Determination of distortion corrections for a fixed length optical cavity pressure standard

Optical gas refractometry has enabled new pressure standards to be developed based on a dual Fixed Length Optical Cavity (FLOC) system. NIST in CRADA (collaboration research and development agreement) partnership with MKS Instruments has created a portable FLOC pressure standard based on gas refractivity. A key challenge for accurate measurements is the characterization of cavity distortions when pressurized. A method for determination of the distortion constants is presented based on using two gases of known refractive index. Using this two-gas technique, the distortions on the portable FLOC pressure standard are corrected so that this correction contributes no more than 1 μPa/Pa of uncertainty (k = 1).