In the framework of the cryogenic cooling system design of the 4T CMS magnet, heat and mass transfer has been experimentally studied at CEA-Saclay on a 9-m high helium two-phase convection loop under atmospheric pressure. The loop includes a 5-m high heated section surmounted by a 4.5-m high collector and is connected to the final CMS phase separator. The heated section of the loop is composed of seven aluminum tubes placed in parallel and heated on one side to reproduce the heating configuration of the magnet cooling system. We focused in this paper on the hydraulic characteristics of the two-phase convection loop. Evolutions of mass flow rate and vapor quality are presented and analyzed as a function of the heat flux with an equations system based on the homogeneous model.