Precision Determination of Adsorption Layers on Stainless Steel Mass Standards by Mass Comparison and Ellipsometry: Part II: Sorption Phenomena in Vacuum

Abstract

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.

Measurement of Newton’s gravitational constant

“…We note that Schwartz [17] has also found a mass increase for stainless steel samples in a vacuum system involving a rotary pump, a turbomolecular pump and a liquid nitrogen cold trap. His samples were 1 kg masses with surface areas differing by a factor of 1.8.…”

“…Schwartz [16] has measured the mass variation per unit area of 1 kg stainless steel objects in air with relative humidity between 3% and 77%. He [17] also has measured the additional mass variation per area due to pumping the system from atmospheric pressure at 3% relative humidity down to 5 × 10 −3 Pa. His samples were first cleaned by wiping them with a linen cloth soaked in ethanol and diethylether and then ultrasonic cleaning in ethanol. After cleaning, they were dried in a vacuum oven at 50 • C. For these cleaned samples, the weight change found for 3% to 50% humidity variation was 11.5 ng cm −2 with an additional change of 29 ng cm −2 in going from 3% relative humidity in air to vacuum (total change of 40.5 ng cm −2 ).…”

Determination of Adsorption Layers on Silicon Sorption Artifacts using the Gravimetric Method

“…The flow rate of either humid or dry air was adjusted using the manual valve of a mass flow controller (MFC). The maximum flow rate of adjustment was approximately 0.03/h for a range of h \ 0.25 and for h [ 0.6 and 0.04/h for a range of 0.25 \ h \ 0.6 [9]. The humidity adjustment took place 1.5 h to attain higher humidity inside the chamber, whereas it took more than 10 h to reach the very low humidity level.…”

Realization of the kilogram by the XRCD method

“…Since variations of the sorption layers on the surface of mass standards have to be taken into account (hysteresis effects) in a pressure range between 0.l Pa and 10 5 Pa, mass comparisons in vacuum were performed in most cases in a pressure range between 10 −4 Pa and 0.1 Pa. In this pressure range, a significant variation of the sorption layers was not observed [65,85,91,92].…”