Advances in humidity measurement applications in metrology: Dissertation

A significant part of the development of humidity measurement applications in metrology is carried out due to the climate change. Within this thesis metrology tools for humidity measurements in energy gases were developed. In addition, a new method and apparatus for radiosonde calibrations were developed and constructed to provide traceability to the System of Units (SI) and thus improve the quality of radiosonde measured humidity data. Metrology tools for humidity measurements of energy gases were developed by studying the water vapour enhancement factor for methane. A new hygrometer-based calibration method was developed and a set-up was assembled for calibrations of liquid microflows from syringe pumps. Metrological validation for the set-up including uncertainty analysis was carried out for flow rates from 0.1 ul min-1 to 10 ul min-1. The set-up was applied in development and construction of a novel apparatus to measure the enhancement factor. The apparatus was metrologically validated by air and methane measurements for pressures up to 6 MPa and dew/frost-point temperatures from -50 °C to +15 °C. Utilising this apparatus, new experimental enhancement factor data was measured. Along with literature data on equilibrium states of water vapour in methane, new literature-based enhancement factors were calculated. The experimental and the calculated data were combined and an equation expressed as a function of pressure and dew-pointe temperature was fitted to the data. The equation covers dew/frostpoint temperature range from -23 °C to +20 °C and pressures below 7 MPa and its expanded uncertainty (k = 2) is 0.23 in the whole range. Radiosondes operate over a wide range of humidity, temperature and pressure values and thus their calibrations should also cover these conditions. However, calibrations performed with traditional methods at cold temperatures and low humidity require a lot of time and therefore a customized apparatus was developed and constructed. The new apparatus enables significantly shorter calibration times within the temperature range from -80 °C to +20 °C and the dew/frost-point temperature range between -90 °C and +10 °C. The apparatus fulfils the requirements set by Global Climate Observing System (GCOS) and its GCOS Reference Upper-Air Network (GRUAN).