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Messages from the
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Stephen Patoray |
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Martin Milton |
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Constant evolution of the International System of Units (SI) The theme chosen for World Metrology Day 2018 is Constant evolution of the International System of Units (SI). This evolution is a culmination of many years of work by a large number of dedicated metrologists to determine the best method of redefining several of the base SI units. This revision will not directly impact legal metrology, since users will be able to obtain traceability to the revised SI from the same sources used at present. However, it does mean that there will be a change in the way we define certain units of measurement and, in some cases, how traceability may ultimately be established. The revised SI will be entirely based on constants of nature. While this may seem to be a big change, it has in fact already happened several times in the recent past, when both the second (1967/68) and the metre (1983) were redefined from being based on the earth’s movement and size to being based on atomic and electromagnetic constants. The significance in this case is that certain concepts that were taught to most of us at school and which were up to now almost carved in stone, may now change. The platinum-iridium (Pt-Ir) prototype that is kept under three locks in a vault near Paris, will go into partial retirement after 137 years of service. Certainly this will mark the end of an era. The SI started out as the MKS system with human sized standards based on what at that time were considered to be invariants of nature: the rotation of the earth, its size, and a weight originally derived from a certain amount of water (later agreed on as a specific amount of Pt-Ir). Better measurements proved over time that those invariants were not in fact as invariant as previously thought; that fact, together with the technological advances applied to measurements that have gradually allowed for much more accurate realization of the units, are the main reasons for the change. Now the last of those original standards will be replaced by a definition based on a fundamental constant of nature. Although 'Le Grand K' is the most famous of the SI artifacts, changes will also happen in other units. The kelvin will no longer depend on a property of water, the ampere will not be based on a definition that is very difficult to realize, and the mole will change to a more practical definition. In addition, the revised definitions of the kilogram, ampere, kelvin and mole will have no impact on the definitions of the second, the metre and the candela. As stated earlier, we do not expect there to be an impact on legal metrology, but it is a significant change in the thinking and the methods of all of us who have been working with these units for many years. We invite you to take a few minutes to review the many documents available on the BIPM website related to this topic. We also hope you enjoy celebrating World Metrology Day with us again this year and look forward to once again marking the importance that metrology plays in the activities of our daily life
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Using the rules of nature to create the rules of measurement The International System of Units (SI) is the accepted set of units for all applications of measurement worldwide. Since it was first given the name SI nearly 60 years ago, improvements have been agreed to it whenever it has been possible to exploit advances in measurement technologies to address new requirements. In November 2018, the General Conference on Weights and Measures is expected to agree one of the most significant changes to the SI which will base it on a set of definitions each linked to the laws of physics. This historic change towards using the laws of nature in the definitions will eliminate the final link between the SI and definitions based on physical artefacts. Following the revisions, the kilogram will be linked to the exact value of the Planck constant rather than the International Prototype of the Kilogram, as sanctioned by the 1st CGPM in 1889. For over 200 years, a collective ambition for the “metric system” has been to provide universality of access to the agreed basis for worldwide measurements. The definitions expected to be agreed in November will be a further step towards this goal. They are based on the results of research into new measurement methods that have used quantum phenomena as the basis for standards that are fundamental. Great attention has been paid to ensure that these new definitions will be compatible with the current ones at the time the change is implemented. The changes will be unnoticeable to all but the most demanding users. Whilst providing the necessary level of continuity for existing users, the changes have the advantage of being able to embrace future improvements in measurement methods to meet the needs of future users because they are based firmly on the laws of physics. The new definitions will use ‘the rules of nature to create the rules of measurement’ linking measurements at the atomic and quantum scales to those at the macroscopic level. As science and technology progress, the demands for measurements to underpin new products and services will increase. Metrology is a dynamic branch of science and the steps taken by the BIPM and the wider metrology community to advance the SI in 2018 will underpin these requirements and meet these needs for many years to come.
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