  Dual Fabry-Perot-Cavity-based Optical Refractometry (DFCB-OR) have been shown
to have excellent potential for characterization of gases, in particular their
refractivity and density. However, its performance has in practice been found
to be limited by drifts. To remedy this, drift-free DFPC-OR (DF-DFCB-OR) has
recently been proposed. Suggested methodologies for realization of a specific
type of DF-DFCB-OR, termed Fast Switching DFCB-OR (FS-DFCB-OR), have been
presented in an accompanying work. This paper scrutinizes the performance and
the limitations of both DF- and FS-DFCB-OR for assessments of refractivity and
gas density, in particular their precision, accuracy, and temperature
dependence. It is shown that both refractivity and gas density can be assessed
by FS-DFCB-OR with a precision in the 10$^{-9}$ range under STP conditions. It
is demonstrated that the absolute accuracy is mainly limited by the accuracy by
which the instantaneous deformation of the cavity or the higher order virial
coefficients can be assessed. It is also shown that the internal accuracy, i.e.
the accuracy by which the system can be characterized with respect to an
internal standard, can be several orders of magnitude better than the absolute.
It is concluded that the temperature dependence of FS-DFCB-OR is exceptionally
small, typically in the 10$^{-8}$ to 10$^{-7}$/C range, and primarily caused by
thermal expansion of the FPC-spacer material. Finally, this paper discusses
means on how to design a FS-DFCB-or system for optimal performance and
epitomizes the conclusions of this and our accompanying works regarding both
DF- and FS-DFCB-OR in terms of performance and provides an outlook for both
techniques. Our works can serve as a basis for future realizations of
instrumentation for assessments of gas refractivity and density that can fully
benefit from the extraordinary potential of FPC-OR.
