Cylinder type and proper weighting (SPLIT FROM 'Pull Dumps - Lose them')

[boulderjohn]

I think that all was just about the fact that you need to add more lead weight with an AL than with a steel.

Just to clarify...
In almost all cases, if you dive an aluminum tank on one dive and a steel tank on the other, and if all other factors are the same, you will need to carry more lead to compensate for the greater buoyancy of the aluminum tank. There should be no dispute about that.

What I challenged was the assertion that divers are overweighted because they use aluminum tanks. What took me a gazillion posts to try to explain was that, yes, you will need more lead to be properly weighted with an aluminum tank, but you are not therefore required to put on more lead and become overweighted. There is a difference between having enough lead and having too much lead. If you carry too much lead you are overweighted, whether wearing an aluminum tank of a steel tank.

 

[TMHeimer]

John, of course, excellent point. Would you agree with this? :
Previously I said that since a steel tank may have more gas, you are therefore carrying more weight in gas than with an AL. But, I think that difference is a minor point. Since a steel tank is negatively buoyant when empty and an AL is a few pounds positive when empty, the steel tank of similar size MUST weigh more than the AL one. This, I think is the major reason why you need 5-7 more pounds with the AL. As opposed to a relatively minor difference in amount of gas. What think you?

 

[boulderjohn]

Since a steel tank is negatively buoyant when empty and an AL is a few pounds positive when empty, the steel tank of similar size MUST weigh more than the AL one.

Of course.

If two tanks have the same volume, and one sinks and the other floats, the one that sinks has to be heavier.

 

[Bob DBF]

if two tanks are the same size and weigh the same, one could still be more or less buoyant than the other, thus altering your total body/kit weight, correct? If they weigh the same and are the same size, would that not mean they displace the same amount of water, thus sink (or float) the same?

You are correct that tanks of the same size and weight will have the same buoyancy. SCUBA tanks however vary in size, weight, and pressure, which is why I gave the the sizes of the two 80 cuft tanks. The size difference is because the same weight of steel can hold a higher pressure than that of AL.

Still comes to the same "total package" weight, no? I don't think you're any lighter walking to the shore entry.

A Catalina C80 weights 31.6# and +4# buoyancy empty. ( Size 26 X 7.25)

A Faber MP 80 weights 32# and -7# buoyancy empty. ( 20 x 7.25 )

The tank weights are almost equal but you can leave almost 11# of lead in your vehicle when you walk to the beach so the load is less. Depending on the tank manfacturer the numbers can change.

Your kit and the amount of air is the same in both tanks, difference is that the steel tank is smaller and therefore more dense so you don't have to carry the weight of the buoyancy difference with the AL tank across the beach.



Bob

 

[TMHeimer]

You are correct that tanks of the same size and weight will have the same buoyancy. SCUBA tanks however vary in size, weight, and pressure, which is why I gave the the sizes of the two 80 cuft tanks. The size difference is because the same weight of steel can hold a higher pressure than that of AL.





Your kit and the amount of air is the same in both tanks, difference is that the steel tank is smaller and therefore more dense so you don't have to carry the weight of the buoyancy difference with the AL tank across the beach.



Bob

Yes, you don't have to carry the extra lead. But are you saying your total weight of body/kit/tank will be less if you use steel? That would go against Archimedes, no?

 

[BRT]

Agree on all points. But as you say, a steel tank of the same size can (safely) hold more gas. That gas has weight, and you are carrying that weight--instead of the extra lead you need for the AL tank, which has less gas. Still comes to the same "total package" weight, no? I don't think you're any lighter walking to the shore entry.

In the first example I gave the tanks had about the same capacity, so almost the same weight of air and the total weight loss was over 8 lb. I don't see how you can say you are not lighter walking to the entry unless you think a Spare Air 2.7cf holds over 8 lb of air.
In the second example you are carrying almost exactly 1.8 lb more air in the 22.6cf so you are the same total weight going in but if you stay down 29% longer you will come up the beach lighter.

 

[BRT]

Yes, you don't have to carry the extra lead. But are you saying your total weight of body/kit/tank will be less if you use steel? That would go against Archimedes, no?

I'm trying to figure out how to post a picture of a steel hp100 beside an al80 but don't know how. The Steel 100 looks like a miniature in the picture. You do not go against Archimedes because the steel tank is smaller on the outside even though it is bigger on the inside. I don't have any steel 80's but they look like dwarfs alongside AL80's. And the pictures don't tell the full story because the steel tanks are round on the bottom with a floodable base and the aluminums are square on the bottom. Your total weight getting into the water is less with an HP 80 and almost identical with an HP100 with 29% more air. Once you try this you won't want to go back. I just weighed a full HP100 and an empty AL80. The 100 with it's almost 9 lb of air weights 43 lb, including the boot. The AL80 empty weights 33 lb with no boot. The steel tank on it's boot is 2" shorter than the aluminum tank on it's base and they are so close to the same diameter that you couldn't guess if one was bigger around.

 

[Bob DBF]

Yes, you don't have to carry the extra lead. But are you saying your total weight of body/kit/tank will be less if you use steel? That would go against Archimedes, no?

Archimedes, when used to be neutral in the water, is about the weight volume ratio, the smaller tank changes the volume, and to maintain a proper ratio you need less weight as well. Your body and kit are the same but the tank is different.

We need to be doing this in person and with beers in hand. With the empty bottles and a tub of water we could work out examples.


Cheers

Bob

 

[TMHeimer]

Archimedes, when used to be neutral in the water, is about the weight volume ratio, the smaller tank changes the volume, and to maintain a proper ratio you need less weight as well. Your body and kit are the same but the tank is different.

We need to be doing this in person and with beers in hand. With the empty bottles and a tub of water we could work out examples.


Cheers

Bob

Agree on both accounts.

 

[boulderjohn]

You do not go against Archimedes because the steel tank is smaller on the outside even though it is bigger on the inside

It is not just the internal volume that makes a difference. The HP 100 is filled to a greater pressure, so it takes less tank volume to hold more air. If you filled an HP 100 to 3000 PSI, like an AL 80, it would not have a whole lot more gas than the AL 80.