With the aim to achieve a new primary standard for calibration of ultra-low gas flows (leaks) the research in determination of gas density using optical refractometry has progressed, resulting in an evaluation of the stability of the prototype and a refined theoretical analysis. Furthermore, a renewed literature search has revealed more relevant work and further knowledge is gained from these papers. The assembled prototype was known to be of proportions far from optimum, however the assumption that it would be sufficient to reach applicable results was too optimistic. The conclusion is to continue with the efforts, however with a new and improved prototype and an adjusted choice of mirror parameters, and this report summarizes the work as of spring 2010.