possible to unscrew the cap from the container should the abovementioned components not engage immediately with each other the initial rotation of the knob 12 will be sufficient to bring such engagement about it will be clear from the aforegoing that the cap 10 offers valuable safety guarantees avoiding the risk of accidents due to sudden escapes of pressurized steam from the container or in the most serious case explosion of the container due to excessive pressure in particular the combination of the manometer with the system for mechanically uncoupling the closure member from the knob gives enhanced security since the operator is first given a visual indication of the dangerous situation and if that goes unnoticed the operator is still protected by the mechanical uncoupling obviously the vent hole 46 represents yet another safety feature the location of the needle 39 and of the scale 41 at the top of the cap allows them to be read immediately the graduated scale 40 is preferably divided into two bands namely a green colored band corresponding to zero pressure or low pressures in the container followed by a red colored band corresponding to higher pressures in the container at values at which the container must not be opened it may be remarked that the high degree of safety is provided with a cap having the same dimensions as known caps of this type thanks to the compact dimensions of the manometer 26 components the path followed by the tube 32 of the manometer 26 for collecting the pressure signal from upstream of the diaphragm 20 is particularly advantageous inasmuch as it does not obstruct the correct working of the mechanical uncoupling system the slot 33 in.