PRECISE MASS AND VOLUME DETERMINATION OF THE SILICON SINKER FOR SEA WATER DENSIMETRY

Accurate density measurement for seawater is demanded to revise the absolute salinity correlation used in the Thermodynamic Equation of Seawater 2010 (TEOS-10), which is widely used to calcurate the global ocean circulation. In NMIJ, a hydrostatic weighing apparatus for sea water densimetry was developed to provide CRM for sea-water density with relative uncertainty of 1 ppm. For this purpose, a 100 g silicon single-crystal sinker was newly developed for buoyancy force measurements. The volume of the sinker should be determined with better uncertainty than 1 ppm. PFM (pressure of floatation method) provided in NMIJ is effective way to determine the sinker density with about 0.1 ppm uncertainty. For  mass calibration, however, uncertainty in the conventional mass comparison with 100 g OIML weight is larger than 0.05 mg which corresponds to 1.2 ppm of sea water density measured. In the present study, therefore, mass measurement based on sub-multiple procedure. Namely, a set of 500 g, 200 g, 200 g, 100 g disc weight made of silicon single crystal was prepared. Their masses were calibrated by sub-multiple measurement with 1 kg silicon sphere (NMIJ-S5) having extremely small uncertainty of mass (17 μg) and volume (0.15 ppm). Design of the silicon disc weights, procedure of the sub-multiple calibration, and thus determined sinker mass/volume are presented.