REENTRANT RADIOFREQUENCY RESONATOR FOR AUTOMATED PHASE-EQUILIBRIA ANDDIELECTRIC MEASUREMENTS IN FLUIDS

A reentrant rf cavity resonator has been developed for automated detec tion of phase separation of fluid mixtures contained within the cavity . Successful operation was demonstrated by redetermining the phase bou ndaries of a CO2+C2H6 mixture in the vicinity of its critical point. W e developed an accurate electrical model for the resonator and used he lium to determine the deformation of the resonator under pressure. Wit h the model and pressure compensation, the resonator was capable of ve ry accurate dielectric measurements. We confirmed this by remeasuring the molar dielectric polarizability A(epsilon) of argon and obtained t he result A(epsilon)=(4.140+/-0.006) cm(3)/mol (standard uncertainty) in excellent agreement with published values. We exploited the capabil ity for accurate dielectric measurements to determine the densities of the CO2+C2H6 mixture at the phase boundaries and to determine the dip ole moment of 1,1,1,2,3,3-hexafluoropropane, a candidate replacement r efrigerant. Near the operating frequency of 375 MHz the capacitor in t he resonator has an impedance near 14 Ohm. This low impedance is more tolerant of electrical conductivity within the test fluid and in paral lel paths in the support structures than comparable capacitors operati ng at audio frequencies. This will be an advantage for operation at hi gh temperatures where some conductivity must be expected in all fluids . Of further value for high-temperature applications, the present rf r esonator has only two metal-insulator joints. These joints seal coaxia l cables; neither joint is subjected to large mechanical stresses and neither joint is required to maintain precise dimensional tolerances. The resonator is rugged and may be operated with inexpensive electroni cs.