards (CMS), Industrial Technology Research Institute (ITRI) Bldg. 16, 321, Sec. 2, Kuang Fu Road, Hsinchu City, 30011, Taiwan (ROC) * LHenryChow@itri.org.tw [Received: ; Revised: ; Accepted: ] Abstract In ISO/IEC Guide 99:2007, i.e. the International Vocabulary of Basic and General Terms in Metrology- 3rd edition (VIM 3), the term "traceability" is replaced by "metrological traceability", giving it a new definition as property of a measurement result which can be related to a reference. In essence, "metrological traceability" can offer an evidence of measurands tracing to the primary standards which can realize the SI units, and offer a documented unbroken chain of calibrations, thus considered as one of the most important terms in VIM 3. National Measurement Laboratory (NML, Chinese Taipei) has long operated its main mission of calibration implemented along with peer assessed traceability of its measurement systems, which demonstrate a calibration hierarchy conventionally in schematic approach. In dealing with definition of the new term "metrological traceability" in VIM 3, this paper elaborates in taking additionally a newly mathematical approach rather than schematic approach only to realize the practical interpretation of "metrological traceability" to show how the unbroken calibration chain is functioning seamless and robust on the gauge block measurement system in NML. Through such study activities, we well assure our strong confidence on technology inheritance of gauge block and the other measurement systems with sufficient metrological know-how in NML, which can continually pass to each entry level metrologist. 1. Indroduction Towards increasing demands of metrological standards with associated metrological traceability, measurement uncertainty and nominal properties in emerging industrial and societal applications, such as material metrology and biological metrology, the International Organization for Standardization and the International Electrotechnical Commission publishing ISO/IEC Guide 99:2007 [1] cancels and replaces the second edition of the VIM and it is equivalent to the third edition of the VIM (VIM 3). In VIM 3, the term "traceability" is replaced by "metrological traceability", giving it a new definition as property of a measurement result which can be related to a reference. In essence, "metrological traceability" can offer an evidence of measurands tracing to the primary standards which can realize the SI units, and offer a documented unbroken chain of calibrations, thus considered as one of the most important terms in VIM 3. International Laboratory Accreditation Cooperation (ILAC) also proposed six important elements to confirm the definition of "metrological traceability" [2]. Metrology Society of India, All rights reserved

2 Lung-Hen Chow, Yi-Ting Chen, Liang-Hsing Chen and Gwo-Sheng Peng National Measurement Laboratory (NML, Chinese Taipei) has long operated its main mission of calibration implemented along with peer assessed traceability of its measurement systems, which demonstrate a calibration (traceability) hierarchy conventionally in schematic approach. In dealing with definition of the new term "metrological traceability" in VIM 3, a newly mathematical approach is taken in addition to the schematic approach to realize practical interpretation of "metrological traceability" to review how the unbroken calibration chain can be functioning seamless and robust on different measurement systems in NML. Gauge block measurement system is typically demonstrated for such purpose of metrology study activity at NML during the year Definition of the Metrological Traceability in VIM 3 Metrological traceability is defined in 2.41 of VIM 3 as "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", within which measurement result, calibrations and measurement uncertainty are also clearly defined in VIM 3. While "a documented unbroken chain of calibrations" is not formally defined, it needs to address more, which is clearly illustrated in Fig. 1, as a graphical interpretation of metrological traceability. For the definition shown in Fig. 1, metrological traceability requires an established sequence of calibrations, i.e. calibration hierarchy, starting from a reference to the final measuring system, with well documentation applying to calibration procedure and measurement uncertainty, and finally obtains a measurement result from the measuring system. A measurement result is generally expressed as a single measured quantity value and a measurement uncertainty. As to such definition, a 'reference' can be a measurement standard, or a measurement procedure, or a definition of a measurement unit through its practical realization. Fig.1. A graphical interpretation of metrological traceability as to VIM 3 80

3 Practical Interpretation of Metrological Traceability on Gauge Block as to VIM 3 It is also noted that the specification of the reference must include the time at which this 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. Nevertheless, metrological traceability should be the property of a measurement result, instead of the traceability of a measuring system previously misunderstood at NML. Similar to aforementioned definitions, ILAC proposes six important elements to confirm the definition of metrological traceability [2] as; i) an unbroken metrological traceability chain to an international measurement standard or a national measurement standard, ii) a documented measurement uncertainty, iii) a documented measurement procedure, iv) accredited technical competence, v) reference to the SI units, such that the traceability chain must, where possible, end at primary standards for the realization of the SI units and vi) calibration intervals at which calibrations must be repeated depending on such factors of uncertainty required, frequency of use, way of use, stability of the equipment etc. Thus, a measurement well equipped with above six key elements will suffice to give a complete interpretation of metrological traceability in order to relate a measurement result to a reference. 3. Conventional Metrological Traceability Diagram NML is a renowned national metrology Institute (NMI) in Taiwan and a registered calibration laboratory accredited by Taiwan Accreditation Foundation (TAF) which performs its assigned calibration work in compliance with ISO 17025:2005 [3]. Since the chapter 5.6 Measurement traceability of [3] gives requirement for calibration laboratory complying traceability, in Calibration-that mentions "For calibration laboratories, the program for calibration of equipment shall be designed and operated so as to ensure that calibrations and measurements made by the laboratory are traceable to the SI units", NML then designs approach to sketch metrological traceability diagram for their measuring systems in order to enable the concept of measurement realized through a complete metrological traceability chain. Fig. 2. Hardness traceability chain 81

