A Saturation Hygrometer for the Measurement of Relative Humidity Between 95 and 105%

Abstract

A new saturation hygrometer capable of measuring relative humidity between 95 and 105% is described. The hygrometer uses a thermally thin mirror on which condensation is controlled by limiting the exchange of ambient water vapor to condensation sites consisting of sized submicron salt nuclei. The salt-solution droplets formed on the nuclei are prevented from exceeding a reference size by heating the mirror with infrared diodes which are part of a thermo-optical servo system. The temperature increase of the mirror is directly related to the ambient relative humidity. The performance of the hygrometer is modeled for the hypothetical case where the instrument is imbedded in an updraft which passes through the base and the lower portion of a typical continental cumulus cloud. The influences of the thermal properties of the mirror, the heat conduction to the mirror support, the radiative energy exchange between the mirror and the environment, the size of the salt nuclei on the mirror, and the interaction with the cloud droplets are evaluated. The hygrometer is tested in the laboratory by measuring temperature increases of the mirror for infrared-heater outputs, and by exposing it to supersaturations produced by a continuous-flow thermal-gradient diffusion chamber. The theoretical results, the stability of the hygrometer under conditions of supersaturation, its insensitivity to ambient aerosols, and its ease of calibration together indicate that the hygrometer can be developed into a practical sensor for atmospheric use.