S Mizushima

The 8th International Comparison of Absolute Gravimeters 2009: the first Key Comparison (CCM.G-K1) in the field of absolute gravimetry

The 8th International Comparison of Absolute Gravimeters (ICAG2009) took place at the headquarters of the International Bureau of Weights and Measures (BIPM) from September to October 2009. It was the first ICAG organized as a key comparison in the framework of the CIPM Mutual Recognition Arrangement of the International Committee for Weights and Measures (CIPM MRA) (CIPM 1999). ICAG2009 was composed of a Key Comparison (KC) as defined by the CIPM MRA, organized by the Consultative Committee for Mass and Related Quantities (CCM) and designated as CCM.G-K1. Participating gravimeters and their operators came from national metrology institutes (NMIs) or their designated institutes (DIs) as defined by the CIPM MRA. A Pilot Study (PS) was run in parallel in order to include gravimeters and their operators from other institutes which, while not signatories of the CIPM MRA, nevertheless play important roles in international gravimetry measurements. The aim of the CIPM MRA is to have international acceptance of the measurement capabilities of the participating institutes in various fields of metrology. The results of CCM.G-K1 thus constitute an accurate and consistent gravity reference traceable to the SI (International System of Units), which can be used as the global basis for geodetic, geophysical and metrological observations of gravity. The measurements performed afterwards by the KC participants can be referred to the international metrological reference, i.e. they are SI-traceable.The ICAG2009 was complemented by a number of associated measurements: the Relative Gravity Campaign (RGC2009), high-precision levelling and an accurate gravity survey in support of the BIPM watt balance project. The major measurements took place at the BIPM between July and October 2009. Altogether 24 institutes with 22 absolute gravimeters (one of the 22 AGs was ultimately withdrawn) and nine relative gravimeters participated in the ICAG/RGC campaign.This paper is focused on the absolute gravity campaign. We review the history of the ICAGs and present the organization, data processing and the final results of the ICAG2009.After almost thirty years of hosting eight successive ICAGs, the CIPM decided to transfer the responsibility for piloting the future ICAGs to NMIs, although maintaining a supervisory role through its Consultative Committee for Mass and Related Quantities.

Surface layer determination for the Si spheres of the Avogadro project

For the accurate determination of the Avogadro constant, two 28Si spheres were produced, whose macroscopic density, in addition to other values, must be determined. To make a contribution to the new definition of the kilogram, a relative standard uncertainty of less than 2 ? 10?8 has to be achieved. Each silicon surface is covered by a surface layer (SL). Consequently, correction parameters for the SL are determined to be applied to the mass and volume determination of the enriched spheres. With the use of a large set of surface analysing techniques, the structure of the SL is investigated. An unexpected metallic contamination existing on the sphere surface enlarges the uncertainty contribution of the correction parameters above the originally targeted value of 1 ? 10?8. In the framework of this investigation this new obstacle is resolved in two ways. A new combination of analytical methods is applied to measure the SL mass mSL and the thickness dSL, including this new contamination, with an uncertainty of u(mSL) = 14.5??g and 14.4??g, respectively, and u(dSL) = 0.33?nm and 0.32?nm for the 28Si spheres AVO28-S5 and AVO28-S8, respectively.In the second part of the work, the chemical composition of these metallic contaminations is found to be Cu, Ni and Zn silicide compounds. For the removal of this contamination, a special procedure is developed, tested and applied to the spheres to produce the originally expected surface structure on the spheres. After the application of this new procedure the use of x-ray reflectometry directly at the spheres will be possible. It is expected to reduce the uncertainty contribution due to the SL down to 1 ? 10?8.

Determination of the amount of gas adsorption on SiO2/Si(100) surfaces to realize precise mass measurement

The adsorption isotherms on SiO2/Si(100) surfaces were measured using a vacuum mass comparator. Samples with a surface area difference of 816.6 cm2 were used for the measurement, and a substitution weighing method was adopted to reduce the uncertainty due to the drift and non-linearity of the indication of the mass comparator. We measured adsorption isotherms of water vapour on the SiO2/Si(100) surfaces outgassed at a temperature of 500 °C and found that dissociative adsorption caused an irreversible increase of 0.028 µg cm−2 with an uncertainty of 0.004 µg cm−2 (k = 1). We also found that the physical adsorption of water molecules on hydroxylated surfaces had a monolayer capacity of 0.004 µg cm−2 with an uncertainty of 0.002 µg cm−2 (k = 1). In addition, the adsorption isotherms for ethanol vapour and n-octane vapour, which were different from water vapour in adsorption properties, were measured and analysed.

