Determining phase boundaries and vapour/liquid critical points in supercritical fluids: A multi-technique approach

We describe an apparatus used to make precise determinations of the coexistence regimes and critical points of single, binary, and multi-component fluids. The design allows semi-automated measurements to be made over a pressure range of 1-300 bar and a temperature range of-20 to +120°C with a stability of ±2 mK. We describe the use of the apparatus in determining the critical points and phase boundaries of single component and binary mixtures through a number of optical, acoustic, and shear-mode piezoelectric sensor methods. We conclude that the accurate determination of bubble-point lines, dew-point lines and critical points is best achieved by making measurement using a number of complementary techniques.