Adsorption layers on stainless steel mass standards (OIML classes E1 and E2) have been determined directly and precisely by the optical method of ellipsometry as a function of relative humidity in the range 0,03 ⩽ h ⩽ 0,77, the relevant influencing factors being surface cleanliness, roughness, steel composition and ambient temperature. Under the same environmental conditions, two pairs of 1 kg artefacts, having geometrical surfaces differing in area by about δ A = 390 cm2, but the same material properties and surface finish as the mass standards, have been compared on a 1 kg mass comparator. The two independent measuring techniques yield strongly correlated results, the standard uncertainties of the measured surface coverings being
Using the two independent measuring techniques, mass comparison and ellipsometry, described in Part I, adsorption layers on stainless steel mass standards and artefacts of nominal value 1 kg have been determined directly and precisely as a function of pressure in the range 5 × 10-3 Pa ⩽ p ⩽ 1 × 105 Pa. For clean, polished surfaces (average peak-to-valley height Rz ⩽ 0,12 μm) reversible sorption isotherms with a coefficient of (0,024 ± 0,005) μg cm-2 were found due to the transition from normal pressure (relative humidity h = 0,03) to the pressure of 0,1 Pa. For uncleaned surfaces this coefficient rises by a factor of about 2,5. At pressures below 0,1 Pa irreversible adsorption was observed with a constant adsorption rate of about 0,012 μg cm-2 d-1, independent of the surface cleanliness. This sorption effect is attributed to continuous condensation of oil particles originating from the turbomolecular pump used.
In view of the proposed redefinition of the kilogram, in 2005, 2007 and 2010 the CCM adopted recommendations and conditions to be met before the redefinition is implemented. Some of these conditions concern the uncertainty by which the kilogram will be realized after the redefinition. This uncertainty has become a point of discussion in other committees, because the needs for this value were not sufficiently transparent. Here, examples of the realization of the new kilogram with different uncertainties, along with the experimental results available from the Avogadro and watt balance experiments, the uncertainties of mass determinations and their propagation with reasonable assumptions are presented, in order to compare possible realization uncertainties with the requirements in practice. It turned out that the CCM conditions are reasonable, if major changes in the dissemination chain of the kilogram are to be avoided.
The density of ambient air has been determined by a straightforward experimental method. The apparent masses of two artefacts having about the same mass and surface, but different well-known volumes, have been compared by using a 1 kg balance in vacuum and in air. The differences of apparent masses and volumes yield the air density with a relative uncertainty (1σ) of 5 × 10-5. From measurements made using a third artefact, surface sorption effects caused by the change between vacuum and air conditions gave a coefficient of about 0,2 μg cm-2.
Four 1 &g stainless steel we.ghts were iseo to carry oLt comparisons lo inwesligaie the efficiency of dinerent proceaures lor cleaning precision mass standards. Not only were mass compar'sons carried 0-1 on a n gh-resolLt,on 1 6g comparison balance but also tne surface layers of 1 kg mass standards were airectly determ ned lor the first time using an aLtomatic ell'psometer. Both measLr.ng IechniqJes show that u.lrasonic cleaning in ethano. is the most efficient procedue for clean.ng pol shed sta.nless steel sLrfaces. Furlnermore, I1 was possib e to prove that ell psometry as an optical techniq-e of surface analysis s well sLited for non-contact. h gnly sens live ana absolute determination 01 the surface c.eanliness of mass standards.
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