The absolute density of pure water was determined with an accuracy better than 1 part in . 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 0,101325 MPa having isotopic composition equal to that of Standard Mean Ocean Water was found to be 998,9468 kg m with a combined standard uncertainty of 0,0006 kg m . The maximum density at about 4 C, as calculated from our previous measurements of thermal expansion, was found to be 999,9756 kg m with an uncertainty of 0,0008 kg m . * Method of exact fractions.** Temperature = 16 C, pressure = 0,101 325 MPa, no water vapour.
The volume determination of fused quartz spheres used for the absolute measurement of the density of water is described. The diameters of the spheres are measured with an optical interferometer, in which the sphere is placed between two parallel Ctalon plates of accurately known separation, and the diameters are obtained by measuring the gaps between each etalon surface and the sphere. Two wavelengths, 633 nm of a frequency stabilized He-Ne laser and 441 nm of a He-Cd laser, are employed in order to use the method of exact fractions. Details are given of a special arrangement for the interferometer which overcomes the low reflectivity of the surface of the fused quartz. A method to eliminate the spurious effect due to the transparency of the quartz sphere is also described. The diameters (about 85 mm) have been measured with standard deviations of 5 to 12 nm, which correspond to 0.16 to 0.43 ppm ' in terms of the volume. The total uncertainties of the volume are estimated to be 0.26 to 0.48 ppm.
The thermal expansion of pure water having natural isotopic abundance was determined by the dilatometric method in a temperature range from 0 to 85 "C and under a pressure of 101 325 Pa. The following equation was obtained :98152)2 (t + 396,18534) (t + 32,28853) 609 628,6(t + 83,12333) (t + 30,24455) 'where g(r) is the density of water at temperature t, which is expressed in terms of the ITS-90, and Qmax the maximum density. The density ratio which the above equation gives is estimated to have an uncertainty of approximately 1 x 10-6.
The effect of the diffraction of spherical light waves and Gaussian beams in a Saunders-type interferometer was analysed numerically to supplement our previous report on the measurement of the absolute density of water.In that work, buoyancy measurements were taken for fused-quartz spheres submerged in water. In determining the volume of these spheres, a Saunders-type interferometer was used to measure the diameter. However, it was suspected that distortion of wave fronts due to diffraction may have occurred in the interferometer, which could introduce a bias in the sphere volume, and hence in the measured water density.The numerical analysis here shows that although there was significant distortion of the interference pattern near the optical axis, it did not impair diameter measurements, as long as the method of data analysis originally described by Saunders was used. Therefore, our measured values for the absolute density of water remain valid.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.