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Precision Determination of Adsorption Layers on Stainless Steel Mass Standards by Mass Comparison and Ellipsometry: Part I: Adsorption Isotherms in Air

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 <or=0,005 μg cm-2. The sorption behaviour of carefully polished surfaces (average peak-to-valley height Rz ⩽ 0,12 μm) is well described by the BET equation for multilayer adsorption, the adsorption isotherm being of type II according to the BET classification. For clean, well-polished surfaces the BET parameters μm = 0,008 4 μg cm-2 and cB = 8,9 were found; the corresponding coefficient in the limited humidity range 0,30 ⩽ h ⩽ 0,60 is 31 × δh ng cm-2. The sorption behaviour of precision stainless steel mass standards is mainly influenced by the degree of surface cleanliness: uncleaned standards with an absolute surface covering of μ' ⩾ 0,7 μg cm-2 show sorption-induced mass variations which are greater by about a factor of 2,5 relative to the clean surfaces.