the main purpose of the invention is to make available a device for preventing overfilling of containers , in particular devices intended to contain liquefied compressed gas , structurally and functionally conceived so as to avoid all the drawbacks complained of with reference to the prior art cited . this and other purposes that will appear in what follows are confronted and achieved by the invention by means of a device for preventing overfilling of containers accomplished in accordance with the claims that follow . the characteristics and advantages of the disclosure will be better shown by the following detailed description of a preferred example of the device , illustrated by way of example but not limitative , with reference to the units drawn in which : fig1 is a lateral view of a device for preventing overfilling according to the present invention ; fig2 is a frontal - section view of the device of fig1 ; fig3 is a lateral - section view of the device of fig1 ; fig4 is a sectional view corresponding to the one in fig3 , in a different operating state of the device of the present invention ; fig5 and 6 are two lateral sections of two details of the device shown in fig1 , in two respective operating states . in the figures , 1 is a comprehensive indication of a device for preventing overfilling of containers according to the present invention . device 1 is intended to be applied to a container ( cylinder b ) for liquefied gas under pressure . device 1 includes an upper channel 2 which has a longitudinal axis y , connected to the pipe union for supplying and filling of cylinder b . below channel 2 , device 1 comprises a valve 3 a , below which is installed a lower channel 4 with axis x orthogonal to axis y , connected with an internal volume v of the cylinder , of known cylindrical form with rounded bottoms , intended to collect the liquefied gas stored in cylinder b . valve 3 a comprises a valve body 3 , interposed between channels 2 and 4 , in which is defined a gas duct 5 entering the cylinder , extended mainly along the direction of axis y . the gas duct 5 is delimited , in the directions transversal to axis y , by an internal surface 6 of valve body 3 . valve 3 a comprises a rigid closure means 7 having the shape of a truncated cone , on axis y with the tapered part turned toward channel 4 , axially movable in valve body 3 from and toward a valve seat 8 in order to respectively open and intercept gas duct 5 . closure means 7 comprises a bottom surface turned toward channel 2 , from which rises an appendix 9 , having a circular base and longitudinally extended along axis y . valve 3 a furthermore comprises a guide element 10 , in which is defined a cylindrical cavity 11 , along axis y , in which appendix 9 is smoothly contained . guide element 10 , on the side axially opposite to the cavity 11 , comprises a central arch 12 , capable of being entered by the flow of gas entering the cylinder . guide element 10 is integral with valve body 3 , being equipped with a perimetric annular protuberance 13 , partially nested in an annular seat 14 , located on the inner surface 6 of valve body 3 . annular protuberance 13 is transverse to axis y and is provided with a plurality of passages 15 , to allow for the flow of entering gas to valve seat 8 . valve 3 a comprises a control rod 17 , associated with closure means 7 , being constructed as a piece with it . rod 17 is longitudinally extended along axis y , in a position axially opposite with respect to the appendix 9 . according to other variations of the present invention ( not shown ), in place of valve 3 a other types of valves may be used , for example valves with a membranous closure means controlled by means of a control rod extended longitudinally , analogous to rod 17 . control rod 17 crosses channel 4 , above which it is bound in a cylindrical axial guide 18 , integral to valve body 3 and working together with guide element 10 to axially guide closure means 7 . control rod 17 furthermore comprises an end 19 , axially opposite closure means 7 . device 1 comprises an actuator 16 that can exert a thrusting force f on end 19 of rod 17 in order to urge closure means 7 away from valve seat 8 . actuator 16 comprises a float 20 , extended inside the interior volume v of cylinder b , and a cam mechanism 21 to exert the thrust f , in a rising phase of float 20 . thrust f is exerted until it reaches an extreme position preset by float 20 , corresponding to a maximum fill level of cylinder b . mechanism 21 comprises a member 22 , linked to valve body 3 by a hinge 23 , on which a cam profile 24 is defined . the cam profile 24 is compatible with closure means 7 in order to exert thrust f until it meets end 19 of control rod 17 . during the upward - traveling phase of float 20 , end 19 of rod 17 slides on cam profile 24 until it reaches an end point 25 ( fig4 ). cam profile 24 is an arc of a circle with its center on hinge 23 so as to hold closure means 7 at a constant distance from valve seat 8 . cam profile 24 is sized in such a way that when float 20 has reached the extreme position of maximum filling , end 19 is placed in correspondence with point 25 . in this state , thrust f cannot be exerted , and closure means 7 thus becomes subject only to its own force weight p , which causes closure means 7 to fall toward valve seat 8 so as to intercept gas duct 5 . according to a constructible variant ( not shown ) of the present invention , the fall of closure means 7 is caused , in addition to weight p , by a return spring . actuator 16 furthermore comprises a lever mechanism 26 for exerting thrust f on rod 17 in a first stage of descent of float 20 starting from its extreme point of maximum filling . lever device 26 comprises a fulcrum 27 , hinged to member 22 at distance d from hinge 23 , and a first arm 28 extended from fulcrum 27 toward cam profile 24 . first arm 28 [ is ] compatible with closure means 7 by exerting thrust f , until it meets end 19 of control rod 17 and a second arm 29 . fulcrum 27 is interposed between first arm 28 and a second arm 29 , integral to float 20 . first arm 28 and cam profile 24 are positioned from the same part with respect to straight line z , which joins hinge 23 and fulcrum 27 . during the upward - moving phase of float 20 , arms 28 and 29 place themselves in a position aligned with member 22 , in such a way that first arm 28 can rest against member 22 . during the descent phase of float 20 , end 19 of rod 17 slides on first arm 28 until it reaches an end 30 of first arm 28 in correspondence with an intermediate position of float 20 ( fig6 ). in correspondence with the intermediate position of float 20 , the point of end 25 of cam profile 24 and end 30 of first arm 28 are adjacent to each other . besides the intermediate position , in a second part of the upward - moving phase of float 20 , end 19 of rod 17 moves on cam profile 24 . the functioning of lever mechanism 26 described above thus permits the rearming of cam mechanism 21 , in such a way that end 19 of rod 17 can move on cam profile 24 when the level of cylinder b is lower than the intermediate one , in particular when cylinder b is empty . the present invention allows us to obtain a device for preventing overfilling of containers capable of responding quickly when the maximum fill level is reset . this device does not experience variations of pressure during the filling of the cylinder . the reduced number , compared to other known solutions , of valve components permits the achievement of greater simplicity in the functional geometries and , consequently , fewer rejects in production and greater dependability in usage . in this way the invention fulfills the proposed purpose , at the same time achieving numerous advantages , among which are : a ) the use of a valve of the type that is always open allows better performance with respect to implementing the vacuum in cylinders . b ) the phenomenon of interrupting filling before it is completed is reduced ; c ) performance does not depend on the pressure or density of the fluid being handled ; d ) repetitiveness of the filling operations is improved , because of reduction of variability in the maximum level actually achieved inside of the cylinder in several successive filling operations .