Design and performance of an absolute gas refractometer based on   quasi-synthetic wavelength method
  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. 
Jitao Zhang, Pei Huang, Yan Li and Haoyun Wei
physics.ins-det physics.optics