Catalytic oxidation of H2 on platinum: a robust method for generating low mixing ratio H2O standards

Standard reference samples of water vapor suitable for in situ calibration of atmospheric hygrometers are not currently widespread, leading to difficulties in unifying the calibrations of these hygrometers and potentially contributing to observed measurement discrepancies. We describe and evaluate a system for reliably and quantitatively converting mixtures of H2 in air to H2O on a heated platinum surface, providing a compact, portable, adjustable source of water vapor. The technique is shown to be accurate and can be used to easily and reliably produce a wide range of water vapor concentrations (≈1 ppm −2 %) on demand. The result is a H2O standard that is expected to be suitable for in situ calibration of aircraft hygrometers, with an accuracy nearly that of the available H2 standards (≈±2 %).