Overweighted at beginning of dive but underweighted at end in shallows

[RobPNW]

The only time you are ever overweighted at the beginning of a dive is if you're overweighted at the end of a dive.

 

[arew+4]

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.

For Faber hp80 It's a 6 lb delta, -8 to -2. so if your neutral without it, your always a bit negative with it. Like I said.

 

[BRT]

For Faber hp80 It's a 6 lb delta, -8 to -2. so if your neutral without it, your always a bit negative with it. Like I said.

If you wear the same amount of lead. Which even you know not to do.

 

[LFMarm]

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.

** In even more advanced contexts, eg with CCR, you may even get correctly weighted with full tanks as you empty them only in case of bailing out and in that case you can take extreme measures like flooding the CCR or sending to the surface on a line the positively buoyant tanks.

 

[scubadada]

** In even more advanced contexts, eg with CCR, you may even get correctly weighted with full tanks as you empty them only in case of bailing out and in that case you can take extreme measures like flooding the CCR or sending to the surface on a line the positively buoyant tanks.

This is in Basic Scuba

If only we could convince folks that you begin the dive overweighted to cover the weight of the gas that you will consume during the dive. That's why you put some gas into your BC after you descend. It's not rocket science or brain surgery.

 

[The Chairman]

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

In reality, all tanks become more buoyant (less negative) as you empty them. In fact, the larger steel tanks have a bigger swing. No, no normally are somewhat overweighted with a steel tank, so you might not notice it. It's important to adjust your weights after each dive. Take a couple pounds out on each dive. When you can't stay down easily, replace the last two.

Caveat: I can't remember if this was already covered, but a stressed diver is automatically more buoyant. The pre-panic scenario is to breathe with your lungs fuller and fuller. Not exhaling properly causes a buildup of CO₂ which increases anxiety.

 

[Zach The Diver]

A lot of senseless arguing here…

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.

Posters here are more concerned with sharpshooting people over semantics than helping OP out.

 

[RobPNW]

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.

 

[LFMarm]

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.

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.

 

[JMarc]

No, nobody is doubting that all tanks will become “more” buoyant with less gas,

Good. 1.3 kg per m3 of air consumed.

with AL80s the effect becomes more noticeable

Why? The change is the same.

may induce an uncontrolled ascent at the end of a dive if a diver is not properly weighed

That's true for AL, steel, carbon,...

due to it becoming positively buoyant.

It is the whole --- diver, tanks, BCD, weights,... --- that has to be considered, at least if you don't intend to drop the tanks themselves.

The insistence that AL tanks becoming positively buoyant is relevant is still not understandable to me. Not only I consider that fact irrelevant, I still have no clue why some consider it so.