Characterization of the long-term stability of mass standards stored in vacuum by weighing and surface analysis

This paper reports work undertaken to investigate the stability of platinum-iridium mass standards stored in vacuum. Mass standards manufactured from platinum-iridium were stored in vacuum over a period of 42 months and their stability compared with that of similar standards stored in air. Additionally, x-ray photoelectron spectroscopy (XPS) was used to study the surfaces of coupons stored alongside the mass standards in order to characterize changes in the contamination overlayer on the weights and correlate this with the measured changes in the mass of the standards. The stability of weights in air and in vacuum was broadly similar over the period of the test. However, the regular transfer of weights between the two media has been shown to significantly increase the rate of contaminant accretion. There was good correlation between the determination of overlayer accretion by XPS and that measured gravimetrically.