NIST is constructing and testing a new optical refractometer, referred to as the Variable Length Optical Cavity (VLOC), that will redefine not just how pressure is measured, but the entire traceability chain of the Pascal. The critical piece of this project is to base the traceability of pressure measurements to fundamental constants of the universe and not on the physical artifacts, like mercury density as in a manometer. Theoretical quantum mechanics have been used to precisely calculate the refractivity (n-1) of a gas and NIST will experimentally verify these calculations and provide experimental measurements of refractivity for other gasses/mixtures using the VLOC. This is part of a new concept for standards, called the Quantum SI, where the Pascal is traceable through quantum measurements and can be realized anywhere by measuring refractive index. Additionally, with the 2018 redefinition of the Boltzmann constant, the SI will separate ITS-90 from the thermodynamic temperature scale. The VLOC will fill the traceability gap to the SI for realization of thermodynamic temperature. With this both the Kelvin and the Pascal can be realized using the Quantum SI.
The talk will focus on the engineering and construction of the VLOC along with limitations and technical complications that have arisen. Specifically, the distortions of fixed and variable length optical cavities and methods to overcome these limitations. Additionally, the steps required to maintain ultra-high purity gas and the testing and final steps to achieve full operation will be discussed. Upon completion, the published refractivity will become the link for devices around the world to realize the Pascal (and Kelvin).