Measured relationship between thermodynamic pressure and refractivity for six candidate gases in laser barometry

Laser refractometers are approaching accuracy levels where gas pressures in the range 1Pa < p < 1 MPa inferred by measurements of gas refractivity at a known temperature will be competitive with the best existing pressure standards and sensors. Here, the authors develop the relationship between pressure and refractivity p = c(1) . (n - 1) thorn c(2) , (n - 1)(2) + c(3) . (n - 1)(3)+ ..., via measurement at T = 293: 1529(13) K and lambda = 632: 9908(2) nm for p <= 500 kPa. The authors give values of the coefficients c(1), c(2), c(3) for six gases: Ne, Ar, Xe, N-2, CO2, and N2O. For each gas, the resulting molar polarizability A(R) 2RT/3c(1) has a standard uncertainty within 16 x 10(-6) . A(R). In these experiments, pressure was realized via measurements of helium refractivity at a known temperature: for He, the relationship between pressure and refractivity is known through calculation much more accurately than it can presently be measured. This feature allowed them to calibrate a pressure transducer in situ with helium and subsequently use the transducer to accurately gage the relationship between pressure and refractivity on an isotherm for other gases of interest.