Out with the Old, in with the New: The Revised SI for the kilogram

Aletta Karsten, Thapelo Mametja and Henk Potgieter [email protected] Outline

• Background (SI and Revised SI) • What is SI units? • Why change? • How? • When? • South African perspective • Si sphere • Kibble Balance What is a SI unit?

• The SI is defined by the SI Brochure, which is published by the BIPM. • The recommended practical system of units of measurement is the International System of Units (Système International d'Unités, with international abbreviation SI). • This SI consists of a set of base units, prefixes and derived units. • The SI is not static but evolves to match the world's increasingly demanding requirements for measurement. History of SI • First milestone • Creation of the decimal metric system at the time of the French Revolution • Deposition of two platinum standards representing the metre and the kilogram, on 22 June 1799, in the Archives de la République in Paris • Signing of the Metre Convention on 20 May 1875 • Created the BIPM and established the CGPM and the CIPM • Work began on the construction of new international prototypes of the metre and kilogram. • In 1889 the 1st CGPM sanctioned the international prototypes for the metre and the kilogram. • Together with the astronomical second as the unit of time, these units constituted a three-dimensional mechanical unit system similar to the CGS system, but with the base units metre, kilogram, and second, the MKS system. Changes in measurement standards (not new) Mass Redefinition of SI

• Link to fundamental constants of nature • More accurate over time

1889

https://www.bipm.org/en/measurement-units/rev-si/ Which units will be redefined? Why?

• The revision of the SI will ensure that the SI continues to meet the needs of science, technology, and commerce in the 21st century. • Of the seven base units of the SI, only the kilogram is still defined in terms of a material artefact (IPK), kept at the BIPM. • The major disadvantage of the present definition of the kilogram is that it refers to the mass of the artefact which, by its very nature, we know cannot be absolutely stable. Mass – how is it defined? • IPK • Standard since 1889 • Cylinder height and diameter (about 39 mm) • Alloy (90% platinum, 10% iridium) • Kept at the International Bureau of Weights and Measures (BIPM) in Paris • Absolute value is 1 kg (no uncertainty) • All mass measured under SI traceable to IPK

• SA – copy 56 of IPK Mass: Problems with current method

• Change of 50 µg in 100 years • Change in IPK or copies? • Mass will be defined in terms of Planck's constant (h) Why use h?

• h is the fundamental constant of quantum physics • Its SI value is used in many diverse fields of modern science and technology. • Changes to the recommended value of h as experiments improve (as per CODATA updates) are at best annoying and at worst confusing. • The rationale for defining the numerical value of h is similar to that for defining c, but has the specific advantages in electrical metrology given. The Metrology process (does not happen overnight) • Fixed to constant of nature (1991) – Planck's constant (h) • Conditions to be met: • At least 3 independent methods, Standard uncertainty ≤ 5 x 10-8 • At least one result accuracy of 2 x 10-8 (20 µg for 1 kg) • Results from experiments at 95% confidence level • Methods • Kibble (Watt) balance or silicon sphere

I

• Both methods used to determine h (final data were submitted mid 2017 for fixing of h in 2018) The kilogram problem • How do we replace the IPK? • It’s difficult • Option 1 • A recipe for creating artefacts of known mass • Build a sphere out of pure 28Si and then measure its size and hence work out the number of atoms in the sphere. • Called the Avogadro Project • Option 2 • A way of measuring mass in terms of something other than other masses. • Measure gravitational force in terms electrical forces. • Called a Watt Balance, now called a Kibble balance. Si sphere method (Avogadro project)

Possible issues: • Pureness • Roundness/ diameter • Surface layer Avogadro project

NMISA’s contribution: • Monitor mass of sphere • Monitor oxide layer on sphere surface • Calibrate NMISA sphere as density standard Kibble balance • First proposed by BP Kibble in 1975 at NPL • Consist of two measurements/ experiments

Test mass 푚

Fm =mg 푉 B I Fel = BLI 푣

푉 = BL 푣 Weighing experiment Moving experiment mg = IBL U = BLv BL difficult to determine accurately UI = mgv Link between 풉 and 풎 푼푰 풎 = 품풗 풎품풗 = 푼푰 • 푈 (use Josephson effect: U = nhf/2e)

푼′ 푼′ • 푰 = (ohm’s law), 풎품풗 = 푼 푹 푹 • Measure volt drop 푈′ = n’hf ‘/2e across a stable resistor R • R (use Quantum hall effect: 푅= h/αe2)

풉풇풇′ 푛푛′ 풎품풗 = ( ) ퟒ α ퟒ품풗풎 α 풉풇풇′ 푛푛′ 풉 = ( ) 풎 = ( ) 풇풇′ 푛푛′ Before After ퟒ품풗 α redefinition redefinition Current results for Planck’s constant NMISA project

• Two parts • Co-operation with NPL on table top model • Develop a system based on equal arm balance to test concepts • Modular system Components

• Mechanical structure • Electrical measurements • Stable resistor • Quantum Hall • Programable Josephson array • Gravity – how much is the earth attracting the mass piece • Magnetism (coil and magnets) • Alignment (laser system) • Speed measurements (laser system) • Vacuum system • Software control system Some international systems Implementation date

• In October 2017 at its 106th meeting, the CIPM noted that the agreed conditions for the redefinition are now met and decided to submit draft Resolution A to the 26th meeting of the General Conference on Weights and Measures (CGPM) and to undertake all other necessary steps to proceed with the planned redefinition of the kilogram, ampere, kelvin and mole. • The 26th meeting of the CGPM will take place the 13-16 November 2018 in Versailles. • Implementation date: World Metrology Day, 20 May 2019 Changes in realisation

• Current: • The unit of mass, the kilogram, is the mass of the international prototype of the kilogram kept in air under three bell jars at the BIPM. It is a cylinder made of an alloy for which the mass fraction of platinum is 90 % and the mass fraction of iridium is 10 %.

• After 20 May 2019 • The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and ΔνCs. • When the definition of the kilogram is based on an invariant of nature instead of a material artefact, it will be possible to realize the SI unit of mass at any place, at any time and by anyone What happens to the IPK?

• There are no plans to change the storage conditions for the IPK. It will remain at the BIPM and it will not be on display for the general public. • The IPK will retain a bit of metrological interest and therefore it will be monitored very sporadically in the future to avoid as much as possible any surface damage. • Measurements of the mass stability of the IPK in the future may help us extrapolate its mass stability in the recent past. Proposed traceability chain (after 20 May 2019) What will happen to the calibration of my standards?

• After the redefinition of the kilogram, you can continue sending your mass standard to your NMI for calibration or to a secondary calibration laboratory just as you do now. However, the traceability path that your NMI will use to link it to the SI kilogram will change. • The BIPM will organize an ongoing comparison among primary realizations of the kilogram and a consensus value of the kilogram will be determined from it. • Member States not having realizations of the new definition of the kilogram will have direct access to traceability to the same consensus value through the calibration services of the BIPM during the phase where the consensus value will be used. What does it mean for science?

• Opens up new field for research – springboard for future innovation • Multi-national projects – co-operation between research groups and countries • Fundamental constant based units – each country can realise the unit (not dependant on the IPK in Paris) • Realise mass on kg scale on g scale – implications for pharmaceutical research • Manufacturers of scientific instruments What does it mean to the man in the street? The New International System of Units

…for all times and cultures… Kibble balance team

Thank you