Arh. Goodwin et al., REENTRANT RADIOFREQUENCY RESONATOR FOR AUTOMATED PHASE-EQUILIBRIA ANDDIELECTRIC MEASUREMENTS IN FLUIDS, Review of scientific instruments, 67(12), 1996, pp. 4294-4303
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.