A gravimetric apparatus is used to measure the excess adsorption at high pressure. The equipment consists of a Rubotherm magnetic suspension balance, which allows to measure also the density of the fluid. In order to obtain the excess adsorbed amount, the measured weight has to be corrected with a buoyancy term, for which the density of the adsorbing fluid has to be known at each experimental conditions. Therefore the homogeneity of density in the high-pressure cell plays a fundamental role in determining the accuracy of the measured excess adsorbed amounts. This paper is intended to show the impact of the actual approach to thermostating the unit on the density distribution of the adsorbing fluid inside the high-pressure cell. Namely, by changing the inlet position of the heating fluid, large differences in the measured excess adsorption are produced. The closer to the critical point of the fluid, the stronger are these differences. An optimum configuration for our measuring device has been found and it has been used to study the adsorption of carbon dioxide (CO2) on Filtrasorb 400 activated carbon at supercritical and near-critical condition
