Master.........Really?

[JamesBon92007]

The buoyancy "swing" for a steel (72) tank is only about 5-8 pounds but the swing for an aluminum 80 is about 11-13 pounds. When empty, a steel 72 has only about 1 pound of buoyancy and the aluminum 80 about 5 pounds.

For me this all means that when I used to do a lot of beach diving I had a lighter tank to carry (steel 72) and needed less weight on my belt plus I find it easier to weight myself properly when diving without a BC. Since most dive shops (in my experience) fill the 72s to 2475 and the aluminum 80s to 3000 psi the difference in the amount of air in the tank is not very large when it comes right down to it.

When they first came out with aluminum 80s they "promised" that they would last forever, presumably because they won't rust. Currently I own steel 72s from the 60s and 70s that still pass the hydro test and I have condemned 2 out of 3 of my aluminum 80s which I bought in the 80s. I did a visual and would not attempt to fill them ever again. The one that I passed did pass the hydro test but is also needs additional testing (more money) so once it's current hydo expires it will become scrap aluminum.

 

[Colliam7]

The buoyancy "swing" for a steel (72) tank is only about 5-8 pounds but the swing for an aluminum 80 is about 11-13 pounds.

This statement, as written, is simply incorrect. The buoyancy swing can only reflect the withdrawal of air from the cylinder, and the decrease of total weight associated with that withdrawal. The characteristics of the metal do not change as the pressure in the cylinder changes, nor is cylinder wall expansion (and, consequently, external volume) an issue for either cylinder (if that was the case, the cylinder would be condemned in any event).

Using your numbers (from a previous post in this thread) for capacity: an AL80 filled to the specified working pressure of 3000psi, holds 77.4 cf of air. An 'old' steel 72, filled to the working pressure of 2250+10% (2475psi) holds 71.2 cf.

1 cubic foot of air, at standard temperature and pressure, assuming average composition weighs approximately 0.0807 lbs.

When the AL80 is filled to the working pressure, the weight of the cylinder increases by 6.24 lbs, reflecting the infusion of 77.4 cf of air. When the LP72 is filled to the working pressure +10%, the weight of the cylinder increases by 5.74 lbs, reflecting the infusion of 71.2 cf of air. It is not physically possible that the 'swing for aluminum 80 is about 11-13 pounds'.

What does change are the trim characteristics of the two types of cylinders, based on distribution of weight along the axis of each cylinder. And, an AL80 will change from negatively bouyant to positively buoyant when it is emptied, whereas certain steel cylinders may simply become less negative as they are emptied. But, the quantitative magnitude of change - the swing - is still a reflection of the weight of air air withdrawn from the cylinder.

 

[rongoodman]

Not entirely pointless... diving on the east coast of Bonaire last week I wanted to get eye-to-eye with a big sea turtle resting on the reef, without touching the reef myself. The guide was (rightfully) very clear about not even 1 finger touching. The photos came out fairly well, but better control would have been nice. Not uber yet

If you don't have a pair of stiff fins, such as the Slipstreams, then get some. Then learn frog kicks, back kicks, and helicopter turns, so you can control your position precisely without using your hands.

 

[JamesBon92007]

This statement, as written, is simply incorrect. The buoyancy swing can only reflect the withdrawal of air from the cylinder, and the decrease of total weight associated with that withdrawal. The characteristics of the metal do not change as the pressure in the cylinder changes, nor is cylinder wall expansion (and, consequently, external volume) an issue for either cylinder (if that was the case, the cylinder would be condemned in any event).

Using your numbers (from a previous post in this thread) for capacity: an AL80 filled to the specified working pressure of 3000psi, holds 77.4 cf of air. An 'old' steel 72, filled to the working pressure of 2250+10% (2475psi) holds 71.2 cf.

1 cubic foot of air, at standard temperature and pressure, assuming average composition weighs approximately 0.0807 lbs.

When the AL80 is filled to the working pressure, the weight of the cylinder increases by 6.24 lbs, reflecting the infusion of 77.4 cf of air. When the LP72 is filled to the working pressure +10%, the weight of the cylinder increases by 5.74 lbs, reflecting the infusion of 71.2 cf of air. It is not physically possible that the 'swing for aluminum 80 is about 11-13 pounds'.

