ABSTRACT
In an attempt to quantify the surface-related influence of changes in ambient relative humidity, temperature, and pressure on the “apparent mass” of platinum-iridium prototype mass standards, an experimental study was carried out at Bureau International des Poids et Mesures in France. The changes in mass difference with temperature and pressure were similar to those in a previous study; however, the coefficient obtained for relative humidity was about ten times greater than that in the previous measurements made entirely at atmospheric pressure. The mass of the individual samples was determined in at least three weighing operations, after the measuring equipment reached a suitable air-conditioned state and the samples were cleaned and arranged. Long-term stability of the mass of high-precision mass standards, such as Pt-Ir prototype kilograms, depends essentially on surface adsorption effects. At the surface of platinum-iridium mass artifacts, three main types of contamination are thought to occur: sorption of water vapor; carbonaceous contamination; and mercury contamination.
