A 50-litre standard hydrogen gas cylinder was temporarily placed and used in a laboratory. The hydrogen gas was used for a flame ionization detector (FID) in gas chromatography (GC) instrument. On January 20, 2015 the safety relief valve on the pressure regulator blew open and released of about 340 g of hydrogen into a laboratory. The gas cloud did not ignite so there was no injury or damage. A full investigation with a complete course of action and reconstruction is presented to verify the cause of leakage and estimate the gas concentration of the dispersion and gas cloud. Preliminary simulation of a likely explosion pressure is given in the paper. If the gas cloud had ignited, the explosion pressure would most likely cause significant structural damage.