What does change are the trim characteristics of the two types of cylinders, based on distribution of weight along the axis of each cylinder. And, an AL80 will change from negatively bouyant to positively buoyant when it is emptied, whereas certain steel cylinders may simply become less negative as they are emptied. But, the quantitative magnitude of change - the swing - is still a reflection of the weight of air air withdrawn from the cylinder.

Yes, the difference in the weight of the compressed air is not all that great however, for the examples given, the aluminum tank is larger and displaces more water so it has a greater buoyancy force being exerted upon it by the water.

 

[dberry]

Yes, the difference in the weight of the compressed air is not all that great however, for the examples given, the aluminum tank is larger and displaces more water so it has a greater buoyancy force being exerted upon it by the water.

This is true for the tanks, but in a real life situation wouldn't a diver add more lead to remain properly weighted when using an aluminum tank? That would cancel the difference in buoyancy completely.

The physics involved is pretty straightforward, although I agree it's a bit counterintuitive (especially when you read tables listing tank characteristics - which wouldn't consider a diver aiming for neutral buoyancy by adding more lead.)

Consider: two tanks - 1 steel, 1 aluminum - both filled with the identical volume of air, say 80 cf (which weighs 6 lbs.) Put them into a tank of water and add just enough weight to both (on cam bands) to give both rigs neutral buoyancy. There will be MORE lead on the Al tank than the steel. Both tanks are now stable in the water column.

Drain all the air out of both tanks. Both rigs have exactly the same loss of weight - 6 lbs. Both will be positive by 6 lbs - that is they will want to float to the surface with EXACTLY the same force.

To be clear: the larger external volume of the Al tank vs. the steel has been counteracted by the extra lead we added to achieve neutral buoyancy. The amount of extra lead added to the Al tank was "tuned in" to achieve the same level of buoyancy (in the water) as the steel.

To be sure, the TRIM of the rigs would change if the center of mass of the shifts, and that'll depend on the tanks, valves, and the placement of the original lead. I'd wager that shift in trim is what most divers dislike about switching between steel and Al tanks. (Other than lugging more weight when out of the water.)

Cheers,
Don

PS I dive with about half my lead on the tank cam bands and a steel bp, so maybe I don't notice the difference in trim as much?

 

[JamesBon92007]

Drain all the air out of both tanks. Both rigs have exactly the same loss of weight - 6 lbs. Both will be positive by 6 lbs - that is they will want to float to the surface with EXACTLY the same force.

Well, the way I understand it is they will not want to float to the surface with exactly the same force due to other factors such as gravitation acceleration, the buoyancy force. If you have a steel tank and an aluminum tank that weigh the same and displace the same amount of water then that would seem to be true. I have never read it (that I can recall) but I'm wondering if the specific gravity of aluminum vs steel is also a factor. Does anyone happen to know if it takes less positive buoyancy to raise a 10 pound block of aluminum vs a 10 pound block of iron? The specific gravity of aluminum is only 2.7 but the specific gravity of iron is 7.87. That would seem to indicate that it would take about 3 times as much positive buoyancy to raise the iron. Given that the aluminum would be larger it would displace more water and the buoyancy force exerted upon it by the water would also be greater.

 

[dberry]

I'll stand corrected on this, but I'm reasonably confident that as long as we consider the diver + rig as a single unit, after adding the extra lead to the Al tank, it's as I described. I *think* the only difference that isn't compensated for is the volume of the extra lead weights (which is very minor). The external volume of the tank isn't changing while draining the gas, and we've already corrected for the extra water displaced by the Al tank with the extra lead.

It's true that the empty Al tank itself (freed from the extra lead on the cam bands) would head for the surface, while the steel tank would sink. And if you put that extra lead anywhere but on the tank (cam bands) you'd might feel the tank pulling up on a loose BP/W and your trim would change, but in terms of the combined diver + rig, I don't see how emptying the tanks can lead to difference between steel and aluminum tanks (assuming both rigs are neutrally weighted at the start.)

