At the DIMNP (Department of Mechanical, Nuclear and Production Engineering) laboratories of University of Pisa (Italy) a pilot plant called HPBT (Hydrogen Pipe Break Test) was built in cooperation with the Italian Department of Fire Brigade. The apparatus consists of a 12 m3 tank which is fed by high pressure cylinders. A 50 m long pipe moves from the tank to an open space and at the far end has an automatic release system that can be operated from remote. A couple of flanges have been used to house a disc with a hole of the desired diameter, so that it could be changed easily during tests. The plant has been used to carry out experiments of hydrogen release and its ignition. During the experimental activity, data have been acquired about the gas concentration and the length of release as function of internal pressure and release hole diameter. The information obtained by the experimental activity will be the basis for the development of a new specific normative framework arranged to prevent fire and applied to hydrogen. This study is focused on hydrogen concentration as function of wind velocity and direction. Experimental data have been compared with theoretical and computer models (such as CFD simulations).