Steel cylinders can hold the same amount of air as aluminum cylinders, but with a smaller size and/or lower pressure?

[Wibble]

You’ve probably worked out there’s two systems in use worldwide.

The Europeans and much of the rest of the world refer to the wet volume (e.g. the amount of water the cylinder would hold if the valve was removed and it was filled with water — in litres) and the max working pressure in bars (one bar is one 'atmosphere')

Thus you’d have a

• 12 litre 232 bar cylinder which would hold 232 x 12 = 2,784 litres.
• an 11 litre aluminium cylinder holding 11 litres at 207 bar (= 2,277 litres)

The great thing about this is it’s really easy to work out how much gas you have from the pressure if it’s not been filled to the working pressure. For example the shop gives you 150 bar in an 11 litre cylinder, then you have 150 x 11 = 1,650 litres. Is that enough gas for the dive you want to do?

The US uses an archaic system of the amount of gas (in cubic feet) that the cylinder should contain if it’s filled to it’s working pressure, e.g. 80 cubic feet. The really confusing thing here is the working volume is often modified by some random pressure words, such as LP or HP (low/high pressure) but without specifying what that means. Bottom line, you don’t know how big the tank is nor the in-water buoyancy characteristics without looking it up in some table.

 

[Ukmc]

This is from the PADI website:

"How much your cylinder holds depends on its internal capacity and its rated working (maximum) pressure. One cylinder can have a higher
working pressure than another, but hold less air because the internal capacity is lower."

"but hold less air because the internal capacity is lower."

Well, wouldn't you expect that anyway?

Yes, that is a true statement. You can have a 50 cu ft cylinder with a working pressure of 3000 (hold 50 cu ft give or take if rated pressure is 50) or 80 cu ft cylinder with a working pressure of 2700 ( if rated 2700, then will have 80 cu ft give or take) . The second cylinder has lower working pressure but higher capacity. So can hold bigger amount of gas.
What PADI is trying to tell you is that a combination of two is what determines your available gas. And yes, metric system and rating tanks by volume of water makes it for an easier calculation.

 

[admikar]

No burst discs ? Why do they think their dangerous ? Guess if your careful filling tanks and don't leave them in the heat/sun you'd be OK ?

If your tank is filled to its rated pressure, amount of heat it requires to exceed pressure it can actually take will be your bigger concern than exploding tank.

 

[admikar]

To give more meaning, rated pressure for my steel tanks is 230 bars, test pressure it needs to withstand without plastic deformation is 348 bars (note: not the burst pressure).
If we take ISA as standard (15C/59F), to get my tank to test pressure will require a temperature of 152C/305F.

 

[OTF]

The idea of burst discs is that they will rupture in case the pressure is too large, instead of having the tank exploding.
But in reality it is much easier that a burst disc ruptures due to corrosion, leaving the diver out of gas.
Statistics show that tanks do not explode due to overpressure.
So a burst disc creates a significant risk of failure whilst providing protection against a very rare risk, which is better prevented with our mandatory hydro test every 2 years.
Here we do not have VIP either...

How often to burst disks actually fail due to corrosion, especially during dives? I haven't seen or heard of many, if any, incidents.

 

[Angelo Farina]

To give more meaning, rated pressure for my steel tanks is 230 bars, test pressure it needs to withstand without plastic deformation is 348 bars (note: not the burst pressure).
If we take ISA as standard (15C/59F), to get my tank to test pressure will require a temperature of 152C/305F.

Test pressure is 150% of rated pressure.
And the explosion pressure is much higher, possibly double!
So, for reaching the hydro test pressure by heating, also the absolute temperature must grow to 150% of the ambient temperature.
If we have the tank filled at its rated pressure at 300 K, you must heat it to 450 K for reaching the hydro test pressure.
That is 150°C above the normal ambient temperature of 27°C.
I do not think that the sun can raise the tank temperature up to 177 °C even if it is painted black...
So the risk of overpressure is substantially not existent. Hence the burst disc is just a possible failure point, better an European valve with no burst disc...

 

[Angelo Farina]

How often to burst disks actually fail due to corrosion, especially during dives? I haven't seen or heard of many, if any, incidents.

Usually they burst during filling.

 

[Wibble]

Question: what would be the fracture pressure of, say, a standard 232 bar 12 litre steel cylinder?

The test pressure's stamped at 348 bar, but that's for measuring deformation in the hydro test. What sort of pressure would be the destruction pressure, i.e. "bang" or at least a loud hiss? Someone must have this and the manufacturers will definitely put samples through to destruction pressures (hydraulically tested using water, not gas!).

 

[Alekseolsen]

A bit of interested read here:

https://www.researchgate.net/publication/225895117_Cracking_in_aluminum_gas_cylinders_A_review_of_causes_and_protection_measures

A lot of bad alloy AL tanks though

 

[Angelo Farina]

Question: what would be the fracture pressure of, say, a standard 232 bar 12 litre steel cylinder?

The test pressure's stamped at 348 bar, but that's for measuring deformation in the hydro test. What sort of pressure would be the destruction pressure, i.e. "bang" or at least a loud hiss? Someone must have this and the manufacturers will definitely put samples through to destruction pressures (hydraulically tested using water, not gas!).

As an engineer I was trained to use a safety factor of 3 when designing pressure vessels.
So the stress in the metal will reach the tensile strenght of the material at 232×3=696 bar. That is twice the hydro test pressure.
What happens to the tank at this pressure can be either an abrupt explosion or, more likely for steel, a plastic deformation.
In the second case the tank expands. This reduces the internal pressure, so the tank can just "expand a little" and stop.
Alu tanks, instead, more likely incur in a fragile destructive breakup.