Primary Gas Pressure Standard Passes Next Stress Test

Abstract

In 2020, Nature Physics 16, 177 (2020) presented a primary gas‐pressure standard, based on temperature and electrical measurements and ab initio calculations of the thermophysical properties of helium. It is compared against the world's most accurate primary mechanical pressure standard and the test is successful with a relative uncertainty of 5 parts per million (ppm) at about 7 MPa. This is feasible for two reasons. First, the experimental setup developed for the determination of the Boltzmann constant is capable of achieving a combined relative uncertainty on the level of 2 ppm and the primary mechanical pressure standard 1 ppm. The latter consists of two pressure balances with five piston‐cylinder assemblies. Second, the uncertainties of the theoretical calculations in 2020 are on the level of about 4 ppm. Within the last two years, significant improvement has been achieved for all theoretical quantities involved, and the uncertainty contribution of the theory is below the level of 1 ppm. With doubled sensitivity, the thermodynamic gas‐pressure standard is compared to the most accurate mechanical pressure standard. This stress test for theory and experiment is once again successful.

Three-body potential and third virial coefficients for helium including relativistic and nuclear-motion effects

“…The improved accuracy of the theoretical values of C ( T ) presented here resulted in a more accurate primary standard for pressure based on measurement of gas properties. 105…”

Four-Body Nonadditive Potential Energy Surface and the Fourth Virial Coefficient of Helium

“…The Hamiltonian matrix includes interactions of the permanent atomic dipoles and pseudostates of atoms 1 and 2 with those of atoms 3 and 4. Atoms in a pair, (1,2) and (3,4), are assumed not to polarize each other. This asymptotic function involves a quadrupole−quadrupole interaction energy, which decreases as the inverse fifth power of the distance from 1−2 to 3−4.…”

“…In recent years, the accuracy of these temperature and pressure determinations has been greatly improved by the ability to compute properties of noble gases, particularly helium, at low and moderate pressures based on ab initio quantum calculations. 1 Example applications include a primary gas-pressure standard with relative uncertainties as small as 5 ppm (1 ppm = 10 −6 ) at pressures up to 7 MPa, 2,3 dielectric-constant gas thermometry in relation to determination of the Boltzmann constant, 4 and refractive-index gas thermometry at temperatures below 25 K that is able to measure the thermodynamic temperature with uncertainties on the order of 0.1 mK. 5 These first-principles methods all make use of the virial expansion, in which gas nonideality is expressed as a power series in the molar density ρ…”

First-principles calculation of the frequency-dependent dipole polarizability of argon

“…which is the basis for the new primary gas-pressure standard established in 2020 [1,2]. Indeed, according to the recent revisions of the fundamental constants [3][4][5], both k and N A have fixed predefined values.…”

“…Therefore, by measuring the temperature and electric permittivity of a gas [1,[6][7][8][9], the macroscopic pressure p can be found, as long as the atomic polarizability is known. By progressive improvements to the experimental setup and accuracy of the polarizability determined from theory, the new pressure standard is competitive with the best mechanical pressure measurements, as illustrated with the recent stress test [2].…”