  Dual Fabry-Perot cavity based optical refractometry (DFPC-OR) has a high
potential for assessments of gas density. However, drifts of the FP cavity
often limit its performance. We show that by the use of two narrow-linewidth
fiber lasers locked to two high finesse cavities and Allan-Werle plots that
drift-free DFPC-OR can be obtained for short measurement times (for which the
drifts of the cavity can be disregarded). Based on this, a novel strategy,
termed fast switching DFPC-OR (FS-DFPC-OR), is presented. A set of novel
methodologies for assessment of both gas density and flow rates (in particular
from small leaks) that are not restricted by the conventional limitations
imposed by the drifts of the cavity are presented. The methodologies deal with
assessments in both open and closed (finite-sized) compartments. They
circumvent the problem with volumetric expansion, i.e. that the gas density in
a measurement cavity is not the same as that in the closed external compartment
that should be assessed, by performing a pair of measurements in rapid
succession; the first one serves the purpose of assessing the density of the
gas that has been transferred into the measurement cavity by the gas
equilibration process, while the 2nd is used to automatically calibrate the
system with respect to the relative volumes of the measurement cavity and the
external compartment. The methodologies for assessments of leak rates comprise
triple cavity evacuation assessments, comprising two measurements performed in
rapid succession, supplemented by a 3rd measurement a certain time thereafter.
A clear explanation of why the technique has such a small temperature
dependence is given. It is concluded that FS-DFPC-OR constitutes a novel
strategy that can be used for precise and accurate assessment of gas number
density and gas flows under a variety of conditions, in particular
non-temperature stabilized ones.
