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View Full Version : Why are there no 120cf Aluminum Tanks?



Vercingetorix
07-30-2008, 08:12
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
07-30-2008, 08:39
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
07-30-2008, 14:02
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
07-30-2008, 16:27
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
07-30-2008, 16:39
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
07-31-2008, 22:47
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
08-01-2008, 07:45
Thicker walls should not make them more positive. Las time I checked, aluminum is denser than water.

Vercingetorix
08-01-2008, 08:02
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.

Because they would be huge and weigh a ton.That's what I was thinking. Just needed confirmation from wiser, more experienced, divers.

navyhmc
08-01-2008, 10:07
I'll go with the heavier than practical crowd too.

wmspdi
08-01-2008, 21:23
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.

Daddy Diver
08-02-2008, 01:34
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.

The swing should be no different betwen tanks regardless of the material. 120cf of air (when measured at Standard Temperature and Pressure) has a given mass. The bouancy of a tank is a function of its mass, pluss the mass of the air inside versus the mass of the water displaced by the shape of the tank. Since neither the tank's mass or shape changes during the dive, all changes in the bouyancy of the tank are a result of changes in the mass of the air inside the tank. So assuming you use the same amount (as measured by mass) of air between two tanks, the delta would be the same and so also would the delta in bouyancy.

My guess is that to make an aluminum tank of sufficient strangth and volume to hold 120cft of stp air would require too large a cylinder to make it practical.

cummings66
08-02-2008, 18:42
Exactly and I'll add my 2 bits in saying the swing is the same regardless of material. Think about it, what changes? Only the air consumed and nothing else.

The difference and I think what's confusing some people is that you normally will need more weight with an AL cylinder to get neutral, but that has no effect on the swing.