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 < < 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 , via measurement at 293.1529(13) K and = 632.9908(2) nm for ≤ 500 kPa. The authors give values of the coefficients , , for six gases: Ne, Ar, Xe, N, CO, and NO. For each gas, the resulting molar polarizability has a standard uncertainty within 16 × 10· . 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 with helium and subsequently use the transducer to accurately gage the relationship between pressure and refractivity on an isotherm for other gases of interest.