2airishuman
Contributor
Modern HP cylinders (that is, anything except the very thick walled 3AA3130 and 3AA3000 cylinders that have been out of production for decades) are made with different steel that is stronger than the steel used in 3AA cylinders.
They are also heat treated, and are subject to destructive test requirements for each batch of cylinders.
Though there are technical differences, the PST, Worthington, Asahi, and Faber special permits are largely the same and use broadly similar steels with similar test requirements. Asahi made a spun cylinder that was heavier due to a thicker base, the Faber permit uses metric units while the PST one uses imperial, the later permits allow ultrasound testing in place of hydrostatic testing, and there are very slight differences in the steel composition.
But none of these are made from the same steel as 3AA cylinders.
All gas cylinders are designed around their test pressure. For 3AA LP steels, that's 4000 PSI. For special permit HP cylinders, it's 5250 PSI. Metallurgically, theoretically, either should withstand repeated stresses to 90% of test pressure. There is some -- really not a lot in the greater scheme of things -- field experience with 3AAs that says that is safe. Whether that field experience is applicable to HP cylinders is, at this point, an open question. I don't want to be part of that experiment, and you'll have to decide for yourself whether you want to be a part of it.
Field experience is vital because cylinders in SCUBA service are subject to things that they don't see in a lab. They are dropped, banged around, they suffer corrosion, they have threads that aren't perfect, some have thicker walls due to inherent uncertainties in the manufacturing process, etc.
As pointed out upthread, there are diminishing returns from increasing the pressure much above 3500 PSI. The ideal gas laws -- that state that the mass of a gas is proportional to volume and absolute pressure and inversely proportional to absolute temperature -- don't hold at that point, because the diameter of a gas molecule becomes large relative to the space between molecules.
Stay safe...
They are also heat treated, and are subject to destructive test requirements for each batch of cylinders.
Though there are technical differences, the PST, Worthington, Asahi, and Faber special permits are largely the same and use broadly similar steels with similar test requirements. Asahi made a spun cylinder that was heavier due to a thicker base, the Faber permit uses metric units while the PST one uses imperial, the later permits allow ultrasound testing in place of hydrostatic testing, and there are very slight differences in the steel composition.
But none of these are made from the same steel as 3AA cylinders.
All gas cylinders are designed around their test pressure. For 3AA LP steels, that's 4000 PSI. For special permit HP cylinders, it's 5250 PSI. Metallurgically, theoretically, either should withstand repeated stresses to 90% of test pressure. There is some -- really not a lot in the greater scheme of things -- field experience with 3AAs that says that is safe. Whether that field experience is applicable to HP cylinders is, at this point, an open question. I don't want to be part of that experiment, and you'll have to decide for yourself whether you want to be a part of it.
Field experience is vital because cylinders in SCUBA service are subject to things that they don't see in a lab. They are dropped, banged around, they suffer corrosion, they have threads that aren't perfect, some have thicker walls due to inherent uncertainties in the manufacturing process, etc.
As pointed out upthread, there are diminishing returns from increasing the pressure much above 3500 PSI. The ideal gas laws -- that state that the mass of a gas is proportional to volume and absolute pressure and inversely proportional to absolute temperature -- don't hold at that point, because the diameter of a gas molecule becomes large relative to the space between molecules.
Stay safe...