BTW, I get that discussions like this based on theory can be trumped by experience, and most SB folks have way more experience than I do. I really don't want to sound cocky. But we're talking about some basic principles; I'll need someone to point out any fallacy I've fallen for.

 

[JamesBon92007]

I'll stand corrected on this, but I'm reasonably confident that as long as we consider the diver + rig as a single unit, after adding the extra lead to the Al tank, it's as I described. I *think* the only difference that isn't compensated for is the volume of the extra lead weights (which is very minor). The external volume of the tank isn't changing while draining the gas, and we've already corrected for the extra water displaced by the Al tank with the extra lead.

It's true that the empty Al tank itself (freed from the extra lead on the cam bands) would head for the surface, while the steel tank would sink. And if you put that extra lead anywhere but on the tank (cam bands) you'd might feel the tank pulling up on a loose BP/W and your trim would change, but in terms of the combined diver + rig, I don't see how emptying the tanks can lead to difference between steel and aluminum tanks (assuming both rigs are neutrally weighted at the start.)

BTW, I get that discussions like this based on theory can be trumped by experience, and most SB folks have way more experience than I do. I really don't want to sound cocky. But we're talking about some basic principles; I'll need someone to point out any fallacy I've fallen for.

I think the standard rule of thumb is that you need about four more pounds of lead with an aluminum 80 compared to a steel 72. It does not make sense to me that if you remove 6 pounds of air from a steel tank you gain 6 pounds of buoyancy but if you remove 6 pounds from an aluminum tank you gain 12 pounds of buoyancy. These numbers are from the book Diving Science. It would seem that the other factors that I mentioned, and possibly others that I didn't, all affect the buoyancy characteristics of our scuba tanks. I believe that the 3-5 pounds of positive buoyancy that is characteristic of some aluminum 80s may be the determining factor. Other aluminum tanks have less positive buoyancy when empty so those would be preferable.


My experience was this: I would buy a nice, new wetsuit and go diving (beach dive, lets say, down to about 40 feet). Before diving I would attempt to determine how much weight I needed. When the dive ended I would decide that I needed, let's say, about two more pounds. On the next dive I might think about it at first just to check and then never think about it again. This is with a steel 72. Then I would see the new, shiny, aluminum tanks on sale at the dive shop. Nobody told me they were more buoyant so I would end up "too light" on my next dive. I would then add a few more pounds for the next dive and end up negative on the bottom and positive at the end of the dive. Now to add a few more pounds on the weight belt. On the next dive I'm even more negative at the bottom. Now I need to go buy a BC and add air to it at the beginning of the dive and let air out at the end. Plus I have more weight to haul up and down the bluffs and I have this huge BC to deal with. Not only that but the aluminum tanks turns out to weigh about five or six pounds more than my steel 72 so I have to carry that too. All of this may not much matter on a dive boat, especially when they provide the tanks and they are already on the boat. But once an activity becomes too much trouble I'm less likely to want to do it.

The thing that puzzles me the most is the posts I've read on ScubaBoard where people say they are wearing something like 45 pounds of lead. Where is that extra 20-25 pounds of positive buoyancy coming from?

 

[Basking Ridge Diver]

The thing that puzzles me the most is the posts I've read on ScubaBoard where people say they are wearing something like 45 pounds of lead. Where is that extra 20-25 pounds of positive buoyancy coming from?

In the dry suit - I wear a constant volume drysuit - Waterproof D1 - when I first started I needed 44 lbs. I am down to about 36 lbs.
The suit is never completely crushed of air - hence the "constant volume" - the honeycomb weave keeps me very warm but it is buoyant at shallow depths. If I don't want to float up at 5 feet I need the extra weight.

Point of reference - in a 7 mill semi dry I needed 24 lbs.

 

[TMHeimer]

If you don't have a pair of stiff fins, such as the Slipstreams, then get some. Then learn frog kicks, back kicks, and helicopter turns, so you can control your position precisely without using your hands.

Yeah I learned the helicopter turn a decade ago. It was nice before I got my split fins (no cramps since). What am I going to do now that needs such precision? I can still frog kick to exercise different leg muscles. Can't back kick though--no big deal. Use of hands has it's place.