Dew points for hydrogen-rich (hydrogen plus propane) and (hydrogen plus n-butane) mixtures determined with a microwave re-entrant cavity resonator

Dew-point measurements were performed for hydrogen-rich binary mixtures with hydrocarbon mole fractions of 0.09995 C3H8, 0.1499 C3H8, 0.0513 n-C4H10, and 0.09888 n-C4H10 using a modified microwave re-entrant cavity apparatus. Isochoric measurements were conducted in the temperature range of (256 to 286) K with pressures up to 6.8 MPa and 2.0 MPa for mixtures containing propane and n-butane, respectively. The combined expanded uncertainties (k = 2) in dew-point temperature and pressure were estimated to be between (0.35 and 2.0) K and (0.0011 and 0.030) MPa. The agreement with predictions of the GERG-2008 equation of state is within 5.5%. However, better agreement with predicted values was achieved using a cubic equation recently developed inhouse. Moreover, experiments with short- and long-term exposure of pressure sensors to hydrogen have shown that pressure measurement, and thus the uncertainty, can be affected; further investigation on this matter is therefore inevitable to improve hydrogen thermophysical property data.