4 Lung-Hen Chow, Yi-Ting Chen, Liang-Hsing Chen and Gwo-Sheng Peng A lately disclosed diagram of traceability chain for hardness measurement traceability in industry [4], shown in Fig. 2 can be referred to investigate how the six key elements of metrological traceability are well presented in such hardness traceability chain, where there are some issues that are worthy of discussion and to be clarified; i) Reliable hardness values, hardness testing machines and hardness calibration machines are mixed placing in the same line of traceability, which we can not make sure whether the measured target is "hardness values" or "machines". The definition tells us: metrological traceability is the property of measurement result that is interested in the quantity intended to be measured, and here shouldn't be "machines; ii) In traceability chain, no explanation is described or illustrated to give appropriate evidence of "an unbroken metrological traceability chain"; iii) No documentation is mentioned for implementing measurement procedure, claimed measurement uncertainty, and final measurement result recorded in each stage of traceability chain; iv) As capable laboratories or institutes for operating "Direct calibration", no direct evidence or documentation is shown or mentioned in traceability chain to demonstrate their accredited technical competence. 3.1 Conventional Approach of NML's Metrological Traceability Diagram During the development of each measurement system for a specific measured quantity of calibration at NML, there will produce two documents; one is Instrument Calibration Technique (ICT) used as a documented calibration procedure, the other Measurement System Validation Procedure (MSVP) used as a documented calibration system evaluation report, within which the claimed uncertainty is recorded. It requires of MSVP that a newly developed or modified measuring system draw a measurement system traceability diagram at NML as part of a documented traceability. Figure 3(a) illustrates a measurement traceability diagram for NML's gauge block comparator, and for small mass measurement system shown in Fig. 3(b). Conventional metrological traceability diagram requested by MSVP at NML is initiated based on the viewpoint of measurement system since the major purpose of MSVP is designed to validate the measurement system and claim its evaluated uncertainty. Thus, a simple and conceptual traceability illustration in Fig. 3(a), or even a little more detailed traceability explanation in Fig. 3(b) cannot fully cover the aforementioned six key elements of metrological traceability. Such conventional approach of metrological traceability diagram is obsolete and cannot comply with the new definition of metrological traceability. 4. New Metrological Traceability Diagram in Mathematical Approach In order to really focus the property of measurement result as metrological traceability is defined in VIM 3, we shall intend to measure only the output quantity Y in the measurement model or equation h(y, X 1,, X n ) = 0, i.e. the measurand, the quantity value of which is to be inferred from information about input quantities in the measurement model X 1,, X n, instead of the consideration of all the affection factor (e.g. temperature, coefficient of thermal expansion etc.) during the whole measurement process. After a close discussion in the metrology study group at NML the beginning of year 2010, we propose another new way of mathematical approach combined with conventionally schematic approach to draw a new metrological traceability diagram for practical interpretation of metrological traceability in compliance with VIM 3. Measurement result is actually measurement result of measurand, in which measured quantity will be expressed usually as a number and a unit (reference), and that is a mathematical representation in itself. Thus we propose to insert "mathematical measurement equation" into conventional traceability diagram to reinforce "unbroken chain of calibrations" at each node of connection between sequence of calibrations as quantitatively and clear documented evidence, where measurement (calibration) system changes as the process. In such approach, the measurand associated with measurement result may 82

5 Practical Interpretation of Metrological Traceability on Gauge Block as to VIM 3 Fig.3(a).Gauge block comparator measurement traceability diagram return back to play obviously a main role of metrological traceability. Gauge block comparator measurement system then is taken as a typical case in this study to demonstrate how this new approach and metrological traceability diagrams (shown in Figs.4-6) practically comply with the new definition of metrological traceability. Figure 4 shows gauge block metrological traceability diagram in mathematical approach, where measurement equations are placed in the right half of figure. In Eq. (1), L x = L r + d 1 (1) where L x at the left of equality, the measurement result of calibrated gauge block is unknown value, L r, the measurement result of standard gauge block and d 1, the measured difference from gauge block comparator at the right of equality being known values. Similarly, in Eq. (2), L r =L N + d 2 (2) where L r at the left of equality is unknown value, L N, the nominal value of standard gauge block and d 2, the measured deviation from gauge block interferometer Fig. 3(b). Small mass measurement system traceability diagram at the right of equality being known values of input quantities. Then again, d 2 becomes an unknown measurand, obtained from and at the left of equality in Eq. (3), d 2 = λ(ε-ξ)/2 (3) where ε is the interference stripe number from the measurement, ξ, the interference stripe number from calculation and the laser vacuum wavelength measurand λ, finally trace and relate to Mise en Pratique, (MeP) of the metre, ƒ r, the frequency value of standard laser (iodine stabilized He-Ne laser) and Δƒ, the measured frequency deviation of beat frequency measurement from Eq. (4), λ = c 0 / (n x ƒ ) and ƒ = ƒ r + Δƒ (4) where f is the frequency value of calibrated laser, n is the refractive index, and c 0 is the velocity of light in vacuum. In mathematical approach and the expression of measurement equations, Fig. 4 has clearly demonstrated a quantitatively unbroken chain of metrological traceability diagram, where the measurement result of each calibration step depends 83

