EMRP-SIB05: Developing a practical means of disseminating the redefined kilogram

• Forschungsstelle: METAS
• Projektnummer: F-5133.30010
• Projekttitel Englisch: EMRP-SIB05: Developing a practical means of disseminating the redefined kilogram

Schlüsselwörter (Englisch)

Kilogram, new definition of the kilogram, dissemination of the unit, surface analysis, sorption, realization of the mass scale in air

Kurzbeschreibung (Englisch)

The unit of mass, the kilogram, is the last of the seven base units of the International System of Units (SI) to be defined in terms of a material artefact rather than by relation to an invariant of nature. Progress is being made towards a redefinition of the kilogram in terms of the Planck constant (h), realized via the watt balance and Avogadro experiments. In order for this redefinition to be feasible, a practical means of linking the new definition to the current mass scale must be in place. The work outlined in this project will develop the procedures necessary to provide this link and to ensure the continuity of the worldwide mass scale. Without this work the uncertainty with which the Planck constant can be fixed will be compromised and furthermore the practical implementation of a redefined kilogram will not be possible.

This project is part of the European Metrology Research Programme (EMRP, http://www.euramet.org/index.php?id=emrp); it is partly funded by the European Union on the basis of Decision No 912/2009/EC.

Projektziele (Englisch)

The project addresses the requirements for the implementation of the redefinition of the kilogram by developing artefacts and protocols which will allow traceability to a vacuum realisation while minimising the uncertainty in the redefinition (the value of the Planck constant) and in the subsequent dissemination of the unit:

• evaluate materials and develop next generation mass standards which are compatible with use in watt balance experiments and that have optimised stability  (a change of less than 1 in 10⁸) for transfer to vacuum and storage in vacuum and inert gas (including dry nitrogen).

• investigate vacuum/air transfer phenomena and derive a procedure, which minimises the uncertainty (a target uncertainty of less than 10 micrograms) in the transfer process.

• use complementary surface analysis techniques to characterise the behaviour of various materials on transfer between vacuum, air and inert gas. The use of small (surface) samples (instead of kilogram artefacts) allows rapid evaluation of a wide range of materials and conditions and maximises the range of surface analysis techniques which can be used. The results will provide an understanding of the physical processes leading to the sorption phenomena.

• produce apparatus for the storage and transfer of mass standards under vacuum or inert gas, investigate the storage of standards in inert media to improve stability and evaluate existing and new cleaning methods for mass standards.

• identify and evaluate the uncertainty components inherent in the traceability to the new definition and in their propagation through the dissemination chain.

Abstract (Englisch)

WP1: Development and evaluation of artefacts suitable to provide maintenance and dissemi-nation of a redefined kilogram
The aim was to identify materials and production techniques for the new generation of mass artefacts to be used in the watt balances as primary standards. For this purpose METAS analysed the chemical composition of metallic samples such as PtIr, AuPt, iridium, stainless steel, silicon, nickel, electroplated gold and electroplated rhodium using x-ray photoelectron spectroscopy (XPS). Additionally, the surface roughness of the samples was measured. Af-terwards, the samples were distributed among the JRP partners for further investigations.

WP2: Development and evaluation of procedures and techniques for the mass transfer be-tween vacuum and air
In the framework of WP2, METAS analysed the sorption characteristics of mass standards made of different materials using vacuum-air transfers. Measurements by means of XPS, quartz crystal microbalance (QCM) and gravimetric weighings were conducted and correla-tions were investigated. Stainless steel kilogram artefacts were used as travelling standards between the JRP partners to validate gas/air/vacuum transfer methods using gravimetric measurements. The sorp-tion coefficients and the absolute mass values in air and in vacuum were determined by each participating NMI. The results were collated by NPL.

