Density of Moist Air Monitored by Laser Refractometry

Air-buoyancy corrections are often a dominant source of error in the transfer of the unit of mass through weighing of nominally equal weights of different densities. The possibilities of monitoring air density through measurement of the refractive index of air are considered. A critical discussion is given of the present knowledge of the refractivities of air and its constituent gases. A simple laser refractometer capable of resolving air-density variations of 0.15 × 10 -3 kg m -3 , corresponding to a change in air buoyancy of 9 μg, is demonstrated in weighings of the Swedish prototype kilogram with stainless-steel standards.