Constant evolution of the International System of Units(SI)

The theme chosen forWorld Metrology Day 2018 is Constant evolution of the International Systemof Units (SI). This evolution is a culmination ofmany years of work by a large number of dedicated metrologists to determine thebest method of redefining several of the base SI units. This revision will notdirectly impact legal metrology, since users will be able to obtaintraceability to the revised SI from the same sources used at present. However, itdoes mean that there will be a change in the way we define certain units ofmeasurement and, in some cases, how traceability may ultimately be established.

The revised SI will beentirely 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 thesecond (1967/68) and the metre (1983) were redefined from being based on theearth’s movement and size to being based on atomic and electromagnetic constants.

The significance inthis case is that certain concepts that were taught to most of us at school andwhich were up to now almost carved in stone, may now change. Theplatinum-iridium (Pt-Ir) prototype that is kept under three locks in a vault nearParis, will go into partial retirement after 137 years of service.

Certainly this willmark the end of an era. The SI started out as the MKS system with human sizedstandards 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 acertain amount of water (later agreed on as a specific amount of Pt-Ir). Bettermeasurements proved over time that those invariants were not in fact asinvariant as previously thought; that fact, together with the technologicaladvances applied to measurements that have gradually allowed for much moreaccurate realization of the units, are the main reasons for the change. Now thelast of those original standards will be replaced by a definition based on a fundamentalconstant of nature.

Although ‘Le Grand K’is the most famous of the SI artifacts, changes will also happen in otherunits. The kelvin will no longer depend on a property of water, the ampere willnot be based on a definition that is very difficult to realize, and the molewill change to a more practical definition. In addition, the reviseddefinitions of the kilogram, ampere, kelvin and mole will have no impact on thedefinitions of the second, the metre and the candela.

As stated earlier, wedo not expect there to be an impact on legal metrology, but it is a significantchange in the thinking and the methods of all of us who have been working with theseunits for many years.

We invite you to takea few minutes to review the many documents available on the BIPM websiterelated to this topic. We also hope you enjoy celebrating World Metrology Daywith us again this year and look forward to once again marking the importancethat metrology plays in the activities of our daily life.

Using the rules of nature to createthe rules of measurement

The International System of Units (SI) is the accepted set of units forall applications of measurement worldwide. Since it was first given the name SInearly 60 years ago, improvements have been agreed to it whenever it has beenpossible to exploit advances in measurement technologies to address newrequirements.

In November 2018, the General Conference on Weights and Measures isexpected to agree one of the most significant changes to the SI which will baseit on a set of definitions each linked to the laws of physics. This historicchange towards using the laws of nature in the definitions will eliminate thefinal link between the SI and definitions based on physical artefacts.Following the revisions, the kilogram will be linked to the exact value of thePlanck constant rather than the International Prototype of the Kilogram, assanctioned 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 basisfor worldwide measurements. The definitions expected to beagreed in November will be a further step towards this goal. They arebased on the results of research into new measurement methods that have usedquantum phenomena as the basis for standards that are fundamental. Greatattention has been paid to ensure that these new definitions will be compatiblewith the current ones at the time the change is implemented. The changes willbe unnoticeable to all but the most demanding users.

Whilst providing the necessary level ofcontinuity for existing users, the changes have the advantage of being able toembrace future improvements in measurement methods to meet the needs of futureusers because they are based firmly on the laws of physics. The new definitionswill use ‘the rules of nature to create the rules of measurement’ linkingmeasurements at the atomic and quantum scales to those at the macroscopiclevel.

As science and technology progress, thedemands for measurements to underpin new products and services will increase. Metrologyis a dynamic branch of science and the steps taken by the BIPM and the widermetrology community to advance the SI in 2018 will underpin these requirementsand meet these needs for many years to come.

这是2018年世界计量日(5月20日）主题，我看不懂英文，看图只知道与国际单位制有关。听量友说是讲国际单位制优化历程。