Overweighted at beginning of dive but underweighted at end in shallows

[tursiops]

I too fail to understand why many experienced divers who post here fail to understand that aluminum tanks become positively buoyant as they empty.

No one denies that AL tanks become positively buoyant as they empty. But that is not the point. The 5-6 pounds lost from an AL80 is also lost from a steel 80.....BOTH tanks get lighter at the end of a dive. THAT is the only point being made. If you are properly weighted with an AL or a steel tank -- i.e., in the vicinity of 5-6 pounds heavy at the beginning of the dive -- then you will be neutral at the end of the dive. It doesn't matter what the tank is made of.

 

[JMarc]

I too fail to understand why many experienced divers who post here fail to understand that aluminum tanks become positively buoyant as they empty.

What I fail to understand is why many experienced divers who post here fail to understand that, for the basic scuba section of this board purpose (*), what is important is not the positively vs negatively buoyant aspect of AL tanks but the change of buoyancy due to air consumption. And AL tank with whatever leads needed to make it match the characteristics of a same capacity steel tank is equivalent to the steel tank. You can welt the weight to the tank, put them on a belt or in your BCD's pocket that doesn't matter.

* In more advanced contexts, especially with multiple tanks, the buoyancy characteristics of the tanks can play a role, forcing you to be always overweight even with no additional weight or to add an unreasonable amount of weight. And if you want for separate parts to have independent buoyancy characteristics as well, that's an additional complexity.

 

[inquis]

BP/W is negative and stays negative. AL80s start negative and switch to positive. Wetsuit starts positive and stays positive. Any individual component's buoyancy is immaterial, as it's the TOTAL buoyancy at the end of the dive that matters.

Think about it, strapping a 2 lb lead brick to an AL80 keeps it (with a reg) from going positive when near empty. Moving that 2 lb from the tank to a trim pocket or weight pocket allows the tank to go positive. BFD, since the total hasn't changed.

 

[tursiops]

@tursiops: On the prior page, in post # 98, you quoted another poster who wrote that aluminum tanks become positively buoyant as they empty and said that's a myth.

Thanks for selectively quoting me so as to make your point. If you want to discuss your error in DMs, fine, but not here.

 

[KenE]

I too fail to understand why many experienced divers who post here fail to understand that aluminum tanks become positively buoyant as they empty.

While this is true versus a steel tank, it does not factor into the problem of buoyancy between the descent and ascent. The issue between descent and ascent is the difference in gas in the tank, regardless of type of tank. The difference is all that matters, not what equipment or whether it goes from negative to positive when considered by itself. You need to consider the whole diver buoyancy between descent and ascent and the buoyancy of just the tank is not the issue.

This fact is important when you change from a steel tank to an AL tank or vise versus, but is not the issue in this case.

 

[Belzelbub]

I too fail to understand why many experienced divers who post here fail to understand that aluminum tanks become positively buoyant as they empty.

I guess the point is that overall, it's irrelevant whether they become positively buoyant or not. You are either weighted properly, or you are not.

What is often stated is that AL tanks are worse than a comparable steel tank when it comes to buoyancy change. An AL80 and a steel 80 both contain about the same volume of gas. At the end of the dive, they will have lost around the same amount gas (barring minor differences related to rated pressures). The weight of the used gas will have the same effect on buoyancy.

The difference is really that the steel starts heavier, so it also ends heavier. You can get by with using less weight, but still need to factor in the buoyancy gained through the dive in your weighting.

 

[BRT]

You are often painfully pedantic, an Al 80 is typically 1.8 pound negative when full, and 3.8 pounds positive at 500 PSI. Since you and the tank maintain volume throughout the dive (barring the generation of, or expulsion of, GI gas) the net effect is a weight loss of 4-5 pounds during the dive, translating into an equal increase in buoyancy.

Ai summary, but easily confirmed by multiple sources.

And what is the buoyancy shift for an HP80? I'll give you a clue. It is slightly more because it has slightly more air and 500PSI is a smaller proportion of its air.

 

[BRT]

I too fail to understand why many experienced divers who post here fail to understand that aluminum tanks become positively buoyant as they empty.

But you are carrying enough weight to offset that buoyancy. It is the change that matters, not the positive or negative buoyancy of the tank when empty.

 

[tursiops]

But you are carrying enough weight to offset that buoyancy. It is the change that matters, not the positive or negative buoyancy of the tank when empty.

 

[KenE]

Don't Feed the Trolls!!