P. Pinot

The BNM Watt balance project

A new watt balance project is now developed by the BNM: the general configuration and the main parts of the experimental set-up in development are presented. Its aim is to contribute to the international effort in monitoring the kilogram towards a new definition of the mass unit with an accuracy of 10-8 or better

First determination of the Planck constant using the LNE watt balance

After separate developments of the different elements with continuous characterizations and improvements, the LNE watt balance has been assembled. This paper describes the system in detail and gives its first measurements of the Plancks constant h. The value determined in air is h = 6.626 068 8(20) × 10−34 Js which differs in relative terms by −0.05 × 10−7 from the h90 value and by −1.1 × 10−7 from that of the 2010 CODATA adjustment of h. The relative standard uncertainty associated is 3.1 × 10−7.

Volume magnetic susceptibility of gold–platinum alloys: possible materials to make mass standards for the watt balance experiment

In 2000, the Bureau National de Metrologie (BNM, France) decided to develop a new watt balance experiment. Among numerous design studies, the choice of the transfer mass is particularly important. Because of the proximity to a source of high magnetic intensity, this mass must have a magnetic susceptibility as weak as possible. Gold–platinum alloy seems to meet this requirement, as well as additional requirements for density and hardness values, making it a possible candidate for mass standard realization. Five different gold–platinum alloys were studied, their volume magnetic susceptibility ranging from −2.8×10−5 to −2.1×10−5 for two of them and from +1.1×10−5 to +8.8×10−5 for the other three.

Theoretical analysis for the design of the French watt balance experiment force comparator

This paper presents a preliminary analysis for designing a force comparator to be used in the French watt balance experiment. The first stage of this experiment consists in a static equilibrium, by means of a mechanical beam balance, between a gravitational force (a weight of an artefact having a known mass submitted to the acceleration due to the gravity) and a vertical electromagnetic force acting on a coil driven by a current subject to the magnetic induction field provided by a permanent magnet. The principle of the force comparison in the French experiment is explained. The general design configuration of the force balance using flexure strips as pivots is discussed and theoretical calculation results based on realistic assumptions of the static and dynamic behaviors of the balance are presented.

Study and comparison of two polishing methods for platinum?iridium surfaces, by means of three characterization techniques

Machining the surface of mass standards is still of great importance. This paper details a comparative study of the roughness of two plane surfaces of a platinum?iridium alloy (90% of platinum and 10% of iridium). Using the BIPM processes, the surfaces were initially machined on a lathe using diamond tools; in addition, one of these was manually polished with a diamond paste.Three techniques for surface characterization are used: shear-force microscopy (ShFM), optical roughness-meter and x-ray reflectometer. The first technique uses the shear-force interaction between the probe and the sample. The ShFM is part of a scanning near-field optical microscope that has the advantage of providing two images simultaneously, a topographical one and a near-field optical one. Only the topographic images will be presented in this paper; the results obtained in optical near-field will be the subject of another publication. To avoid any confusion, we will speak here of ShFM. The topographic images and their associated statistical and physical parameters, such as power spectral densities (PSDs), root-mean-square height, etc, are discussed in this paper. The PSDs of the surface are also determined experimentally from x-ray and visible light scattering measurements using angle-resolved scattering theory. This theory, which is well adapted for visible radiation, is extended to x-rays. The measurements made with the three instruments demonstrate that the two surfaces present a roughness of the same order of magnitude. However, the defects that contribute in a preponderant manner to their roughness spectrum do not belong to the same spatial bandwidth but depend on the polishing process.

The nanometric roughness of mass standards and the effect of BIPM cleaning-washing techniques

A method based on light scattering has been used to characterize the surface roughness of four standard kilograms made from XSH alacrite, in order to determine the root-mean-square height delta and the correlation length sigma. The values of these two parameters, which have been found to lie between 2 nm and 5 nm and between 170 nm and 250 nm, respectively, show the high surface quality that it is possible to obtain by mechanical polishing of XSH alacrite. Cartographies made by light scattering, as well as delta and σ values, have shown that in spite of identical manufacturing conditions the kilograms present a significant difference in surface roughness. We have observed that the lower bases were in general rougher than the upper ones. This was explained by the different histories of the upper and lower bases. On the other hand, the same value of σ was found for both bases of a given kilogram. We have also found that the smoother the kilogram, the smaller the mass loss. This clearly shows the effect of surface roughness on the stability of mass standards. Finally, significant variations in the values of and delta and σ have been observed after the cleaning-washing process at the Bureau International des Poids et Mesures. We have also shown that the variations noted after several successive cleaning-washings are mainly the result of the first of these. This agrees with observations made during earlier gravimetric studies.

The mirage effect to investigate the adsorption of organic molecules on the surface of mass standards

Adsorption and desorption of cleaning products are among the most important contributions to the instability of mass standards. An optical method, the mirage effect, is proposed to investigate the adsorption/desorption of cleaning solvents on the surface of mass standards at conditions close to normal temperature and pressure (NTP). A theoretical model of the mirage effect in the presence of an adsorbable vapour is recalled, relating the amplitude and phase of the mirage deflection to the adsorption isotherm. Representing this isotherm with the BET (Brunauer, Emmett and Teller) equation and introducing an inverse procedure, the parameters characterizing the adsorption process, such as the amount of adsorbed matter in the first monolayer and the differential heat of adsorption, are obtained. Experimental results for the adsorption of acetone, ethanol and isopropanol on XSH alacrite are presented that agree qualitatively with results from more conventional methods.

Minimization of the coil movement of the LNE watt balance during weighing mode and estimation of parasitic forces and torques involved

Alignments of watt balance experiments are necessary to achieve a relative uncertainty at a level of few parts in 108. This paper briefly describes the LNE watt balance and concentrates on adjustments made to minimize the coil movements during weighing mode. The parasitic forces and torques involved in these movements are estimated by a mathematical model. Some of the calculated parasitic forces are compared with an evaluation done by studying the yaw movement of the beam.

Progress on the LNE watt balance project

This paper describes the main progress on the LNE watt balance project since 2004. Development of different parts and structure of the watt balance including starting their assembly are presented.

Thermal desorption mass spectrometer for mass metrology

This article presents a device for the study of physisorbed elements on polished surfaces (diameter ⩽56 mm) of the kind used in mass metrology. The technique is based on mass spectrometry of molecules desorbed after heating under vacuum of the analyzed surface. We describe a first application of the device to study current and future mass standards in order to understand how their surface reactivity depends on storage conditions, cleaning processes, and polishing methods. Surface contamination analysis by thermal desorption mass spectrometry to examine the effect of cleaning on pure iridium is given as an example.