past which the tank should not be filled too. To help to make certain that the tank doesn't accidentally go beyond this point, some cylinder valves have a device known as a burst disk. This is an industry requirement in North America.
A burst disk consist of a thin copper disk held in place with a gasket and a vented plug. If the pressure inside the tanks rises to 140% of the working pressure, the disk ruptures, and the air escapes thought the vented plug. They are designed to rupture at pressures between 125% and 166%.
Burst disk protect against damage that may occur when the tank is over filled. Burst disk usually rupture when full if they are left to sit in the hot sun. In the hot sun tank pressures have been known to rise to several hundred psi. If the burst disk is old - or the burst disk has the wrong pressure rating - this rise in pressure may be sufficient to cause rupture.
In newer burst disk assemblies the vented plug directs the escaping air to both sides of the valves instead of straight out. This safer because the air escapes won't cause the tank to spin.
Keep in mind, when a tank goes through it's normal yearly Visual Inspection, the burst disk is changed at that time.
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