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

[RXTdiver]

From the PADI Online course there are these parts:

1. "Steel cylinders usually hold the same amount of air as aluminum cylinders, but with a smaller size and/or lower pressure"

If a steel cylinder is smaller and is holding the same amount of air (capacity right?), wouldn't the pressure have to be higher instead of lower?​

2. How much air 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.

This implies that the amount of air and the capacity are two different things. I thought that the amount of air a cylinder could hold is
the capacity.


3. Some websites say the amount of air is calculated by pressure x capacity. Others say pressure x size = capacity. The chart below
implies that size and capacity are two different things. These terms seem to be used interchangeably. This has me confused.
a. How is size defined?
b. How is capacity defined?
c. What is the overall equation used to calculate the amount of air in your cylinder? Is it pressure x size or pressure x capacity?

There is a lot going on here. Too many things to keep track of, so if you can answer point by point, that would be very helpful.

 

[Wibble]

Aluminium is not as strong as steel, therefore the walls of an aluminium cylinder will be thicker than the same sized steel cylinder. For the same internal volume the alumninum cylinder will have a bigger external volume. This makes them lighter in the water and is the reason Aluminium 80cf cylinders are used for stage/deco/bailout cylinders.

Also as steel is stronger, a steel cylinder can normally hold more pressure. Examples a typical ali80 can be blown to 207bar (3000 psi) working pressure, but a steel cylinder is normally 232bar (3360) and you can get steel cylinders at 300bar (4350 psi).

Aluminium 80 cylinder is 11 litres of WET volume:

Pressure Bar	Pressure PSI	Gas Volume (litres)
207	3000	2,277 litres

Steel 12 litre cylinder -- is 12 litres of WET volume:

Pressure Bar	Pressure PSI	Gas Volume (litres)
200	2900	2,400 litres
207	3000	2,484 litres
232	3360	2,784 litres
300	4350	3,600 litres

 

[Alekseolsen]

Definitly not written very easily.

They may have higher service pressures, and the wall thickness of steel cylinders are often thinner then AL cylinders, thus more room for breathing gas with the same external measurements.

From an old thread: (Scuba Tank Wall Thickness)
Quote:
It shows steel cylinders ranging from 0.164 - 0.197 inches (4.2 - 5.0 mm).
Typical aluminum cylinders range from 0.465 – 0.608 (11.8 – 15.5 mm), with a few oddballs having thinner walls.

 

[Eric Sedletzky]

It said “and/or”, not both.
However, you’re only seeing the outside dimensions of cylinders not the internal size.
Aluminum cylinders are a lot thicker so the internal space is pretty small compared to steel cylinders which for LP tanks the tank wall can be pretty thin.
Then there’s the whole subject of cave fills.

 

[tursiops]

Aluminium is not as strong as steel, therefore the walls of an aluminium cylinder will be thicker than the same sized steel cylinder.

Yes, PADI is referring to the outside dimensions of the cylinder.

 

[lexvil]

Externally a LP 85 looks so much like an AL 80 that you will more often than not get a 3200 fill on it

 

[The Chairman]

It's great to see so many right answers in here.

 

[Eric Sedletzky]

Externally a LP 85 looks so much like an AL 80 that you will more often than not get a 3200 fill on it

That would make it a 90 something, yes?

 

[lexvil]

That would make it a 90 something, yes?

Close to 100, without doing all of the math, once they mistook an 85 for another HP 100 and I got that one back with 3800

 

[Alekseolsen]

It's great to see so many right answers in here.

For a change?