This paper deals with a numerical simulation of the natural convection air flow and its influence on apparent mass in high precision mass measurements. Results are in reasonable agreement with the experimental values obtained on a 1 kg silicon sphere and a 1 kg mass standard. There is clear evidence that the three-dimensional finite element model studied, allowing inclusions of the boundary and initial conditions determined experimentally, captures the most important features of convection and estimates the relevant corrections.

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I Within the project on the redetermination of the Avogadro Constant through the X-ray-density method, it is necessary to measure the mass of 1-kg silicon spheres with an uncertainty well below l*lO-'. Among the many sources of uncertainty, we investigated the effect of the convective currents caused by temperature gradients inside the weighing chamber.