Oxygen sensitivity of krypton and Lyman-(alpha) hygrometers

The oxygen sensitivity of krypton and Lyman-alpha hygrometers is studied. Using a dewpoint generator and a controlled nitrogen/oxygen flow the extinction coefficients of five hygrometers associated with the third-order Taylor expansion of the Lambert-Beer law around reference conditions for oxygen and for water vapor were measured. Latent heat flux corrections for cross-talk of the sensible heat flux are given as a function of the Bowen ratio. The spread observed in oxygen sensitivities calls for individual oxygen calibrations for each apparatus. It is found that the separation between the tubes is the crucial parameter with respect to the oxygen sensitivity. The oxygen-related nonlinearities in the Lambert-Beer law are stronger than those associated with water vapor. This leads to a strong reduction of the oxygen sensitivity at a more or less constant water vapor sensitivity when the separation between the tubes is doubled from 1.3 to 2.6 cm. This characteristic is supported by a sensitivity analysis of the theoretical response relations of both types of hygrometers, based on multiple emission and absorption lines. The findings of Tanner et al. are confirmed--the results of a previous study strongly overestimate the oxygen sensitivity of today's krypton hygrometers.