Tank comparison Steel V Aluminum

[Scuba.com]

Aluminum Tanks - The aluminum tanks cost significantly less then a steel tank. Aluminum tanks are more buoyant than steel tanks, so you may be finding yourself adding a few extra pounds on your weight belt.
The difference between Pressure & Volume - It should be noted that the pressure of a tank is not a measure of the size or volume of a tank. For example, most aluminum tanks, from the smallest 6 cuft pony bottle to an 80 cuft tank, have a fill pressure of 3000 psi. For a 6 cuft bottle, this means at 3000 psi the tank contains 6 cuft of air. For an 80 cuft bottle this means at 3000 psi the tank contains 80 cuft of air. Tanks come in a variety of combinations of volumes and fill pressures but the cubic foot (cuft) volume is the measure of how much air a tank holds.

Steel Tanks - Steel has a higher density rate than aluminum, which allows the tank to be negatively buoyant when your dive is completed. Steel tanks are available either in high pressure (HP) or low pressure (LP). High pressure tanks are smaller and contain a higher volume of air, and also require a regulator set up for DIN. A HP 100 is slightly smaller than an AL 80. LP tanks are much easier to fill, but are also quite large for the volume of air that they hold.

Valves - A K-valve, which comes on all aluminum tanks and low pressure steel tanks, do nothing other than release or stop the flow of air from the cylinder.
DIN valve - These valves are used on all high pressure tanks - 3300 PSI and above. Regulators used on HP tanks require that your regulator be converted to DIN

 

[Zoomer]

I think I now know where my confusion is..... let me see if I have this correct now, when a manufacturer says that a tank is 80cf... that is 80 cubic feet of air at the tanks rated pressure?
The size of the cylinder just gets smaller as the rated pressure of the tank goes up? Right....

 

[Divin'Papaw]

Valves - A K-valve, which comes on all aluminum tanks and low pressure steel tanks, do nothing other than release or stop the flow of air from the cylinder.
DIN valve - These valves are used on all high pressure tanks - 3300 PSI and above. Regulators used on HP tanks require that your regulator be converted to DIN

Sorry, but your comments here are just not correct. I do use DIN 1st stages on my HP100s and I prefer DIN, but I know many divers who use a Yoke 1st stage on 3442 PSI HP tanks with Thermo Pro DIN/K valves. They work just fine. They are actually designed to be DIN or Yoke (K valve) by using the insert that comes with the valve.

 

[Divin'Papaw]

I think I now know where my confusion is..... let me see if I have this correct now, when a manufacturer says that a tank is 80cf... that is 80 cubic feet of air at the tanks rated pressure?
The size of the cylinder just gets smaller as the rated pressure of the tank goes up? Right....

That's pretty much correct, except for the fact that an AL80 actually holds 77.4 CF and not 80 CF. Isn't that a kicker!

 

[Jimmer]

Valves - A K-valve, which comes on all aluminum tanks and low pressure steel tanks, do nothing other than release or stop the flow of air from the cylinder.
DIN valve - These valves are used on all high pressure tanks - 3300 PSI and above. Regulators used on HP tanks require that your regulator be converted to DIN

Any of the currently manufactured HP tanks (3442 PSI) all come with combination DIN/K valves, which means you don't need a DIN reg to use them. I'm also noticing a lot of aluminum tanks are coming with the same valve. To me it makes sense, that way you don't need specific regs for each tanks.

I think I now know where my confusion is..... let me see if I have this correct now, when a manufacturer says that a tank is 80cf... that is 80 cubic feet of air at the tanks rated pressure?
The size of the cylinder just gets smaller as the rated pressure of the tank goes up? Right....

Yes, 80 cubic feet is 80 cubic feet. So an hP80 will be smaller than an al80, which will be smaller than an LP80.

 

[Jimmer]

Dammit Hoosier! you beat me to it.

 

[Nick_W]

I assume then that it's not the 6351 alloy? Tanks made with 6351 alloy are going to become harder and harder to fill, even if they pass the eddy current test. It's just too risky for most shops.

Maybe the newer 6061 alloy tanks will last longer. I hope so. I own two Luxfer 6061 aluminum tanks and I'd love it if they last 32 years.

Like I said, I'm not picking on you, I've seen the 10-15 years written and said before. I'm just not sure where it comes from. I have several tanks that are beyond that in age so I know I didn't just get that one lucky tank.

I think I have one that is 6351 and a couple others that are 6351 by another name. Either way, I'm sure eventually I won't be able to get them filled any more, but that wasn't really the point. The point was, buy a new AL tank now and it should last you until around 2040 or so.

The way I see it, the number one difference is buoyancy.

Buoyancy isn't an issue for me when I'm using the AL tanks since I only use the AL tanks in warm water. If a person is going to be diving cold water or is maybe a little bit buoyant themselves , then steel is probably the way to go.

If you want to write off AL, go ahead, just don't do it for the wrong reasons.

 

[Divin'Papaw]

Like I said, I'm not picking on you, I've seen the 10-15 years written and said before. I'm just not sure where it comes from. I have several tanks that are beyond that in age so I know I didn't just get that one lucky tank.

I think I have one that is 6351 and a couple others that are 6351 by another name. Either way, I'm sure eventually I won't be able to get them filled any more, but that wasn't really the point. The point was, buy a new AL tank now and it should last you until around 2040 or so.

The way I see it, the number one difference is buoyancy.

Buoyancy isn't an issue for me when I'm using the AL tanks since I only use the AL tanks in warm water. If a person is going to be diving cold water or is maybe a little bit buoyant themselves , then steel is probably the way to go.

If you want to write of AL, go ahead, just don't do it for the wrong reasons.

Thanks for clarifying. The 10-15 year figure may come from the problems with the 6351 alloy that isn't being used any longer. All aluminum tanks that I'm familiar with are now made using 6061 and hopefully they won't have the issues the older alloy ran into in terms of neck crack failures.

 

[Nick_W]

Thanks for clarifying. The 10-15 year figure may come from the problems with the 6351 alloy that isn't being used any longer. All aluminum tanks that I'm familiar with are now made using 6061 and hopefully they won't have the issues the older alloy ran into in terms of neck crack failures.

I think you're probably right on where that figure came from, but it is kind of bogus. As is the misinformed choice by some shops to stop filling them and the borderline fraudulent practice of some shops of telling owners the tank was recalled or must be condemned. It should be noted that of the roughly 25 million tanks made from 6351 alloy, only 15 have ruptured in the United States. Anyone want to venture a guess as to how many steel tanks have ruptured? I'll give you a hint, its more than 15.

Like I said, buoyancy. That is, in my opinion,the only significant thing to think about when picking the material from which your tank is constructed.

Oh, that and if its a pretty color.

 

[Jimmer]

Like I said, buoyancy. That is, in my opinion,the only significant thing to think about when picking the material from which your tank is constructed.

I agree that is the main reason, but I'll add one more reason, and that is capacity options. You have a lot more options for capacity with steel, and even shape and size. ie. HP119, versus HP120 are very different in dimensions but hold the same gas. With aluminum the choices are pretty lean when it comes to the larger sizes, al80, al80, al80, or the odd al100.