  A novel realization of Gas Modulation Refractometry (GAMOR) that outperforms
the original realization [Single Cavity Modulated GAMOR (SCM-GAMOR)], is
presented. The reference measurements are carried out by equalizing the
pressures in the two cavities. By this, the time it takes to reach adequate
conditions for the reference measurements has been reduced. This implies that a
larger fraction of the measurement cycle can be devoted to data acquisition,
which reduces white noise and improves on short-term characteristics. The
presented realization also encompasses a new cavity design with improved
temperature stabilization and assessment. This has contributed to an improved
long-term characteristics. The system was characterized with respect to a dead
weight piston gauge. For short integration times (up to 10 min) it can provide
a response that exceeds that of the original SCM-GAMOR system by a factor of
two. For longer integration times, and up to 18 hours, the system shows, at
4303 Pa, an integration time independent Allan deviation of 1 mPa
(corresponding to a precision defined as twice the Allan deviation, of 0.5
ppm). This implies that the novel system shows a significant improvement with
respect to the original realization for all integration times (by a factor of 8
for an integration time of 18 hours). When used for low pressures, it can
provide a precision in the sub-mPa region; for the case with an evacuated
measurement cavity, the system provided, up to ca. 40 measurement cycles (ca.
1.5 hours), a white-noise limited noise of 0.7 mPa sqrt(cycle), and minimum
Allan deviation of 0.15 mPa. Furthermore, over the pressure range investigated,
i.e. in the 2.8 - 10.1 kPa range, it shows, with respect to a dead weight
piston gauge, a purely linear response. This implies that the system can be
used for transfer of calibration over large pressure ranges with exceptional
low uncertainty.
