Descending Vertically Vs Horizontally

[RIOceanographer]

Notice the steel PST 80cf tank swings from -8.5 lbs full to -2.5 lbs full.
Notice that the Luxfer AL80 swings from -1.4 to +4.40! Same amount of air in the tanks when filled to the rated pressure but look at the buoyancy swings. That has to be accounted for when balancing your rig and determining how much weight you need.

Not to argue but the buoyancy swing per given amount of air actually is the same and those numbers you give actually prove it if you look more closely at the specs.

The AL 80 has a 5.8lb swing, and the steel 80 has a 6.0 lb swing, so there is a 0.2lb difference but this is because the two tanks don't really hold the same volume of air if you look at the specs. The capacity of the AL80 is really only 77.4 cu ft at 3000 psi. The Steel 80 really holds a full 80 cubic feet.

If you do the math the percentage of the air in the AL 80 relative to a steel 80 is:
77.4/80=96.75%

If you multiply the percentage by the swing of the steel tank you get the swing of the AL tank:
0.9675*6lbs=5.8lbs

So in other words the buoyancy swing is the same for tanks with the same volume of air in them.

 

[P_Abercrombie]

What Charlie99 was addressing is this statement....

What Charlie said is true. All tanks swing from a negative weight to a less negative (or more positive) as the air is depleted from said tank. The fact that a steel tank is still negative when sucked down to 500 psi does not discount the fact that it was more negative at the beginning of the dive.

Let's see it in numbers....
Let's say you are using a steel tank that when full is negative 8lbs and when empty is negative 1lbs. Let's also say that you need 5 lbs to be neutral on your safety stop. Okay, your tank will be negative 1lbs when doing your safety stop so that means you need to add 4 more lbs on your belt to get the 5lbs need to be neutral at the safety stop. So now let's look at the beginning of the dive and how much weight you were carrying. 8lbs (from a full tank) and the other 4 you put on a belt. So you see from this example that you are still overweighed at the beginning of the dive the same as if you had been diving one of your Al80's.

If I read Charlie's email incorrectly, my apologies. I think we are all saying the same thing.

 

[pants!]

P, the "swing" of the tank is dependent on one thing and one thing alone: the volume of gas consumed.

 

[P_Abercrombie]

Not to argue but the buoyancy swing per given amount of air actually is the same and those numbers you give actually prove it if you look more closely at the specs.

The AL 80 has a 5.8lb swing, and the steel 80 has a 6.0 lb swing, so there is a 0.2lb difference but this is because the two tanks don't really hold the same volume of air if you look at the specs. The capacity of the AL80 is really only 77.4 cu ft at 3000 psi. The Steel 80 really holds a full 80 cubic feet.

If you do the math the percentage of the air in the AL 80 relative to a steel 80 is:
77.4/80=96.75%

If you multiply the percentage by the swing of the steel tank you get the swing of the AL tank:
And 0.9675*6lbs=5.8lbs

So in other words the buoyancy swing is the same for tanks with the same volume of air in them.

The swing may be the same but the beginning and end results are different when you consider weighting at the beginning of the dive and that depends on the tank you choose. You have to add more lead to account for the positive buoyancy of the aluminum tank at the end of the dive rather than steel...

 

[P_Abercrombie]

P, the "swing" of the tank is dependent on one thing and one thing alone: the volume of gas consumed.

Yes it is. Individual mileage may vary. But you should still account for it: what if you get caught in a current and use more gas then you intended. What if your buddy has problem and needs to share and depletes your tank more than you anticipated but you still have to make a safety stop. You have to account for all this...

 

[Charlie99]

Duh. But if you are properly weighted you will have that lead.

Or put another way, an AL80 + 7 pounds of lead will look just like the PST 80cf steel tank. While this increases the weight on the boat (and more importantly, when trying to climb the ladder back onto the boat) it does not affect the buoyancy CHANGE during the dive.

My original post was because you stated that the buoyancy swing differed according to whether or not the tank itself is postively buoyant when empty. That's NOT relevant.

 

[JeffG]

What if your buddy has problem and needs to share and depletes your tank more than you anticipated but you still have to make a safety stop. You have to account for all this...

Tough to go shopping at the safety stop...at this point, it is what it is. Nothing you can do about it.

 

[P_Abercrombie]

Duh. But if you are properly weighted you will have that lead.

Or put another way, an AL80 + 7 pounds of lead will look just like the PST 80cf steel tank. While this increases the weight on the boat (and more importantly, when trying to climb the ladder back onto the boat) it does not affect the buoyancy CHANGE during the dive.

My original post was because you stated that the buoyancy swing differed according to whether or not the tank itself is postively buoyant when empty. That's NOT relevant.

Duh?!?

Anyway, here is what I said "If you are properly weighted and balanced you'll find that you are overweighted a little at the beginning of the dive to account for the positive swing of the tank (if you have a tank that swings negative to positive as the air is depleted)." Meaning simply this: if you have a tank that become positively buoyant as you deplete it, you have to account for that in you initial weighting. If when empty, the tank is 3 lbs positive then you need to add an extra 3 lbs of weight before the dive to offset this at the end of the dive. Maybe I didn't write it clearly but Jason a few posts above did. Sorry if you misread it.

 

[pants!]

"If you are properly weighted and balanced you'll find that you are overweighted a little at the beginning of the dive to account for the positive swing of the tank (if you have a tank that swings negative to positive as the air is depleted)."

Every tank on earth has a positive swing, and the swing is identical for an HP80 and Al80. You are overweighted by the exact same amount on both tanks.

Exact same goes for an Al 100 vs HP100. The ONLY difference diving these tanks is how much you weigh on land, because you will be overweight at the start of the dive by the EXACT SAME AMOUNT if you're properly weighted.

Your comment will only be correct if you take out your parenthetical if statement. It's true in all cases, in all tanks.

 

[P_Abercrombie]

Every tank on earth has a positive swing, and the swing is identical for an HP80 and Al80. You are overweighted by the exact same amount on both tanks.

Exact same goes for an Al 100 vs HP100. The ONLY difference diving these tanks is how much you weigh on land, because you will be overweight at the start of the dive by the EXACT SAME AMOUNT if you're properly weighted.

Your comment will only be correct if you take out your parenthetical if statement. It's true in all cases, in all tanks.

No; look at the links for the charts I posted. The amount of the swing may be the same but the end result is different. An empty HP80 has more negative buoyancy than an empty AL80; an AL80 is positively buoyant when empty.

If you did two dives, one with the steel 80 and one with the AL80 and you ran down to 500 psi on each dive, you would need more lead at the end of the dive with the AL80 to offset its buoyancy than you would with the steel.

I see how this "if you have a tank that swings negative to positive as the air is depleted" isn't clear; yes all tanks do but some more than others; sorry if I wasn't clear.