Mass measurement of 1?kg silicon spheres to establish a density standard

Air buoyancy causes a significant systematic effect in precision mass determination of 1 kg silicon spheres. In order to correct this effect accurately, mass measurement of the silicon sphere was conducted using buoyancy artefacts; additionally, in order to stabilize atmospheric conditions, we used a vacuum chamber in which a mass comparator had been installed. The silicon sphere was also weighed in vacuum to verify the air buoyancy correction. Mass differences measured in air and in vacuum showed good agreement with each other in spite of the desorption effect from weight surfaces. Furthermore, the result of weighing under vacuum conditions demonstrated better repeatability than that obtained in air.

Final report on the Seventh International Comparison of Absolute Gravimeters (ICAG 2005)

The Bureau International des Poids et Mesures (BIPM), S?vres, France, hosted the 7th International Comparison of Absolute Gravimeters (ICAG) and the associated Relative Gravity Campaign (RGC) from August to September 2005.ICAG 2005 was prepared and performed as a metrological pilot study, which aimed: To determine the gravity comparison reference values; To determine the offsets of the absolute gravimeters; and As a pilot study to accumulate experience for the CIPM Key Comparisons. This document presents a complete and extensive review of the technical protocol and data processing procedures. The 1st ICAG?RGC comparison was held at the BIPM in 1980?1981 and since then meetings have been organized every 4 years.In this paper, we present an overview of how the meeting was organized, the conditions of BIPM gravimetric sites, technical specifications, data processing strategy and an analysis of the final results. This 7th ICAG final report supersedes all previously published reports.Readings were obtained from participating instruments, 19 absolute gravimeters and 15 relative gravimeters. Precise levelling measurements were carried out and all measurements were performed on the BIPM micro-gravity network which was specifically designed for the comparison.

Results of the Seventh International Comparison of Absolute Gravimeters ICAG-2005 at the Bureau International des Poids et Mesures, Sèvres

The International Comparison of Absolute Gravimeters ICAG-2005 was held at the Bureau International des Poids et Mesures (BIPM), Sevres, France in September 2005. The organization of ICAG-2005, measurement strategy, calculation and presentation of the results were described in a technical protocol pre-developed to the comparison. Nineteen absolute gravimeters carried out 96 series of measurements of free-fall acceleration g at the sites of the BIPM gravity network. The vertical gravity gradients were measured by relative gravimeters. For the first time the budgets of uncertainties were presented.

The improvement of the adsorption characteristics of stainless steel surfaces by sputter-deposited films

The amount of reversible water vapour adsorption on surfaces was measured by comparison weighings using samples with a surface area difference of more than 795 cm2. Stainless steel surfaces, single crystal silicon surfaces and sputter-deposited surfaces were chosen as the samples. The sputter-deposited films with a thickness of 25 nm to 28 nm showed characteristics that suggested they were densely packed. It was found that these sputter-deposited films could reduce the monolayer capacity, nm, of the stainless steel surface (0.0238 µg cm−2) by 35%.

Determination of gas adsorption on native oxides formed on Si(100), Si(111) and Si(110) surfaces

We measured gas adsorption on native oxides formed on Si(100), Si(111) and Si(110) surfaces and found that the dependence of the gas adsorption on the surface orientation was insignificant. This fact suggests that the atomic structure of SiO2 near the top surface is almost amorphous. For the physical adsorption of water vapour, we obtained a monolayer capacity of 0.0040 µg cm−2 and a BET parameter of 4.5, which corresponds to 0.0065 µg cm−2 water vapour adsorption at 50% humidity with an uncertainty of 0.0005 µg cm−2 (k = 1). Thus, it can be concluded that the physical adsorption of water on a defect-free surface of a 1 kg silicon sphere amounts to 1.8 µg with an uncertainty of 0.14 µg (k = 1) at 50% humidity. In addition, measurement using ethanol vapour and n-octane vapour showed that gas adsorption decreased after hydroxylation of SiO2/Si surfaces.