Optical n(p, T_90) measurement suite 1: He, Ar, and N_2

Published

November 23, 2023

Author(s)

Patrick Egan, Yuanchao Yang

Abstract

An n(p, T90) measurement suite is reported for the gases helium, argon, and nitrogen. The methodology is optical refractive-index gas metrology, operating at laser wavelength 633 nm and covering the temperature range (293 < T < 433) K and pressures p < 0.5 MPa . The measurement suite produces several things of thermophysical interest. First, the helium dataset deduces the effective compressibility of the apparatus with a relative standard uncertainty of 1.3 × 10^-4 . Next, the argon dataset determines T - T90 with a relative standard uncertainty of about 3 µK·K^-1. (The implementation is relative primary thermometry; T - T90 is the difference between thermodynamic temperature and ITS-90.) Finally, the nitrogen dataset estimates the temperature dependence of polarizability within 3.5 % relative standard uncertainty. As a by-product of the nitrogen and argon measurements, values of the second density virial coefficient B_p(T) are derived with uncertainties smaller than those of previous experiments. More broadly, the work enables conversion of a measured refractivity at known temperature to optical pressure within 3.5 µPa·Pa^-1 across the stated range, albeit traceable to the diameter of a piston-gage.

Citation

International Journal of Thermophysics

Pub Type

Journals

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Keywords

Gas thermometry, Polarizability, Refractometry, Thermodynamic metrology

Citation

Egan, P. and Yang, Y. (2023), Optical n(p, T_90) measurement suite 1: He, Ar, and N_2, International Journal of Thermophysics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=956156 (Accessed October 9, 2025)