If the momentum scale is systematically incorrect, the mass scale will also be systematically wrong. For charged particles, this effect can be caused by an incorrect knowledge of the magnetic field, systematic problems in understanding tracking systems, or an overall scaling of the geometry of the experiment. In reconstructing , a scaling of the geometry is the most likely cause of a systematic error. By measuring the mass differences, we remove these effects on the measurement of the mass. However, a small systematic can survive due to problems in the momentum calculation.