The preparatory work for the new definition of the kilogram was carried out using experiments conducted in a vacuum, whereas mass measurements using the International Prototype Kilogram (IPK) were carried out in air. So it became necessary to develop standards and methods for linking measurements in air with mass measurements in vacuum. This was in order to establish the value of Planck's constant in line with the current definition of the kilogram, on the one hand, and subsequently to ensure the dissemination of the unit of mass in a vacuum to end users, on the other.
OBJECTIVES
Develop and evaluate artefacts suitable for determining Planck's constant and Avogadro's constant in order to ensure traceability to the IPK and ultimately enable the dissemination of the kilogram after its redefinition.
Provide the appropriate procedures and apparatus for mass transfer between vacuum experiments (Kibble balance and vacuum mass comparators) and open-air experiments (comparison with the IPK and dissemination of the unit to end users).
Develop and adapt surface analysis techniques, e.g. X-ray photoelectron spectroscopy (XPS), ellipsometry, contact angle spectrometry (CAS) and surface layer models for contaminant accumulation on mass standards (including silicon spheres).
Assess the mass stability of properly preserved mass artefacts and develop the metrological infrastructure necessary for the maintenance (in the medium term) of the mass unit and its dissemination on the basis of various realisations (via a set of artefacts that can be stored in several national metrology laboratories and key comparisons).
Develop and validate methods for reproducible cleaning (to less than 5 μg) of primary mass standards, including optimisation of non-contact cleaning techniques such as the use of UV-activated ozone and gas plasma techniques.
Identify and evaluate the components of uncertainty inherent in the implementation and their propagation throughout the chain of dissemination for the kilogram and its multiples and submultiples.
SUMMARY AND RESULTS
Before the redefinition of the unit of mass in 2018, the International Prototype Kilogram was stored and used in air. Research work to achieve this redefinition of the unit of mass was carried out using watt balance and Avogadro experiments, which led to measurements being taken in vacuum. It was therefore clear that the redefined kilogram would be materialised in vacuum and would therefore require the transfer of the ‘primary’ kilogram standards from vacuum to air in order to ensure traceability for end users. This would add additional sources of error that needed to be controlled, such as the sorption of surface contaminants and water layers, to already complex measurement procedures. The EMRP project ‘Development of a practical means of disseminating the redefined kilogram (NewKILO)’ led to the development of new mass standards, new methods for cleaning and monitoring them, and procedures and equipment for transferring mass standards between vacuum and air. More generally, the aim was for the results of this project to help ensure that the redefinition brings benefits to the end-user community.
The LNE-Cnam Joint Metrology Laboratory (LCM) was involved in all work packages of this project. The following results only concern the project results to which the laboratory contributed.
With regard to the development and evaluation of artefacts, in order to ensure traceability to the watt balance and Avogadro experiments, the project partners defined the following characteristics for the material:
- low magnetic susceptibility (< 2×10-4) for use in the watt balance;
- a hardness of HV > 200 to facilitate machining and polishing;
- chemical resistance to corrosion and oxidation;
- a homogeneous material without porosity, cavities or trapped gases to ensure long-term stability.
Based on these properties, different materials were selected and compared.
Among the many studies undertaken to characterise these materials, the LCM assessed surface roughness after mirror polishing using an optical roughness tester: no direct link was found with mass stability, but it certainly affects sorption effects.
With regard to the development of procedures and appropriate equipment for mass transfer between vacuum experiments and open-air experiments, the LCM participated in measurements of the sorption coefficient as a function of pressure. It worked more specifically on platinum-iridium. The work showed that there was no variation in mass between 0,1 Pa and 0,001 Pa.
In order to validate and evaluate the reproducibility of the air/vacuum transfer method for mass standards, a comparison of gravimetric measurements of mass standards subjected to air/vacuum/air cycles was undertaken. This comparison was carried out on two groups of three masses. In the first group, two masses were kept in air, one of which had to be cleaned before weighing, while the third was kept in a nitrogen atmosphere. In the second group, all three masses were kept in air. The LCM participated in the measurements related to this group. The sorption coefficients calculated by the various national laboratories differ by up to an order of magnitude. These differences can be explained by the fact that each laboratory uses its own sorption artefacts, which may differ in particular in terms of their polishing.
LCM participated in one of the studies concerning mass transfer between vacuum and/or air and nitrogen. Three different cycles were applied to Pt-Ir and then analysed by TDS (thermo desorption mass spectrometry) at LCM in order to determine their advantages and disadvantages.
For mass stability studies, particularly during storage, cleaning and transport of masses, in 2014 the LCM set up a plasma cleaning device, integrated into the introduction chamber of the TDS device. A study of the effect of different cleaning methods was conducted. The application of air plasma cleaning to Pt-Ir artefacts previously cleaned with ethanol or isopropanol demonstrated the effectiveness of this method, particularly on carbon and hydrocarbon compounds. A gravimetric study observed a variation in the mass of the Pt-Ir artefacts depending on the cleaning process used (BIPM cleaning-washing or plasma air). A series of comparisons was made before and after the two cleaning processes. After each cleaning, a rapid increase in mass was observed, which stabilised after a few hours.
A comparison of acetone and ethanol adsorption on PtIr, iridium and AuPtAgCu alloy surfaces was performed. The adsorption of both acetone and ethanol is lower on pure iridium than on PtIr. With regard to the quaternary alloy AuPtAgCu, the very high adsorption of acetone suggests high surface porosity, which makes it unsuitable for use as a mass standard.
Another aspect of the work concerned medium-term storage and transfer of standards between laboratories with a primary realisation operating under vacuum. In order to evaluate the storage and transfer protocol, the LCM participated in measurements on a set of masses. There is no significant advantage to storing masses in a neutral gas; in fact, it is more advantageous in terms of mass stability to store masses in air. The current method of storing masses in air is therefore recommended.
PUBLICATIONS AND COMMUNICATIONS
PLIMMER M.D., DU COLOMBIER D., IRAQI HOUSSAINI N., SILVESTRI Z., PINOT P. and HANNACHI R., “Apparatus to measure adsorption of condensable solvents on technical surfaces by photothermal deflection”, Review of Scientific Instruments, 2012, 83, 11, DOI: 10.1063/1.4767245.
SILVESTRI Z., AZOUIGUI S., BOUHTIYYA S., MACÉ S., PLIMMER M.D., PINOT P., TAYEB-CHANDOUL F. and HANNACHI R., “Thermal desorption mass spectrometer for mass metrology”, Review of Scientific Instruments, 2014, 85, 4, DOI: 10.1063/1.4870921.
DAVIDSON S., BERRY J., SILVESTRI Z., HOGSTROM R. and GREEN R., “Addressing the requirements for the practical implementation and ongoing maintenance of the redefined kilogram”, 22nd IMEKO TC3 International Conference on Measurement of Force, Mass and Torque 2014, Held Together with TC5 and TC22, Cape Town, South Africa, 3-5 February 2014.
SILVESTRI Z., BOUHTIYYA S., PINOT P. and DAVIDSON S., “How to disseminate the mass unit for the new kilogram?”, International Congress of Metrology, Paris, France, Poster, 21-24 September 2015, DOI: 10.1051/metrology/20150018003.
PARTNERS
- NPL (GB),
- CMI (CZ),
- CNAM (FR),
- DFM (DK),
- EJPD (CH),
- LNE (FR),
- MGRT (SI),
- MIKES (FI),
- PTB (DE),
- SMU (SK),
- TUBITAK (TR),
- INRIM (IT),
- NRC (CA).