Overweighted at beginning of dive but underweighted at end in shallows

[tursiops]

No, nobody is doubting that all tanks will become “more” buoyant with less gas, but with AL80s the effect becomes more noticeable and may induce an uncontrolled ascent at the end of a dive if a diver is not properly weighed due to it becoming positively buoyant.

If you are neutral at the beginning of a dive, you will be 4-6 pounds (or more) buoyant at the end of the dive if your tank is empty, regardless of the kind of tank, AL or steel, large or small. That is very hard to control. If you have an AL80, exactly the same thing is true.

However, if you overweighted at the beginning of the dive, by about 5 pounds or so, then at the end of the dive you will be neutral, regardless of the kind of tank, AL or steel.

If you are greatly overweighted at the beginning of the dive, by about 10 pounds or so, you will sink like a rock, have to add considerable air to your BCD to stabilize at depth, and fight Boyles Law as you ascend and descend during the dive. At the end, with an empty tank, you will still be 5 pounds heavy and will need air in your BCD to hold a safety stop. With an empty AL tank, you will be also be 5 pounds heavy at your safety stop.

Conclusion: Be 5 pounds heavy at the beginning of the dive, and don't worry about what your tank is made of.

 

[Belzelbub]

No, nobody is doubting that all tanks will become “more” buoyant with less gas, but with AL80s the effect becomes more noticeable and may induce an uncontrolled ascent at the end of a dive if a diver is not properly weighed due to it becoming positively buoyant.

Not really. If underweighted by a few pounds, you are underweighted by a few pounds. Choice of tank material does not make it more or less noticeable.

There is a common misconception that steel tanks have less swing than AL. That’s simply not true. The swing is all about the weight of the gas that is breathed. AL tanks tend to go positive when empty. To compensate for that, the diver wears more ballast than they would need for a steel.

 

[inquis]

nobody is doubting that all tanks will become “more” buoyant with less gas, but with AL80s the effect becomes more noticeable

By your logic, a wetsuit should be the kiss of death, as it's also positively buoyant and very much so!

The reason a wetsuit is not, is because we carry lead or other negatively buoyant things (e.g. steel backplate) to offset that positive buoyancy. The EXACT same thing is done to offset the positively buoyant empty AL80 tank.

Again, the only problem is when the sum of ALL the buoyancies goes positive. We're talking about the tank, gas, suit, plate, lead, your body... everything in total.

 

[rsingler]

Thirteen pages.

How, exactly, at we helping beginning divers understand buoyancy with this thread? This is the Basic Forum, after all. Recall the title of this thread.

I am struck by the degree of support for the notion that AL80's are somehow "different " from steel (though yes, there is a subtle trim effect that is greater, but not buoyancy).
I am more struck by the persistence of the discussion.
Those of you arguing that, from a buoyancy standpoint, some tanks are different from others should take a pause, go back to the Encyclopedia of Diving or other source, and relearn the basics.

If anything, this discussion is just one more data point that shows that two weekends to complete Open Water Diver is not enough. And I won't even touch on the requirements of some organizations to complete "Advanced" OWD.

 

[lowwall]

If anything, this discussion is just one more data point that shows that two weekends to complete Open Water Diver is not enough.

2 weekends? How quaint.

 

[Zach The Diver]

Obviously a decrease in an equivalent volume of gas between two different tanks (regardless of material) will decrease the weight of those tanks and enhance buoyancy characteristics regardless of the material of the tank. Identical reduction of gas= identical force. I’m not arguing against this.

At the end of the day, though, aluminum is characteristically MORE buoyant than steel, meaning that- all other variables equal- more weight is technically required to maintain neutral buoyancy.

What I do agree with is that this should not be an issue for a diver with any level of experience.

I was wrong about saying that aluminum somehow made the perceived changes in buoyancy more pronounced.

Conclusion: Be 5 pounds heavy at the beginning of the dive, and don't worry about what your tank is made of.

You misunderstood what I said originally but I agree with this. This is the advice I follow when switching between my HP100 Steel Doubles and my AL80 Doubles. This is scarcely a concern for any experienced divers.

By your logic, a wetsuit should be the kiss of death, as it's also positively buoyant and very much so!

The reason a wetsuit is not, is because we carry lead or other negatively buoyant things (e.g. steel backplate) to offset that positive buoyancy. The EXACT same thing is done to offset the positively buoyant empty AL80 tank.

Again, the only problem is when the sum of ALL the buoyancies goes positive. We're talking about the tank, gas, suit, plate, lead, your body... everything in total.

That’s obvious and you’re splitting hairs. Of course you have to consider the whole dive system lol. You’re arguing against something that I didn’t even say to just argue.

Idk if I’m missing something but basic physics doesn’t discount density and weight of different tank materials. What does check out is that the difference is rather minimal and that it rarely matters, unless you’re a newer diver with the concerns OP had.

If there is a significant hole in my knowledge please let me know (I ain’t perfect).

 

[Zach The Diver]

Thirteen pages.

How, exactly, at we helping beginning divers understand buoyancy with this thread? This is the Basic Forum, after all. Recall the title of this thread.

I am struck by the degree of support for the notion that AL80's are somehow "different " from steel (though yes, there is a subtle trim effect that is greater, but not buoyancy).
I am more struck by the persistence of the discussion.
Those of you arguing that, from a buoyancy standpoint, some tanks are different from others should take a pause, go back to the Encyclopedia of Diving or other source, and relearn the basics.

If anything, this discussion is just one more data point that shows that two weekends to complete Open Water Diver is not enough. And I won't even touch on the requirements of some organizations to complete "Advanced" OWD.

Good lord at that last paragraph man. Smug much?

 

[Zach The Diver]

The risk of uncontrolled ascent is exactly the same for any tank for a given amount of gas consumed during the dive. This is not an opinion, it’s physics. Different tanks have different trim full vs empty but the difference of upward force between beginning and end of a dive is only driven by the amount of gas used.

The composition of the cylinders holding that volume of gas also plays a role.

 

[BRT]

Seems a person would just need to know their appropriate weight for every tank configuration. Yes identical tank capacity will drop the same amount on a dive but different tank material will change the starting weight needed. And tanks of different capacity may drop different amounts on a dive depending on whether you use the extra capacity and have longer dive times at similar depths and task loading.

True. You weight for the end of the dive not the beginning. That is up to your BCD.

 

[BRT]

The composition of the cylinders holding that volume of gas also plays a role.

Some tank materials lose weight at the end of a dive? No, it is only the air in the tank that changes.