How thick are the walls of a tank?

[SparticleBrane]

Sorry for the bad shots, they were taken with my cell. These are shots of an al80 cut in half.

 

[alphonsis]

Yeah, a friend of mine has an older steel 72 (65) and it stands w/o a boot and has a concave bottom.

 

[spectrum]

Now how much of that aluminum is needed for structural purposes and how much has been added just to get the weight (buoyancy) to be reasonable?

Pete

 

[Walter]

I seriously doubt they add any material just to make them less buoyant.

 

[oxyhacker]

You were looking for a wrong answer?

Cool. About 3/4 inch for aluminum and 1/8 for steel. That's what I was looking for.

-Charles

 

[Jimmer]

I seriously doubt they add any material just to make them less buoyant.

???????? adding Aluminum wouldn't make it less bouyant.

 

[Luis H]

Now how much of that aluminum is needed for structural purposes and how much has been added just to get the weight (buoyancy) to be reasonable?

Pete

Gee...you had to ask that.

Well, lets take the AL 80 as an the example.
The calculated stress (per CFR49 equation, not the hoop stress) during hydro test is 31,872 psi. If I recall correctly (I am at home, no engineering material data here) the yield stress for 6061-T6 is 35,000 psi.

The hydro test stress is about a 10% under the yield. Since the hydro is intended to test for the material yield properties, IMO it is over design by this amount. This is not necessarily the same as intended safety factor, but it turns out to work like that.

That being said, the aluminum T6 heat treatment condition can easily degrade with heat (in theory even a super fast fill can cause damaging localized heat spots). Therefore, that extra safety factor may be just a good idea with this alloys.


IMO, buoyancy was definitely a consideration with aluminum tank design. The 1972 aluminum 72 cu ft (at a working pressure of 2475 psi) didn't last very long. They were so buoyant you had to put a 5 lb weight in the boot or the bottom of the tank was going to float to much.


Added: Those aluminum 72 were horrible. If I recall correctly, they were buoyant by a couple of pounds when they were full (and they were very tall, before you even put a boot with weight in the bottom). If your back back wasn't tight they would hit you on the head, and the waist belt was always pulling on you. I don't think they were produced for much more than a year. I was surprised to see two of those tanks that one of the vintage equipment divers was using during our gathering in 2006 in Florida (Sand Dog III). It was definitely a low point in Scuba tank design.

 

[Luis H]

???????? adding Aluminum wouldn't make it less bouyant.

Yes, it would. The specific gravity of aluminum is higher than one.

 

[Jimmer]

Sorry my mistake, I read the original post wrong, thought it read "more buoyant" for some reason. Time I get my eyes tested.

 

[Luis H]

Sorry my mistake, I read the original post wrong, thought it read "more buoyant" for some reason. Time I get my eyes tested.

I figured that (I read your profile).
Now I bet you just hope that non of your engineering professors see that post.