WP3: Surface effects and dynamic changes on the artefact surface between vauum, air and selected gases
The aim was to use a range of complementary surface analysis techniques to characterise the surface effects on artefacts stored in and transferred between air, vacuum and inert gas.
For that purpose metallic samples were exposed to cyclic venting with vacuum-N2-vacuum, vacuum-N2-air-vacuum and vacuum-air-vacuum. The effects of the three transfer processes on the surface contamination were investigated by using XPS. In addition, all samples were cleaned by hydrogen plasma, UV/ozone and nettoyage-lavage in order to correlate the influence of the cleaning methods with the materials and transfer processes. Based on these results a ranking list of best materials and processes for future mass standards could be generated.

WP4: Evaluation of the mass stability of artefacts with a focus on storage, cleaning and transport methods
The aim was to address the on-going requirements for improvements and developments to conventional mass standards, storage and transfer methods and cleaning processes. METAS and its JRP partners applied various cleaning techniques to a variety of sample materials to study material-specific (re-)contamination and cleanliness of the surface. Furthermore, different storage conditions for the samples were tested after cleaning. The findings showed significant differences between the efficiency of the cleaning techniques and the achieved cleanliness. It was found that hydrogen plasma was non-abrasiv and that UV/ozone oxidized the surface.
To evaluate transport methods, stainless steel kilogram artefacts were stored either in air or in a nitrogen-filled container and transported among the participating NMIs. The artefacts were transferred via a glove-box to a mass comparator running under vacuum to carry out gravimetric weighings.

WP5: Identification and evaluation of the uncertainty components inherent in the mise-en-pratique and in their propagation through the dissemination chain
One of the aims was to find a mathematical model which can be used to quantify the change in mass of a standard when transferred from air to vacuum and vice versa. For that purpose, surface artefacts of gold, stainless steel and rhodium were exposed to a number of vacuum-air transfers. By means of gravimetric weighings and QCM measurements reversible and irreversible sorption could be distinguished. Mathematically, the change in mass due to sorp-tion effects could be described by a combination of an exponential and a linear term.

Umsetzung und Anwendungen (Englisch)

The results obtained in this project contributed much to the technological knowledge about manufacturing, cleaning and storing mass artefacts. The gained knowledge and experience will be applied primarily at the BIPM (Bureau International des Poids et Mesures) in Paris and national metrology institutes around the world for the realisation of the kilogram by means of watt balances and the dissemination of the mass unit. Mass standards and surface artefacts of different materials provide the basis for this. In doing so, material-specific contamination is taken into account. Moreover, mass comparisons and long-term stability can be monitored and cross-checked experimentally when using artefacts of different materials in order to take necessary corrective measures.

Publikationen / Ergebnisse (Englisch)

• Fuchs P, Marti K, Russi S 2013 Removal of mercury contamination on primary mass standards by hydrogen plasma and thermal desorption Metrologia 50 73-82.
• Marti K, Fuchs P, Russi S 2013 Cleaning of mass standards II: A comparison of new techniques applied to actual and potential new materials for mass standards Metrologia 50 83-92.
• Fuchs P, Marti K, Russi S 2014 Traceability of mass in air to mass in vacuum: results on the correlation between the change in mass and the surface chemical state Metro-logia 51 376-386.
• Fuchs P, Marti K, Russi S 2014 UV/ozone cleaning of mass standards: results on the correlation between mass and surface chemical state Metrologia 51 387-393.
• Marti K, Fuchs P, Russi S 2015 Traceability of mass II: a study of procedures and ma-terials Metrologia 52 89-103.
• Fuchs P, Marti K, Russi S 2012 New instrument for the study of ‘the kg, mise en pra-tique’: first results on the correlation between the change in mass and surface chemical state Metrologia 49 607-614.
• Marti K, Fuchs P, Russi S 2012 Cleaning of mass standards: a comparison of new and old techniques Metrologia 49 628-634.
• Fuchs P, Marti K, Russi S 2012 Materials for mass standards: long-term stability of PtIr and Au after hydrogen and oxygen low-pressure plasma cleaning Metrologia 49 615-627.
• Fuchs P, Marti K, Russi S 2013 Gewichtige Arbeiten für eine Neudefinition des Kilo-gramms METinfo, Vol. 20, No. 1/2013 10-14