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So how does that work, is the 300psi rating calculated by the difference between internal and ambient pressure? Does it not matter whether the greater pressure is internal or external?
Oh no, it totally matters. In the case of a receiving tank, rated for 300 psi internal pressure vs 1 atm external. The limiting factor there would be tensile strength, or how well the material resists being pulled apart. Sticking it underwater with 1 atm internal would test a combination of compressive and tensile strength, but more compressive (if it were a perfect sphere, it would just be compressive). Good news is, steel is a relatively good choice for both.
Which was one of the complaints about the material choice for the Titan; Carbon fiber has high tensile strength, but low compressive strength. The strength of the hull had more to do with the resin than the carbon fiber itself. In fact, I'd be curious to know if there was even a benefit to using carbon fiber over regular fiberglass. That and it's hard to inspect for fatigue compared to other materials like steel.
Case and point: Deepsea Challenger, the sub used by James Cameron to go to the bottom of the Marianas Trench, had a pressure hull made of steel.