  We present a refractometer which is capable of measuring the refractive index
of gas with an unambiguous range of 1.000395 and uncertainty of 3.1E-8 at 633
nm absolutely. The measurement range is extended by means of a group of vacuum
tubes according to the principle of quasi-synthetic wavelength (QSW) method.
The basic principle of the QSW method and the design of the gas refractometer
are presented in detail. The performance of the refractometer has been verified
by the measurement of dry air, nitrogen gas and ambient air under different
environmental situations. The gas-filling or pumping process is not needed
during the measurement, so that we can complete a measurement within 70
seconds. Comparing with previous ones, the refractometer reported here has
integrated virtues of large unambiguous range, fast speed, high accuracy, and a
simple instrumentation design.
