Measurement of the molar polarizability of argon

RISE and Umeå University are world leaders in developing methods based on Fabry-Pérot refractometry in order to measure and realize the SI unit for pressure, the pascal. A large part of the development is done together with European colleagues, in projects such as MQB Pascal.

Read more about MQB Pascal

The properties of the gas is the major limitation

The new methods developed has been so successful that it is no longer the experimental systems themselves that limit the accuracy of the realisation. The major limitation is instead how well the molar polarizability of the gas used is known.

The molar polarizability of a gas is a way of describing how sensitive the gas is to electric fields on a molecular level. To realise pressure with as high an accuracy as possible, the molar polarizability needs to be known with as little uncertainty as possible.

New method

The purpose of the project is to use a new method based on a Fabry-Pérot refractometer with ultra-high resolution to investigate how accurately we can measure the molar polarizability of argon. The goal is to do so with an uncertainty of less than 5 parts per million, which would create the conditions for the best realisation of pascal to date in the important pressure range between 1 Pa and 30 kPa.

If the method works as intended, it can also be used to measure the molar polarizability of other gases, such as nitrogen which is the most popular gas for pressure measurements.