Characterization of the Humidity Calibration Chamber by Numerical Simulations

At the Centre for Metrology MIKES of VTT Technical Research Centre of Finland (VTT MIKES), we have been developing a humidity calibration apparatus for radiosondes within an EMRP Project Metrology for Essential Climate Variables. The minimum air temperature and absolute humidity are -80°C and 2.576 × 10^-4g·m^-3 (corresponding the dew-point temperature -90°C), respectively. Recent developments for the apparatus extend its pressure operation range down to 7 hPa (abs). When operating in such dry conditions, the efficiency in calibration is highly limited by the time of humidity stabilization in a measurement chamber: Because the water vapor pressure is very low, the adsorption and desorption of water molecules at the chamber walls have a significant effect on the spatial and temporal humidity differences in the chamber. Inhomogeneity in humidity field inside the calibration chamber increases calibration uncertainty. In order to understand how varying parameters such as pressure, temperature, inflow speed and geometry of chamber effect on stabilization time of humidity field, computational fluid dynamics simulations were developed using Comsol software. Velocity and pressure of fluid, water vapor diffusion, temperature as well as adsorption/desorption of water molecules on the chamber walls were included in the simulations. Adsorption and desorption constants for water on the measurement chamber wall were determined experimentally. The results show that the flow speed and the surface area are the dominant parameters affecting the stabilization time of a calibration chamber. It was also discovered that more homogenous water vapor concentration field is obtained at low pressures.