Measured relationship between thermodynamic pressure and refractivity for six candidate gases in laser barometry

Abstract

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=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 ≤ 500 kPa. 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.

The Short-Term Performances of Two Independent Gas Modulated Refractometers for Pressure Assessments

“…For more detailed theoretical descriptions of the Lorentz–Lorenz equation and the equation of state, and for expressions valid for higher pressures, the reader is referred to the literature, e.g., [ 2 , 5 , 16 , 27 , 29 , 30 , 31 , 32 ].…”

Refractive-index gas thermometry

“…Recent, highly-precise measurements of the refractivity of Ne, Ar, Xe, N 2 , CO 2 , and N 2 O at the optical wavelength λ = 633 nm are traceable to the calculated refractivity of helium at the same wavelength. This work yields the dynamic molar polarizability of these gases at optical frequencies with a relative standard uncertainty of 1.6 × 10 −5 , thereby advancing the prospect of optical RIGT at pressures less than 500 kPa (Egan et al 2019).…”

Экспериментальная Реализация Спектрального Сенсора Коэффициента Преломления На Основе Отражательного Интерферометра

“…Как известно, коэффициент преломления воздуха с высокой степенью линейно пропорционален давлению по закону n a (p) = n a0 − c p при давлениях меньше атмосферного [7], где n a0 = 1.0002926 RIU, c = 2.8 • 10 −9 RIU/Pa, p -давление. Существуют оптические методы измерения давления -например, основанные на изменении набега фазы в интерферометрах из-за изменения коэффициента преломления [8,9].…”