[en] Measurements of the linear gas density in the neutraliser of the FAR neutral deuterium beam injection test line have been made, using the charged products of D(d,p)T fusion reactions. They show a disagreement with the linear density expected from conductance measurements made in the absence of beam. Non-molecular flow cannot explain this effect, but a model of gas heating seems to be able to account for it. For the JET neutral beam line, this means an efficiency loss of about 20%. Due to the narrow transverse dimension of its neutraliser, the effect on the future Tore-Supra neutral beam injection line should not be as dramatic