Evaluating Calibration Uncertainty and Response Time of RS41 Humidity Sensors Under a Ventilation Speed of 5 m s−1

Abstract

Some commercial radiosondes use heating-type humidity sensors to prevent condensation and improve response time during soundings. However, the heating process affects the temperature and relative humidity (RH) in the ground facilities where they are tested. In this study, we conduct a test of the humidity sensor in a commercial radiosonde (Vaisala RS41) to assess its RH measurements and response time. An upper air simulator (UAS) is used to control the air ventilation speed to 5 m s⁻¹ and adjust the ventilation direction to 0°, 45°, and 90° relative to the boom plane, thereby inducing convective cooling relevant to sounding conditions for testing heated humidity sensors. The temperature and RH ranges covered by our tests were −67°C to +20°C and 10% rh to 90% rh, respectively. Results indicate that the temperature measured in the test cell by a calibrated reference thermometer aligns with the temperature measured by the RS41 temperature sensor within their respective uncertainties. The mean difference in RH between the UAS and three RS41 units is less than 3.1% rh, with a maximum standard deviation of 2% rh. Furthermore, the response time of the RS41 humidity sensors during water sorption and desorption was measured. The response curves are fitted using a double exponential function with two time constants (short and long ones). As the test temperature decreases, both time constants increase. The response curves are formulated for their reconstruction and subsequently time-lag correction. The results of this work can contribute to enhance the traceability of radiosonde RH measurements to the International System of Units.