Why are there no 120cf Aluminum Tanks?

[Vercingetorix]

The subject line says it all. At 3000 pounds, I'm guessing the walls would be massively thick (compared to steel) accompanied by a tank that weighs more than steel? Or maybe because the swing weight between full and empty would be huge.

TIA.

[gregor]

The swing weight would be exactly the same as any other 120 cubic foot tank, its the loss of air that causes the change in weight, the air doesn't know if the tank is steel or aluminum.

As to why they don't make them, i'm sure it has to do with the amound of material in them. Just thinking out loud, but the strongest parts of the tank are probably the top and bottom, the longer a tank gets, the more "middle" they have, and the only way to make that area strong enough is to thicken the walls, so i'd guess that as tanks get bigger the walls have to get thicker to give it the structural integrity in the "middle", and a 120 would be too thick, and too heavy.

I've seen a chopped HP120 and the walls are really pretty thin compared to the thickness of the walls in an Al80.

[rawalker]

Originally Posted by gregor

The swing weight would be exactly the same as any other 120 cubic foot tank, its the loss of air that causes the change in weight, the air doesn't know if the tank is steel or aluminum.

As to why they don't make them, i'm sure it has to do with the amound of material in them. Just thinking out loud, but the strongest parts of the tank are probably the top and bottom, the longer a tank gets, the more "middle" they have, and the only way to make that area strong enough is to thicken the walls, so i'd guess that as tanks get bigger the walls have to get thicker to give it the structural integrity in the "middle", and a 120 would be too thick, and too heavy.

I've seen a chopped HP120 and the walls are really pretty thin compared to the thickness of the walls in an Al80.

I believe that although the swing weight would be the same the swing buoyancy of the aluminum tank would be greater due to the lighter weight of the aluminum in relation to the mass of the tank. This would require a greater amount of balast to compensate as the tank empties.

[CompuDude]

Because they would be huge and weigh a ton.

Al.100s are not very popular for this same reason.

Al. does not scale up quite as well as steel.

[digitalman]

If I want/need that much air, I'd double up some 63's. The weight would probably be similar and the tanks would be shorter than a 120 would be, unless they made it 10" diameter.

[wmspdi]

I used Aluminum 100s in Cozumel and was surprised how much heavier they were (out of water) than my PST E7 100's. I can't imagine the weight of an aluminum 120. With the displacement from the thicker walls mentioned above they might even start off positive (when full) and go up from there.

[DivingCRNA]

Thicker walls should not make them more positive. Las time I checked, aluminum is denser than water.

[Vercingetorix]

Originally Posted by wmspdi

With the displacement from the thicker walls mentioned above they might even start off positive (when full) and go up from there.

My neutral 80cf AL tanks are heavier that standard AL tanks. The neutral have thicker walls (to hold 3300 psi) versus standard (with 3000 psi). My tank starts heavier and ends at neutral buoyancy.

Originally Posted by CompuDude

Because they would be huge and weigh a ton.

That's what I was thinking. Just needed confirmation from wiser, more experienced, divers.

[navyhmc]

I'll go with the heavier than practical crowd too.

[wmspdi]

My neutral 80cf AL tanks are heavier that standard AL tanks. The neutral have thicker walls (to hold 3300 psi) versus standard (with 3000 psi). My tank starts heavier and ends at neutral buoyancy.

I was thinking more about the overall size of the 3000 psi 120 cf aluminum tank and water displacement. Most of my post certification diving has been with steel tanks so I guess I really didn't think it through far enough.