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 1 Pa < 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 $p=c_1\cdot (n-1)+c_2\cdot {(n-1)}^2+c_3\cdot {(n-1)}^3+\cdots$ , via measurement at T = 293.1529(13) K and λ = 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, N₂, CO₂, and N₂O. For each gas, the resulting molar polarizability $A_R\equiv \frac{2RT}{3c_1}$ has a standard uncertainty within 16 × 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.