Special issue on density

Abstract

This special issue on density was undertaken to provide readers with an overview of the present state of the density standards for solids, liquids and gases, as well as the technologies developed for measuring density. This issue also includes topics on the refractive index of gases and on techniques used for calibrating hydrometers so that almost all areas concerned with density standards are covered in four review articles and seven original articles, most of which describe current research being conducted at national metrology institutes (NMIs). A review article was invited from the Ruhr-Universität Bochum to highlight research on the magnetic suspension densimeters.In metrology, the determinations of the volume of a weight and the density of air are of primary importance in establishing a mass standard because the effect of the buoyancy force of air acting on the weight must be known accurately to determine the mass of the weight. A density standard has therefore been developed at many NMIs with a close relation to the mass standard. Hydrostatic weighing is widely used to measure the volume of a solid. The most conventional hydrostatic weighing method uses water as a primary density standard for measuring the volume of a solid. A brief history of the determination of the density of water is therefore given in a review article, as well as a recommended value for the density of water with a specified isotopic abundance.The most modern technique for hydrostatic weighing uses a solid density standard instead of water. For this purpose, optical interferometers for measuring the diameters of silicon spheres have been developed to convert the length standard into the volume standard with a small uncertainty. A review article is therefore dedicated to describing the state-of-the-art optical interferometers developed for silicon spheres. Relative combined standard uncertainties of several parts in 108 have been achieved today for measuring the volume and density of silicon spheres. These technologies are currently being used not only for establishing a solid density standard, but also for determining the Avogadro constant by the x-ray crystal density method, where the density, molar mass and lattice constant of a silicon crystal are measured based on the definition of the SI units. Considering that much of the present research on the Avogadro constant has been undertaken to replace the present definition of the kilogram with a new definition based on a number of atoms, it is satisfying to note that the most accurate density standard may contribute to a new definition of the kilogram.Differential density measurements by hydrostatic weighing and by the pressure of flotation method developed for measuring the density differences between silicon crystals and solids are given in a review article and three original articles, where combined standard uncertainties of a few parts in 108 have been achieved in measuring relative density differences. These technologies are being used not only for the determination of the Avogadro constant, but also for evaluating defects in silicon crystals used in the semiconductor industry.Another important liquid used in the density standard is mercury because the pressured standard determined from mercury column barometers, the molar gas constant determined from an acoustic resonator, and the Josephson constant determined from a mercury voltmeter are all dependent on the density of mercury. A review article is therefore dedicated to an overview of the history, recommended value and recent progress in the measurement of the density of mercury.This special issue also features the technologies developed for measuring the thermodynamic properties of fluids. New instruments with a magnetic suspension balance have substantially improved the uncertainty in measuring the density of fluids at elevated pressures and temperatures. Two review articles and an original article are therefore dedicated to describing the history, principle and recent progress in magnetic suspension densimeters. When the diamagnetic properties of organic liquids are taken into account, an unprecedented uncertainty of a few parts in 106 can be achieved for density measurements at high pressures.The papers collected in this special issue reflect the general trends in research on density standards. I hope that this special issue will stimulate new directions of research in the density standard, and also help to create more accurate, reliable and convenient methods for many applications. Thanks are addressed to all the authors who have contributed to this special issue on density, to the referees who have given their time, and also to the former editor Professor Martin and the present editor Dr~Williams for their support and useful suggestions in collecting the articles.