Determination and correction of mass exchange errors in the NRC watt balance

We have investigated mass exchange corrections in the NRC watt balance. Two effects arising from mass exchanges during the weighing phase produce significant systematic errors in 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 An effect due to tilting of the beam required an estimated correction of (0.238 ± 0.018) μW/W and an effect due to tilting of the flat, which supports the beam, required an estimated correction of (-0.636 ± 0.056) μW/W.