In the US, the bulk of SCUBA cylinders in service are aluminum, not steel. In the case of aluminum cylinders in a fire, the cylinder wall itself will typically fail before the pressure in the cylinder becomes high enough to rupture the burst disk. Most house fires tend to burn at between 1000 °F, and 2000 °F, yet the wall of an aluminum 80 SCUBA cylinder when exposed to a temperature exceeding 265 °F can begin lose its annealed strength and will fail (i.e. explosively rupture) above 350 °F.
Consider an AL-80 when filled to it's service pressure at 3000 psi and 70 °F; using a formula derived from Gay-Lussac's Gas Law, when the cylinder is heated to a temperature of 400 °F the gas inside will have a pressure of approximately 4,600 psi. The standard AL-80 burst disk is rated for 5,000 psi. In a house fire, the side wall of aluminum cylinder can (and probably will) explosively rupture before the burst disk, especially if the cylinder is not filled to it's maximum service pressure. For example, if a partially filled cylinder contains 1,000 psi at 70 °F, it will be approximately 1500 psi at 350 °F or 2,800 psi at 1000 °F. That's more than enough kinetic energy to do serious damage when the cylinder wall ruptures. Experts don't consider the burst disk to be protection against overpressure explosions of aluminum cylinders exposed to fires. For the application of PRD's in SCUBA cylinders, the burst disk required by law in North America is a safety hazard (having been injured more than once by PRD's that failed during fills).
