Aluminum 80 tank has more liters than steel 80 tank?

[James Ocean]

Hi all,

If you look at the specs, an XS Scuba 80 aluminum tank has 77.4cu ft/11.1 liters - Luxfer-Cylinders-Aluminum — XS Scuba

An XS Scuba 80 Steel tank has 81.7cu ft/10.2 liters - Steel Cylinders — XS Scuba

So the aluminum tank has less cu ft, but higher liters.
Whereas the steel tank has more cu ft, but less liters.

Why is this? Does this mean the aluminum tank has greater air capacity?

 

[BRT]

Higher pressure on the steel tank. 3442 vs 3000.

 

[JackD342]

The cubic feet measurement is compressed air at service pressure. The liters measurement is the volume of the cylinder at ambient pressure. Think of it as the liquid volume. They have two different service pressures, (3000/3442psi) so there is no contradiction.

Folks who work in bar rather than psi use a different calculation method. Each bar is a multiple of the “empty” volume.

 

[James Ocean]

Thanks for the info. So given that, they should last approx the same time?

 

[RainPilot]

Thanks for the info. So given that, they should last approx the same time?

If filled to their correct pressures to start then yes.

Generally, steel is stronger than aluminium, so the walls can be thinner and/or they can take a higher pressure. They will also (usually) be lighter and smaller than an aluminium tank of the same capacity, but heavier in the water since they have a smaller total volume and thus less positive buoyancy when submerged.

 

[CodeS]

Do the calculations in metric.

For example an 11.1L tank (water capacity) filled to 207 bar = 11.1*207 = 2 297.7 litres

If the same tank has a poor fill to 180 bar then the calculation is - 11.1*180 = 1998 litres

The same method works for all tanks and can easily be done on the fly.

 

[almostDIR]

yep that's why Europeans like the metric system and litres x bars instead of the cubic feet stuff.

another thing is that your actual fill pressure is not exactly the same that would result the tank containing exactly 80cf or gas which makes the whole "tank containing always 80cf or 40cf of gas" stuff pretty irrelevant and unnecessarily confusing especially if the max fill pressures of the tanks are different and the actual fill pressure varies.

with litres and bars the calculations are so simple that you can even do them underwater in a stressful situation and it is easier to blend mixes with them or vip gas and so on.

 

[Capt. Bas]

And by size an Al80 is matching a metric steel 12ltr almost perfectly even by weight.

 

[Kevrumbo]

It's easier to fundamentally explain and conceptualize the rated volume and service pressure of a scuba cylinder based on the European/Asian surface atmosphere reference convention of 1 bar: The common AL80 tank has a metric cylinder rating factor of 11 liters/bar, or in other words, at the surface of 1 bar, if you pour water into the cylinder, the measured volume it can contain is 11 liters. (It's easier to work with Metric Cylinder Ratings like 11L/bar at a 1 bar surface pressure, rather than cf-per-psi like 0.025 cf/psi at 14.7psi surface pressure Imperial reference for the AL80 tank).

However, when pressurized with breathing gas to any value up to its recommended Service Rating (207 bar for the 11L per bar Alu cylinder in this example ), a cylinder carries an equivalent volume of free gas much greater than its water capacity, because the gas is compressed to several hundred times the standard surface atmospheric pressure of 1 bar (as opposed to water which is incompressible). So if you have a gas pressure reading of 200 bar in your AL80 tank, you have a total available free gas volume of 200 bar multiplied-by 11 liters/bar or 2200 liters.

 

[StefinSB]

What’s a “litre”? Same as a liter?