Absolute volume determination of a silicon sphere with the spherical interferometer of PTB

For an absolute measurement of a density standard, mass and volume are measured separately. The absolute determination of the volume is still the quantity limiting the uncertainty. Lowest uncertainties are achieved by interferometric measurements of the diameter of spheres made from a silicon single crystal. So far, measurements have been based on a plane interferometer. At PTB, we demonstrate a new method of determining the diameter using a spherical interferometer, which yields more and particular information about the resulting volume.