Pressure reducer suitable for mounting on cylinders (5, 15, 25) provided with connection and closing means, wherein: said connection means comprise an external thread (5b, 15b, 25b) and a first annular flat surface (5a, 15a, 25a) coaxial with said external thread (5b, 15b, 25b); said closing means comprise a needle valve (5c, 15c, 25c), also coaxial with said external thread (5b, 15b, 25b); said pressure reducer is contained in a housing provided with an internal thread and an second annular flat surface, coaxial with said internal thread, the connection with said cylinders (5, 15, 25) taking place by screwing and said needle valve (5c, 15c, 25c) being opened by the pressure exerted by a pin, present on said housing, wherein the pressure reducer comprises an annular gasket and a specific seat of said housing into which it fits to obtain the seal, the pressure reducer being configured so that after clamping has taken place, said second annular flat surface of said housing is in contact with said first annular flat surface (5a, 15a, 25a) of said cylinder (5, 15, 25),) and the surface of the annular gasket comes into contact with said first annular flat surface (5a,15a,25a) of said cylinder (5,15,25), and said second annular flat surface of the housing and said first annular flat surface (5a, 15a,25a) of said cylinder (5,15,25) lie on the same plane, said annular gasket being deformed due to said clamping and exerting, through spring reaction on the bottom of said seat and on said first annular flat surface (5a, 15a, 25a) of said cylinder (5, 15, 25), the force required to obtain, between said cylinder (5, 15, 25) and the housing of the device pressure-seal of the gas drawn up, wherein the gasket has sufficient flexibility-to completely enter the seat when clamping takes place without being subjected to permanent deformation, and is sufficiently stiff to produce, as a consequence of the deformation imparted, the force required to obtain the seal. said connection means comprise an external thread (5b, 15b, 25b) and a first annular flat surface (5a, 15a, 25a) coaxial with said external thread (5b, 15b, 25b); said closing means comprise a needle valve (5c, 15c, 25c), also coaxial with said external thread (5b, 15b, 25b); said pressure reducer is contained in a housing provided with an internal thread and an second annular flat surface, coaxial with said internal thread, the connection with said cylinders (5, 15, 25) taking place by screwing and said needle valve (5c, 15c, 25c) being opened by the pressure exerted by a pin, present on said housing, wherein the pressure reducer comprises an annular gasket and a specific seat of said housing into which it fits to obtain the seal, the pressure reducer being configured so that after clamping has taken place, said second annular flat surface of said housing is in contact with said first annular flat surface (5a, 15a, 25a) of said cylinder (5, 15, 25),) and the surface of the annular gasket comes into contact with said first annular flat surface (5a,15a,25a) of said cylinder (5,15,25), and said second annular flat surface of the housing and said first annular flat surface (5a, 15a,25a) of said cylinder (5,15,25) lie on the same plane, said annular gasket being deformed due to said clamping and exerting, through spring reaction on the bottom of said seat and on said first annular flat surface (5a, 15a, 25a) of said cylinder (5, 15, 25), the force required to obtain, between said cylinder (5, 15, 25) and the housing of the device pressure-seal of the gas drawn up, wherein the gasket has sufficient flexibility-to completely enter the seat when clamping takes place without being subjected to permanent deformation, and is sufficiently stiff to produce, as a consequence of the deformation imparted, the force required to obtain the seal.