Metrologia PAPER Recent citations SI traceability and scales for underpinning - Amount of substance and the mole in the SI atmospheric monitoring of greenhouse gases Bernd Güttler et al - Advances in reference materials and To cite this article: Paul J Brewer et al 2018 Metrologia 55 S174 measurement techniques for greenhouse gas atmospheric observations Paul J Brewer et al - News from the BIPM laboratories—2018 Patrizia Tavella et al View the article online for updates and enhancements. This content was downloaded from IP address 205.156.36.134 on 08/07/2019 at 18:02 IOP Metrologia Bureau International des Poids et Mesures Metrologia Metrologia Metrologia 55 (2018) S174–SS181 https://doi.org/10.1088/1681-7575/aad830 55 SI traceability and scales for underpinning 2018 atmospheric monitoring of greenhouse © 2018 BIPM & IOP Publishing Ltd gases MTRGAU Paul J Brewer1,6 , Richard J C Brown1 , Oksana A Tarasova2, Brad Hall3, George C Rhoderick4 and Robert I Wielgosz5 S174 1 National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, United Kingdom 2 World Meteorological Organization, 7bis, avenue de la Paix, Case postale 2300, CH-1211 Geneva 2, Switzerland 3 National Oceanic and Atmospheric Administration, 325 Broadway, Mail Stop R.GMD1, Boulder, CO 80305, United States of America P J Brewer et al 4 National Institute of Standards and Technology, 100 Bureau Drive, MS-8393 Gaithersburg, MD 20899-8393, United States of America 5 Printed in the UK Bureau International des Poids et Mesures, Pavillon de Breteuil, F-92312 Sèvres Cedex, France E-mail: [email protected] MET Received 21 June 2018, revised 2 August 2018 Accepted for publication 6 August 2018 10.1088/1681-7575/aad830 Published 7 September 2018 Abstract Paper Metrological traceability is the property of a measurement result whereby it can be related to a stated reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty. The stated reference can be the International System of Units, and 1681-7575 more specifically a realisation of a primary standard with its value and uncertainty expressed in SI units, which offers many benefits such as long term stability and the ability to reproduce the standard within its stated uncertainty at any time. Alternatively, measurements can be 5 made traceable to an empirical scale, an approach that has evolved for practical reasons to meet the needs of certain measurement communities. We explore the benefits and drawbacks of these two measurements systems as they have evolved for greenhouse gas measurements S174 at background levels and outline the advantages of using elements from each to secure a more robust measurement infrastructure. In particular, this work examines the importance S181 of compatibility, the accuracy of measurement results and the benefits of comparisons with independent primary standards. Keywords: metrology, traceability, calibration, SI, scale, atmospheric monitoring Introduction the measurement uncertainty. The reference in a measurement system, to which measurements are made traceable, can be The goal of implementing metrological traceability within a realised in different ways. Here we consider two approaches measurement system, is to ensure that measurements can be for a measurement system for greenhouse gas amount-of-sub- comparable and compatible anywhere at any time [1]. A met- stance fractions in the atmosphere. The first provides trace- rological traceability chain [1] must be established from the ability to the international system of units (SI) and the second (value of the) measurement result to the value embodied in, or to a defined scale. In particular, we consider the advantages carried by a material, or obtained by a measurement procedure and disadvantages of each approach and how the measure- [1]. Metrological traceability is not just a simple statement ment system may ultimately benefit from an approach that [2]. It must be based on an unbroken chain of quantity values combines elements of each. in a specified material or measurement, each contributing to There are some prominent examples of the use of scales within metrology, for example for temperature measurements 6 Author to whom any correspondence should be addressed. 1681-7575/18/05S174+8$33.00 S174 © 2018 BIPM & IOP Publishing Ltd Printed in the UK Metrologia 55 (2018) S174 P J Brewer et al [3]. The unit of the fundamental physical quantity known as statement will include all sources of uncertainty, up to and thermodynamic temperature (T), is the kelvin (K), defined as including the realisation of the SI units themselves. The the fraction 1/273.16 of T at the triple point of water. Triple- degree of equivalence of measurement standards can then be point-of-water cells provide a convenient realisation of this calculated by the difference in their values, to the extent to definition. For temperatures other than the triple point of which this difference is smaller than the combined uncertain- water, direct measurements of thermodynamic temperature ties of the values of the standards. require a primary thermometer based on a