Methods to determine water vapour sorption on mass standards

The purpose of this work is to study the effect of water vapour sorption on mass standards by means of two methods: (a) a global and direct gravimetric method determining the water vapour deposit on the total surface area of mass standards; (b) an optical and indirect ellipsometric method permitting the evaluation of the adsorbed water thickness on the localized surface of mass standards. The coefficient of water vapour adsorption in moist air and the amount of water desorption from air to vacuum were determined on the surfaces of mass standards. Measurements were achieved by both methods using the same samples of Pt/Ir, stainless steel and silicon. Results obtained on stainless steel showed a good agreement between the two methods. For the Pt/Ir and the silicon samples, the sorption effect evaluated by the gravimetric method was about twice that inferred from ellipsometry.