spacing element 31 or it may have lower size but bigger than that of opening 32 and be fixed to spacing element 31 in such a way to completely close said opening 32 in its upper part fig3 illustrates the case in which sheet 33 has the same lateral size as the first sheet and the spacing element sheet 33 may be metallized on its face 34 orientated outwards on its face 34 u2032 orientated towards the inside of the device that is in operation towards chamber 12 or both materials thickness values and production methods of the elements making up device 30 first sheet adhesive deposited in discontinuous fashion on a face of this spacing element and second sheet are equivalent to those of the analogous elements in device 20 the only difference between device 20 and device 30 is that in the latter as said spacing element 31 has the same lateral size of first sheet 21 this entails that spacer 31 is not contained in the assembly between the first and second sheets so that all layers of the device must be fixed to the next ones with adhesives possibly having recourse to the micronails piercing aided coupling technique cited above in cases in which adhesion of spacing element 31.