The use of hydrogen as an energy carrier is a real perspective in Europe since a number of breakthroughs obtained in the last decades open the possibility to envision a deployment at the industrial scale if safety issues are duly accounted. However, on this particular aspects, experimental data are still lacking especially about the explosion dynamics in realistic dimensions. The purpose of this paper is to provide a set of totally new and well instrumented hydrogen - air vented explosions. Experiments were performed in a large explosion chamber within the scope of the DIMITRHY project (sponsored by the National French Agency for Research). The 4 m3 rectangular experimental chamber (2 m height, 2 m width and 1 m depth) is equipped with transparent walls and is vented (0.25 and 0.5 m2 square vents).. Six pressure gauges were used to measure the overpressure evolution inside and outside the chamber. Six concentration gauges were used to control the hydrogen repartition in the vessel. The hydrogen-air cloud was seeded with micro particles of ammonium chloride to see the propagation of the flame, the movement of the cloud inside and outside the chamber. The incidence of reactivity, vent size, ignition position and non homogenous repartition of hydrogen received a particular attention.