Elimination of Mass-Exchange Errors in the NRC Watt Balance

We report on the evaluation, correction, and subsequent elimination of the mass-exchange errors in the National Research Council (NRC) of Canada watt balance. Two effects arising from mass exchanges during the weighing phase initially produced significant systematic errors in the measurements of the Planck constant (h). These effects were estimated, and corresponding corrections were applied to measurements of h made at NRC between August and September of 2011. The effect due to the tilting of the beam required an estimated correction of (0.238 ±0.018)μJ · s/(J ·s), and the effect due to the tilting of the flat, which supports the beam, required an estimated correction of (-0.636 ±0.056)μJ · s/(J ·s). After the initial measurements were completed, we made several modifications to the balance which achieved a reduction of the two errors to negligible levels. A second set of measurements, made with the modified apparatus in May and June of 2012, shows very good agreement with the first measurements.