Determination of the absolute density of water at 16 °C and 0,101 325 MPa

Abstract

The absolute density of pure water was determined with an accuracy better than 1 part in 106. The method used was hydrostatic weighing in which a fused-quartz sphere, whose volume was measured by optical interferometry, was weighed in water. The density obtained in the hydrostatic weighing was corrected for impurity effects using differential densimetry and, using mass spectrometry, was further corrected for isotopic composition. A total of eighteen samples was measured using three spheres. The density of pure water at 16 °C (ITS-90) and 0,101 325 MPa having isotopic composition equal to that of Standard Mean Ocean Water was found to be 998,9468 kg m-3 with a combined standard uncertainty of 0,0006 kg m-3. The maximum density at about 4 °C, as calculated from our previous measurements of thermal expansion, was found to be 999,9756 kg m-3 with an uncertainty of 0,0008 kg m-3.