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What is a Burst Disk ?
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All Scuba cylinders have a maximum working pressure past which the tank should not be filled. To help to make certain that the tank doesn't accidentally go beyond this point, 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%. For example, let's say your tank is rated to 3000 psi working pressure. This tank may rupture the burst disk anywhere from 3750 psi to 4980 psi. Quite a large range. For this reason dive shops will not go beyond the working pressure of the tank.
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. Because the burst disk has a tolerance to burst at higher pressures, this is usually not a problem and seldom occurs. In the hot sun tank pressures have been known to rise to several hundred psi. this is another reason dive shops keep the initial pressure low. If the burst disk is old - or the burst disk has the wrong pressure rating - this rise in pressure may be sufficient to cause a 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 system keeps the tank from spinning out of control if a rupture occurs.
Keep in mind, a yearly Visual Inspection should be done on all active diving tanks. At that time, the burst disk is changed