Direct Determination of Air Density in a Balance Through Artifacts Characterized in an Evacuated Weighing Chamber

This paper describes a simple device which permits mass comparisons in air without appeal to the correction for air buoyancy. The device consists of a canister which is evacuated and weighed on a laboratory balance with a mass inside. A second weighing of another mass in the evacuated canister provides the desired mass comparison. The method was used to determine the mass difference between two stainless steel weights of widely differing densities. With knowledge of this mass difference and of the volume difference one may, by a simple air weighing of the two objects, determine directly the density of the air in the balance case. Densities of air determined by this method were compared with those calculated from the barometric pressure, the temperature, and the relative humidity of the laboratory air. The experimental and calculated values agree throughout to within 1.0 μg cm^-3 (where the normal air density is about 1.2 mg cm^-3). The calculated and experimental values of day-to-day fluctuations in air density agree to within 0.5 μg cm^-3.