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<1MPa 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=c1⋅(n−1)+c2⋅(n−1)2+c3⋅(n−1)3+⋯, via measurement at T=293.1529(13)K and λ=632.9908(2)nm for p≤500kPa. The authors give values of the coefficients c1,c2,c3 for six gases: Ne, Ar, Xe, N2, CO2, and N2O. For each gas, the resulting molar polarizability AR≡2RT3c1 has a standard uncertainty within 16×10−6⋅AR. 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.