Measuring the thermo in thermodynamic

In 2019, scientists redefined the thermodynamic unit of temperature, the kelvin, in terms of a fundamental constant of nature, the Boltzmann constant. This redefinition bolstered researchers’ efforts to develop next-generation methods of measuring the unit.

Rourke reviews refractive-index gas metrology, a technique that uses microwave or optical refraction to directly measure thermodynamic temperature, revealing the state-of-the art in the field and the best reference data currently available

“Readers of this review will learn both about the basic concepts and latest leading-edge developments related to the applications of refractive-index gas measurements,” author Patrick Rourke said. “This review also offers a one-stop shop for the current best reference data relevant to a variety of fields, particularly physical properties of gases, such as the electric polarizability, magnetic susceptibility, refractivity, and higher dielectric and density virial coefficients.”

Beyond temperature, accurately measuring refractive index can determine gas pressure or humidity, and can help scientists better understand chemical properties at a quantum level. However, advancement is becoming limited by the inaccuracy of some of the reference data needed to analyze the measurements. The review encourages scientists to eliminate these inaccuracies to improve the metrology instrument’s applications.

“The key future directions to focus on are new improved experimental measurements and theoretical calculations of the reference data that underpin refractive-index gas metrology,” Rourke said. “Readers should also stay tuned for further developments in microwave refractive index measurements of temperature and optical refractive index measurements of pressure.”

Source: “Perspective on the refractive-index gas metrology data landscape,” by Patrick M. C. Rourke, Journal of Physical and Chemical Reference Data (2021). The article can be accessed at https://doi.org/10.1063/5.0055412 .