International comparison of volume measurement for a 1 kg silicon sphere between KRISS and NMIJ

1 kg single-crystal silicon spheres are presently used as primary density standards in many countries. The absolute density of the spheres is determined from the measurements of their mass and volume in conformity with the definitions of the SI base units. Since the mass of the spheres is almost 1 kg, a mass comparison with the prototype of the kilogram can be performed with very low uncertainty. Absolute volume measurements for the spheres therefore have a crucial role in realizing a reliable density traceability system. To confirm the reliability of the volume measurement, the volume of a silicon sphere was measured independently using optical interferometers at the Korea Research Institute of Standards and Science (KRISS, Korea) and the National Metrology Institute of Japan (NMIJ, Japan). An optical interferometer with an etalon scanning system was used at KRISS. On the other hand, an optical interferometer with an optical frequency scanning system was used at NMIJ. The volume was measured at 20 °C and 0 Pa, and the results are in agreement with each other within their uncertainties. Details of the two interferometers and the comparison results are described.