Abstract
A new electronic mass comparator, capable of automatic weighing in vacuum, was introduced into the National Research Laboratory of Metrology. The standard deviation of 0.1 μg, a factor of 3 smaller than the standard deviation of the best previous measurement using a mechanical balance, was achieved for the mass comparison between two 1-kg weights. For eight days, mass difference of two 1-kg weights with the surface area difference of 66.7 cm2 was measured in vacuum. We observed an outgassing rate of 9×10-6 Pa·3·s-1·m-2 after an evacuation for 3.9-6.8 hours, and below 6×10-9 Pa·m3·s-1·m-2 after an evacuation for 107 hours. Residual gas in the vacuum chamber, which housed the electronic mass comparator, was analyzed to monitor the oil contamination. At a total pressure of 5.5×10-4 Pa, we observed the main peaks due to hydrocarbon at 41, 43, and 57 amu with a partial pressure of 3×10-6 Pa.
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