Flame acceleration and explosion of hydrogen/air mixtures remain key problems for severe accident managementin nuclear power plants. Empirical criteria have been developed in the early 2000s by Dorofeev and colleagues [1]providing effective tool to discern possible FA or DDT scenarios. A huge experimental database, composed mainlyby middle-scale experiments in smooth tubes and obstacle-laden ducts, has been used to validate these criteria. Inthese devices the position of the reaction front is usually detected by photo-diodes or photomultiplier tubes uniformlydistributed along the tube axis. As a result, only a coarse representation of the velocity profile can be achieved.In this paper we develop a new technique to track the flame position along the tube at any time. This method consistsin performing time-resolved IR absorption measurements by doping the fresh mixture with an alkane. The velocityprofile is then derived by measuring the variation of the extension in depth of the unburnt gas along the tube axis.Correction factors are eventually drawn from the comparison between longitudinal (IR absorption measurements) andcross-sectional (photomultiplier tubes) flame velocity diagnosis techniques. Finally, experimental results are comparedto numerical simulations [2] and analytical models proposed in the literature [3].