well-understood In the field of gas analysis, reference materials are usu- physical system whose temperature may be derived from ally disseminated in high pressure cylinders using gravimetric measurements of other quantities. In practice, primary ther- methods to assign the reference value. There is a broad con- mometry is difficult and time consuming and not a practical sensus amongst metrologists that ‘verification’ is required to means of disseminating K. As an alternative, the International ensure the ‘primary method’ is behaving as expected [7]. The Temperature Scale provides an internationally accepted recipe implication is that within gas metrology, amount-of-substance for realising temperature in a practical way. The present fraction values can be realised and their uncertainties calcu- scale is the International Temperature Scale of 1990 (ITS- lated with traceability to the mole. This is achieved via mass 90) (Recommendation 5, CI-1989). The values assigned to and purity measurements, tables of atomic weights and the the fixed-point temperatures used in ITS-90 and pre-ITS-90 molar mass constant Mu. These values should then be verified scales were based on the state-of-the-art knowledge of T at by comparison against an independently prepared gas mixture the time of inception of each scale. Estimates of the differ- of similar composition. This process is intended to demon- ences between T and the ITS-90 (T90) are available with strate that errors in preparation, loss of material to cylinder consensus estimates provided for T − T90, for selected meas- walls, and reaction of gaseous components within the mixture, urements from 4.2 K to 1358 K, as well as a recommendation which would compromise the ‘complete understanding’ of the for analytic approximations to T − T90 for the range 0.65 K to method are well understood. It is agreed that this verification 1358 K. The values of T − T90 are small, and would be neg- step provides a good demonstration that the value and uncer- ligible for the vast majority of users. Considerable research tainty have been properly assigned, based on a gravimetric activities have gone into understanding the factors that can value fully traceable to the SI [8]. The general methods for lead to variations in the fixed points (e.g. impurities and iso- the realisation of gas standards with assigned amount-of- topic composition), so that the scales can be realised inde- substance fractions is described within the written standard pendently in National Metrology Institutes (NMIs) around ISO 6142-1:2015 [8]. This methodology has been success- the world achieving comparable results. As will be explained fully used by NMIs to develop standards for greenhouse gases later, this differs from the scale approach developed for green- for different measurement communities from emission levels house gases, where the scale is realised by one unique set of down to background atmospheric levels and below. Standards standards, held by one institute. produced by such methods can routinely be offered with rela- tive standard uncertainties of 0.1%, but for special applica- tions and over particular amount-of-substance fraction ranges Discussion these can be reduced to 0.025% or below. Traceability to the SI for the amount-of-substance fraction of greenhouse gases in air The scale approach The role of a NMI is to provide national standards for mea- An alternative approach is a measurement system based on surement and in many cases manage the national measure- traceability to a ‘scale’, often a family or collection of gas ment system comprising calibration laboratories, certification standards, based on an agreed reference value or reference and accreditation bodies, and legal metrology [4]. It is also method. This system offers superior precision as compariso ns to carry out the necessary comparisons with other NMIs, are made to one source and the absolute accuracy of the now through the International Committee for Weights and artefact has limited impact on measurement compatibility, Measures’ Mutual Recognition Arrangement (CIPM-MRA) provided that all measurements are traceable to the same ref- [5], to demonstrate international equivalence of the national erence, and that reference is stable. Within the greenhouse standards. In most cases this is based on their traceability to the gas field (particularly at the global scale), this practice arose SI. This system was adopted at the 10th General Conference partially out of the realisation that gas amount-of-substance on Weights and Measures (CGPM) on which to establish a fraction measurements could be performed with precisions practical system of measurement for international use [6]. that were often better than the uncertainties associated with Today the SI is the fixed reference point for almost all modern gas mixture preparation. In addition, the key quantity to be science and technology. The base units are regarded as dimen- measured was the relative difference in amount-of-substance sionally independent and form a coherent set of derived units.