Accurate vapor pressure measurements of supercooled water in the temperature range between 252 K and 273 K

The accurate knowledge of the saturation vapor pressure of liquid water and ice has a great importance in a host of applications where humidity measurements are involved since it enables the correct conversion among different physical quantities that quantify the water vapor amount of a humid gas mixture. More specifically, accurate saturation vapor pressure measurements of supercooled water are strongly required in meteorology and atmospheric sciences, especially in cloud microphysics studies. Considering the importance of this kind of measurements and the limited availability of experimental data in the literature, further measurements in the metastable region were carried out at the Istituto Nazionale di Ricerca Metrologica (INRIM). The sample cell used in the experiment is a U-shaped capillary tube made of borosilicate glass, whose temperature is kept constant by immersion in a thermostatic bath with a high temperature stability (about 1 mK). The cell is connected to a capacitive diaphragm pressure gauge measuring the equilibrium water vapor pressure, that varies between about 116 Pa at T = 252.25 K and 615 Pa at T = 273.25 K. In the present work, the measuring method and the experimental setup are described and the measurements results reported. The results obtained in the whole temperature range are compared with those available in the literature and with the commonly adopted saturation vapor pressure formulations. A complete uncertainty budget is also given, resulting in an expanded relative uncertainty ranging from 0.06% at 273.25 K to 0.20% at 252.25 K.