Automated isochoric apparatus for the measurement of density of fluid mixtures at temperatures from 298.15 K to 773.15 K and pressures up to 40 MPa: results for helium and for nitrogen

A new fully automated apparatus for the measurement of the densities of gaseous fluid mixtures is described. The apparatus is intended for use in the temperature range 298.15 K to 773.15 K at pressures up to 40 MPa. The instrument is based upon the principle of near-isochoric measurement using a spherical pressure vessel and considerable care has been taken to ensure high precision in each of the primary measured quantities and in the evaluation of the volume changes of the pressure vessel with both temperature and pressure. In particular, a special interface vessel was included in the design which serves to isolate the pressure transducer from the test fluid and its varying conditions of temperature, while simultaneously reducing the spurious volumes in the system. As a result of these features it is estimated that the uncertainty in the measurement of gaseous densities is typically ±0.05 per cent, although it will depend upon the substance studied as well as the conditions. Experimental results for the amount-of-substance density are presented for helium along three isochores and for nitrogen along five isochores. In both, the measurements serve to confirm the correct and satisfactory operation, but for nitrogen the present results at the highest temperatures yield amount-of-substance densities with an error that is estimated to be smaller than those of earlier studies.