and resistor 17 heating them is located within a perimeter defined by an annulus 11 c having sufficient width to ensure the effective bonding of membrane 1 for instance by gluing to the walls of the vacuum tight chamber of the leak detector advantageously said annulus 11 c will be substantially u201c cold u201d with respect to areas 15 since it is not run through by resistor 17 thus the adhesion of membrane 1 to the chamber walls will not be harmed as better shown in fig3 resistor 17 comprises a film 17 a of a conductive material preferably chromium or in the alternative copper or aluminium and is bonded to membrane 1 through a layer of adhesive material 17 b for example of titanium conductive layer 17 a is moreover coated with a protecting layer 17 c for instance of gold referring to fig4 there is schematically shown a leak detector generally denoted 31 detector 31 comprises a vacuum tight chamber 33 obtained by means of a hollow cylindrical body 39 one end of which is connected to the suction port of a vacuum pump 37 for example an ionic pump the other end of chamber 33 is separated from the outside environment by a gas selective permeable membrane 1 of the kind described with reference to the previous figures advantageously said membrane 1 is bonded to cylindrical body 39 defining chamber 33 along circular rim 41 of said cylindrical body 39 membrane 1 is preferably bonded to said rim 41 by gluing peripheral annulus 11 c of membrane 1 in the alternative membrane 1 may be glued to a metal ring subsequently brazed to rim 41 of chamber 33 membrane 1 is preferably.