Hygrometric applications of microwave quasi-spherical resonators

In this work, triggered by the initial suggestion and the preliminary results obtained at the National Institute for Standards and Technology (NIST), we have explored the possible application of a microwave resonant cavity to realise a reference standard hygrometer. Particularly, we have measured the microwave resonance frequencies of a small volume (69 cm^3) triaxial elipsoidal cavity to determine the water vapor mole fraction x_w of H2O/N2 and H2O/air mixtures. The mixtures were prepared using the INRiM standard humidity generator for frost-point temperatures T_fp in the range between 241 K and 270 K and a commercial two-pressure humidity generator operated at dew-point temperature T_dp between 272 K and 291 K. Following these tests, we demonstrated the performance of the QSR operated as a condensation hygrometer. For this demonstration, a suitable experiment was prepared in cooperation with the National Physical Laboratory (NPL), which allowed to compare the performance of the QSR with that of a calibrated chilled-mirror hygrometer. Finally, with the aim of reducing the uncertainty currently associated to the literature values of the constants in the Debye equation, we have measured the dielectric constant of pure water vapour in the temperature range from 320 K to 470 K at pressures up to 1.7 MPa. As a whole, the results obtained in these experiments demonstrated the validity and satisfactory performance of the microwave technique for hygrometry.