How thick are the walls of a tank?

[spectrum]

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.

So if I'm reading you right there really isn't much extra aluminum in the AL80 but it is better proportioned than the AL72.

 

[Luis H]

So if I'm reading you right there really isn't much extra aluminum in the AL80 but it is better proportioned than the AL72.

You could probably kind of conclude that.

Is that non committal enough.

I didn't design the thing...I am just reverse engineering it.

Some engineers will consider 10% a considerable added safety factor on a very predictable structure, like a pressurized cylinder (IMO, it is basically extra above the CFR requirements). The CFR does not specify any material derating for mishandling or abuse, heat, etc.

 

[charlesml3]

You were looking for a wrong answer?

Yea. I plan to take this misinformation and spread it around every scuba related forum I can find. It'll generate more flames than all the threads on "Should I get an Air II?", "What size pony should I buy?" and "Why should I take DIR training?" ever did. My plan is coming together. It'll be awesome. Now I need to perfect my Dr Evil laugh.

-Charles

 

[Gilldiver]

Luis, I checked the specs and MIL-H-6088 will allow 6061-T6 to be fabricated at temps up to 350F for a maximum cumulative time of 60 minutes.

So spot heating should never be a problem as the thermal conductivity of the aluminum walls should make it just about impossable to reach that temp unless you use a secondary heating source.

Pete

 

[spectrum]

You could probably kind of conclude that.

Is that non committal enough.

Now you're talking like me.

 

[Jimmer]

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

Or the company I just interviewed with haha

 

[Luis H]

Luis, I checked the specs and MIL-H-6088 will allow 6061-T6 to be fabricated at temps up to 350F for a maximum cumulative time of 60 minutes.

So spot heating should never be a problem as the thermal conductivity of the aluminum walls should make it just about impossable to reach that temp unless you use a secondary heating source.

Pete

Well...I have not performed any analysis (as in transient heat transfer analysis) to confirm or deny that statement. I thought that I presented my statement related to this point with enough uncertainty to leave it open with no definitive conclusion. Your statement is probably correct, but I read somewhere something saying that it was theoretically possible. I probably shouldn’t repeat it with out confirmation, but at the time the source sounded credible so I didn’t question it.


I have been doing some transient heat transfer (lumped heat capacitance) calculation based on several simplifying assumptions to compare the theoretical temperature decay profile of a scuba tank filled in air versus a tank filled in a water bath. The calculations were for both a steel tank and an aluminum tank. I am doing this for PSI / PCI.

One of the interesting observations is the response difference due to the thermal capacitance of the aluminum.

Aluminum thermal properties (6061-T6)
Thermal conductivity: k=104 Btu/ (hr*ft*F)
Thermal capacitance: C=0.23 Btu/(Lb*F)

Chrome-molly Steel
Thermal conductivity: k= 27 Btu/ (hr*ft*F)
Thermal capacitance: C = 0.1 Btu/(Lb*F)

The conductivity of the aluminum is almost 4 times that of steel, but the wall thickness is about 3 times as thick. The thermal capacitance of aluminum is a bit over twice as much as steel and the mass of an aluminum tank is almost 25 % higher than a steel tank. So there is a lot more thermal capacitance in an aluminum tank.

By observation, I have consistently noticed that an aluminum tank, that gets a quick fill, will not seem hot on the outside until much later than a steel tank. The air convection coefficient (inside and out) would be the same on both types of cylinders.

I haven’t precisely measured it, but my perception is that it can be easily 5 or even 10 minutes later that an aluminum tank outer surface is at its highest temperature.

The bottom line is that I don’t have any specific conclusions about how hot an aluminum tank can get in normal use, but 350F (or 300F from a different source) is a fairly low temperature.


Oh, I have been meaning to ask you. How is your regulator working?

 

[Gilldiver]

I haven't had a good chance to run it again at depth yet, 5' in a pool just doesn’t cut it. Work and work travel is really getting in my way this summer. Turkey 3 weeks ago Poland tomorrow with a China trip in the wings. Globalization has a big down side. I figure that each "week" trip costs me 3 weekends.

I'd like to go over the adjustment process you mentioned one time and perhaps get a new spring. An IP of close to 200 psi just doesn’t seem right to me.

As for the heat treatment, I quoted the MIL-spec (which has been superseded by an AMS) but in practice we don’t like to see any aluminum alloy exposed to above 250F after treatment and aging. And 250 will burn you real well, so if you can touch it without blistering, you are OK as far as aluminum heat treatments are concerned.