Development of a double cap on the humidity sensor in radiosondes for improving ventilation

Although a protective cap on the humidity sensor of radiosondes is an effective tool to prevent the sensor from wetting by rain and/or clouds, the cap is thought to be responsible for poor ventilation and thereby induces a time delay in the measurement of relative humidity in upper air. In the present study, a new type of the protective cap having a double cap configuration has been developed for improving the ventilation of air. The double cap consists of an outer cap and an inner cap with holes and windows on the top and sides of each cap, respectively, that are designed not to face each other, between two caps for blocking the penetration of water droplets. The wetting of the double cap was tested by artificial water droplets as well as during flying tests on rainy days and the humidity sensor in a double cap was not wet in both cases. In addition, the response of sensors to indoor humidity changes as well as during balloon-borne measurements was found to be faster in the case of double caps than conventional closed caps, suggesting the enhancement of ventilation when using the double cap. The present study provides an efficient way to improve the performance of humidity sensors of radiosondes by simply modifying the design of protective caps.