6 Lung-Hen Chow, Yi-Ting Chen, Liang-Hsing Chen and Gwo-Sheng Peng on and traced to the measurand and measurement result of the previous step. The measured quantities to be traceable, from the right of equality in this equation to the left of equality in the previous measurement equation are tail-to-head interconnected in green line, finally connected to the practical realization of a measurement unit for checking whether the element of an unbroken chain is achieved both graphically and in mathematical approach. Figure 6 shows complementary illustration onto gauge block metrological traceability in documentation for other important traceability elements, including documented expanded uncertainties associated with documented ICT and MSVP in each traceability step at NML. Figure 5 illustrates gauge block system's auxiliary measurement parameter traceability diagram at NML, where the laboratories operating calibrations of relevant auxiliary parameters are all inner laboratories of NML and accredited by TAF, and that means all such inner laboratories' technical competence accredited too, with calibration intervals indicate on their calibration certificates issued by them. In summary, NML's gauge block metrological traceability in new approach combined with different diagrams complementary each other shown in Figs. 4-6, fully explicitly presents six important elements of metrological traceability. 5. Further Discussion on Measurement Equation for the Interpretation Comparing with previous viewpoint on the evaluation of measurement uncertainty towards measuring system and from NML's gauge block calibration system evaluation report or MSVP, measurement equation in Fig. 4 was originally expressed in a more complicated form; d = L(1 + αθ) L (1 + α θ ) (5) 1 s s s where L is the dimension of calibrated gauge block at 20ºC, α is thermal expansion coefficient of calibrated gauge block, θ is temperature difference of calibrated gauge block with respect to that 20ºC and L s,α s and θ s are similar ones symbolized for standard gauge block. Fig. 4. Gauge block metrological traceability diagram (in mathematical approach) 84

7 Practical Interpretation of Metrological Traceability on Gauge Block as to VIM 3 Fig. 5. Gauge block system's auxiliary measurement parameter traceability diagram- Fig.6. Gauge block metrological traceability diagram (in documentation) 85

8 Lung-Hen Chow, Yi-Ting Chen, Liang-Hsing Chen and Gwo-Sheng Peng Equation (5) is then deduced to obtain as; Ls(1 + αsθs) + d1 L= = Ls + d1 + L s( αθ s s αθ) + (1 + αθ ) (6). In Eq. (5), it is of much confusion in finding which parameter is the measurand or output quantity intended to be measured, and which ones are input quantities. In Eq. (6), the measurand L though placed correctly in the left-hand side of equation, the adding complicated affection factors, α and θ seem inappropriately to become among many input quantities, such that it is so difficult to trace the outcome of each calibration to the outcome of the previous calibration in a calibration hierarchy. Thus, a simple and correct mathematical equation to represent a measurement model such as Eqs. (1-4) indicated in Fig. 4 of the gauge block traceability will play an important role as giving complementary merits that conventional metrological traceability diagram cannot address about for practical interpretation and checking of an "unbroken" metrological traceability chain. 6 Conclusion Through the study activities for drawing new metrological traceability diagrams in mathematical approach that new edition of VIM 3 requires, we will well assure ourselves on technology inheritance of gauge block and the other measurement systems with sufficient metrological know-how in NML, which can continually pass to each entry level metrologist. Besides, since "reference to the SI units" is one of metrological traceability elements, the degree of complexity for derived quantity such as fluid flow would be much more significant than that for base quantity. We will keep elaborating to further and deepen the concept of unbroken chain in metrological traceability to every measurement system of any kind quantities operating at NML. References [1] ISO/IEC GUIDE 99:(E/F), International Vocabulary of Metrology - Basic and General Concepts and Associated Terms (VIM), (2007). [2] ILAC P-10, ILAC Policy on Traceability of Measurement Results, (2002). [3] ISO 17025, General Requirements for the Competence of Testing and Calibration Laboratories, (2005). [4] Alessandro Germak, Konrad Herrmann and Samuel Low, Traceability in Hardness Measurements: from the Definition to Industry, Metrologia, 47 (2010) S59-S66. 86