Apparatus for wide-ranging, high-accuracy fluid ( p, ρ, T) measurements based on a compact two-sinker densimeter

Abstract

An apparatus for high-accuracy fluid ( p, ρ, T) measurements over wide ranges of temperature, pressure, and density (90 K to 520 K; 0 MPa to 40 MPa; 5 kg · m −3 to 1700 kg · m −3) is described. The densimeter is based on the Archimedes (buoyancy) principle and is a two-sinker system incorporating a magnetic suspension coupling. The density is obtained directly, without the need for calibration fluids. The complete apparatus, including the thermostat, pressure and temperature measuring systems, auxiliary systems, and computer control, is described. The determination of the sinker volumes and calibrations of the measuring systems are detailed. The traceability to national standards and fundamental SI quantities is emphasized. The uncertainty ( k = 2) in density is 0.0025% + 0.0006 kg · m −3 at 293 K and 0.1 MPa. The uncertainty increases to 0.015% + 0.001 kg · m −3 at the upper and lower temperature limits of the apparatus and a pressure of 40 MPa, primarily due to increased uncertainties in the sinker volumes. The uncertainty in temperature is 0.004 K and that in pressure ranges from 0.005% to 0.05%. Density measurements on high-purity nitrogen and helium demonstrate the performance of the densimeter and confirm the stated uncertainties. New data are presented for a high-density fluoroether; these measurements demonstrate the capabilities for liquids.