Development of a low-temperature low-pressure humidity chamber for calibration of radiosonde humidity sensors

Abstract

A low-temperature low-pressure humidity chamber has been developed at the Korea Research Institute of Standards and Science for the calibration of radiosonde humidity sensors. The humidity generator operates in a two-temperature and two-pressure mode in which a saturator and a test chamber are separately immersed in two baths. The saturation performance of the saturator is first evaluated at a low temperature (−75 °C) and at atmospheric pressure. In this condition, the uncertainty of the frost-point temperature by the saturator is 53 mK with the coverage factor k = 2. Then, a chilled mirror hygrometer is calibrated for the compensation of low-pressure effects such as a pressure drop along the tubing line inside the hygrometer. Significant differences between the frost-point temperature at the hygrometer inlet and that measured by the hygrometer are observed at a pressure lower than 20 kPa. An empirical formula for compensating the low-pressure effects on the hygrometer is obtained and, subsequently, is used for the validation of the frost-point temperature generated at a low temperature and low pressure in the test chamber. The corresponding uncertainty due to the calibration of the hygrometer including the compensation of low-pressure effects is 306 mK at k = 2. A representative humidity generation and its validation by the hygrometer is demonstrated at a temperature (−70 °C) and two pressures (4.6 kPa and 20 kPa) in the test chamber. Uncertainty budgets on relative humidity in the test chamber are presented from 10 %rh to 40 %rh with a 10 %rh interval at both pressures. Consequently, the combined uncertainty is less than 2%rh (k = 2) which includes uncertainty factors such as the saturation efficiency, the adsorption/desorption hysteresis, the validation by the hygrometer measurement at low pressures, and so on. The newly developed humidity chamber enables the calibration of radiosonde humidity sensors at low temperature and low pressure conditions that mimic upper air environments in a traceable manner to the International System of Units (SI).

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

“…The saturation vapour pressure of water is controlled by the temperature and pressure of the saturator. The saturator system employed in this study has been described previously by Lee et al (2019a)). Radiosondes are installed in the test chamber inside the climate chamber.…”

“…Subsequently, the saturator generates a well-defined partial water vapour pressure in the gas depending on its temperature and pressure (Choi et al, 2015). Three and five sub-saturator coils made of stainless steel are present in the pre-saturator and main saturator, respectively (Lee et al, 2019a). Owing to the separation of the main saturator from the pre-saturator, most of the heat exchange between the gas and ice/water occurs in the pre-saturator.…”

“…Table 3 summarizes the uncertainty budget for relative humidity generation by the UAS. The same models of the saturator, thermostatic bath, and thermometers as those used in our former work are adopted (Lee et al, 2019a). Therefore, the uncertainty values related with the saturator are consistently available.…”

“…The slow response of humidity sensors, especially at low temperatures, induces a time lag in the measurement (Miloshevich et al, 2004). To evaluate the calibration uncertainty, several groups have reported the development of low‐temperature low‐pressure humidity chambers, including KRISS (Lee et al, 2019a), the Istituto Nazionale di Ricerca Metrologica (INRiM) in Italy (Cuccaro et al, 2018), and the VTT in Finland (Sairanen et al, 2014, 2015). According to the GRUAN, the required accuracy of the water vapour measurement in the upper air is 2% in terms of the mixing ratio (GCOS, 2007).…”

Development of upper air simulator for the calibration of solar radiation effects on radiosonde temperature sensors

“…Although the saturator is not used in the current study, the UAS can potentially control the humidity in the test chamber by using the saturator in the thermostatic bath. The potential operation scheme for humidity control in the UAS test chamber was described in a recent report in which a two-temperature, two-pressure humidity generator was used for humidity generation and its validation at low temperatures and low pressures (Lee et al, 2019).…”