The selection of water property formulae for volume and flow calibration

摘要

When employing either gravimetric or volumetric methods for the calibration of flowmeters, volumetric measures and proving tanks, knowledge of the density of water is required. In addition, when measuring flow of water and calibrating flow meters, the compressibility, viscosity and speed of sound may also be needed. In the scientific literature, different formulae are used to express the properties of water. This is particularly noticeable in the case of equations for the density of water as given in standards. Different expressions have been referenced and included in International Standards Organisation, Organisation for International Legal Metrology documents and petroleum measurement standards from the American Petroleum Institute and the Energy Institute based on when the standard was prepared or which group prepared it. This variation does not generally give rise to significant differences in density; however, inconsistencies may cause problems for trade when small differences in the value of a product occur. In this paper, some of the most commonly referenced expressions have been given and compared. Finally, recommendations have been suggested to allow the most appropriate selection of formulae to suit the application. The effect of contamination has been examined and this has been extended to include salt water, which now requires high standards of measurement for the oil industry. The aim of the paper is to provide a set of practical formulae which can be used to reduce differences in measurement, ensure low uncertainties for measurement and provide an understanding of and reference source for the identification of unattributed formulae where they occur. The practical formulae provided here will apply equally to measurement at the highest levels of volume determinations, through to industrial and scientific volume and flow measurement applications using water.