Comparison of two methods for determining the sorption correction for a vacuum-realized kilogram

The International System of Units (SI) is expected to be revised in 2018, and in this new system, the unit of mass, the kilogram, will be realized in a vacuum environment. In order to transfer the vacuum realization to atmospheric pressure air, the effects of the sorption of atmospheric contaminants must either be corrected for or eliminated. NIST has constructed a system that directly compares a mass in vacuum to a mass in air, thereby eliminating the need for sorption correction. We describe the operation of this system and compare results to those obtained from sorption corrections.