A method to measure the density of seawater accurately to the level of 10 −6

A substitution method to measure seawater density relative to pure water density using vibrating tube densimeters was realized and validated. Standard uncertainties of 1 g m −3 at atmospheric pressure, 10 g m −3 up to 10 MPa, and 20 g m −3 to 65 MPa in the temperature range of 5 °C to 35 °C and for salt contents up to 35 g kg −1 were achieved. The realization was validated by comparison measurements with a hydrostatic weighing apparatus for atmospheric pressure. For high pressures, literature values of seawater compressibility were compared with substitution measurements of the realized apparatus.