Dissemination of the density unit using 1 kg spheres made of natural silicon by pressure-stabilised hydrostatic weighing

Since the establishment of the revised SI the unit of mass “kilogram” can be realised by two independent primary methods (the “Kibble balance method” or the “X-ray crystal density method”, XRCD) related to the Planck's constant. In addition, the XRCD method is also a primary realisation of the density unit using spheres of highly enriched Si28. In contrast to the realisation method based on very expensive Si28 spheres, a more cost-effective method has been developed to disseminate the density unit using spheres of monocrystalline natural silicon. A well characterised Si28 sphere will be used as reference for the calibration of the density of spheres made of natural silicon. So far, the use of a pressure-stabilised hydrostatic weighing apparatus seems to offer the best possibility to perform the necessary density comparison measurement with smallest uncertainties. This apparatus is developed to ensure high pressure and temperature stability by shielding the atmosphere inside the comparator from the environment. The concept of a pressure-stabilised hydrostatic weighing system is presented here.