Calibration of RS41 humidity sensors by using an upper‐air simulator

Abstract The upper‐air simulator (UAS) was developed at the Korea Research Institute of Standards and Science to test commercial radiosonde sensors under representative conditions. In this framework, humidity generation is performed through a two‐temperature mode in which the temperatures of the saturator and the test chamber are controlled independently. The operation range of the UAS in terms of the temperature, dew/frost‐point temperature, and relative humidity (RH) is −70 to +20°C, −80 to +20°C, and 5%–100%, respectively. The total combined uncertainty of the UAS humidity generation is within 1.1 %RH at a coverage factor k = 2. As a proof of concept, the calibration of Vaisala RS41 humidity sensors is demonstrated. The maximum difference between the humidity measured using the RS41 sensors and UAS reference humidity is approximately 3 %RH at −67°C, which gradually decreases to approximately 1 %RH as the temperature is raised to 21°C. The repeatability of a single RS41 unit and reproducibility of three different units are 0.5 and 0.9 %RH, respectively, in terms of the standard deviation. The uncertainty of an RS41 humidity sensor is 0.9–1.4 %RH (k = 2) at −67°C. Realizing the calibration of commercial radiosondes by using the UAS can enhance the traceability to the International System of Units and emphasize the role of metrology for meteorology and climate research.