Sudden and uncontrolled acsent.

[pants!]

wait... you're right

luxfer 80 gains 5.8
luxfer s80 gains 5.9
catalina s80 gains 5.8
catalina c80 gains 5.7

Enlightening, isn't it?

 

[H2Andy]

Enlightening, isn't it?

i knew the answer all along, but was trying to confuse everybody else

:14:

(ya...right)

 

[Soggy]

hmmm... ok... i'm missing something then

Yes, you are...

so when a Luxfer 80 goes from -3.6 to 2.2 bouyancy (net gain 5.8 lbs)

and a Pressed Steel 80 goes from -5.5 to -1 (net gain 4.5 lbs)

they have both gained the same bouyancy?

Those numbers are never very accurate. Stop and think about this. really stop and think...

What is the difference between an empty tank and a full tank? Well, a full tank has air in it and an empty one doesn't.

So, what causes the tank to shift buoyancy when going from empty to full? Air has weight to it...specifically 1 lb per 12.5 cft of gas.

Now, using basic reasoning skills, how could the material the tank is made of (assuming it is not elastic, like a balloon), POSSIBLY affect the buoyancy swing if the *only thing changing* is the quantity of gas in the tank?

Now, the reality is that an aluminum 80 doesn't actually hold 80cft at 3000 psi, it holds 77.4 cft at 3000 psi and a steel 80 holds 80 cft at 2640 or 3500 depending on if it is a HP or LP tank. That means the al80 actually has less of a swing than the steel tank, but only because the quantity of gas in the tank is less.

 

[darylm74]

Hello,



I am new on this forum, and saying Hello Underwater World to everyone here.
I also have a question.



Last weekend myself and two of my friends were scuba diving in Tobermory, Ontario, Canada
We were down at 55 feet, right at thermocline debth, sitting there for a minute or two when we decided it was time to head back to shore.
We still had about 5-7 minutes worth of air remaining before 500psi, and were relaxed and not presured to go up. We were playing in the water, and did a flip flop.
My first flip was okay, but on the second one I have noticed that one of my friends was acsenting very rapidly, and I was shooting up as well.
I imidiatelly started dumping air from my BCD and managed to stop and hover at about 10 feet depth. Two of my friends however shoot up and could not stop their acsent until they reached the surface.

My question is why has it happened?

Its not the first time I was at that depth, but it is the first time I moved so fast upwards with no apperant reason.

Thanks all,

Evric

You mentioned that you were sitting around the thermocline. Is it possible that you were sitting slightly below it with proper buoyancy and when you did the flips you dipped back up into the warmer side of the thermocline? I just now got into diving doubles and never noticed a huge difference at the thermocline when diving single, but when I am in doubles (which is new to me) dropping down and hit the thermocline, if I don't start adding air I drop like a rock. I notice this effect more probably because of the weight of the doubles. On the ride back up, having a bigger wing, if I don't start dumping air, I will rise like a rocket.

Like I said I notice this much more since I just got into doubles not long ago and have quite a bit more weight and lift capabilities than I do in my single wing. Since you are still working on your skills, it could be a smiliar situation.

 

[Bill51]

Ding ding ding! Brilliant observation and almost certainly true.

Overweighted, lots of extra air in the BC, acrobatics which result in the diver losing SA and ascending a bit = diver in uncontrolled ascent.

Now, toss in just a tad of vertigo and it’s very easy to kick yourself up when you think you’re kicking horizontal.

 

[H2Andy]

So, what causes the tank to shift buoyancy when going from empty
to full? Air has weight to it...specifically 1 lb per 12.5 cft of gas.

the tank gets lighter as it gets emptier, thus needing to displace less water
to "float", but since it retains the same overall dimensions, it displaces MORE
water than it needs to "float", and thus becomes positively bouyant?

which means the only significant factor as to gaining bouyancy is how
much air is used from the tank

but isn't it true that if you start with a heavier steel tank, you will end up
with more overall negative bouyancy, given the same amount of air used
simply because you started with more negative bouyancy to begin with,
in absolute terms?

in other words, if your steel tank makes you 20 lbs negative at the
beginning of the dive, and 14 lbs negative at the end of the dive,

whereas your aluminum tank makes you 14 lbs negative at the start
of the dive, and 8 lbs negative at the end of the dive

you are more likely to be underweighted at the end of the dive
with the aluminum tank, assuming you started both dives with the
same ammount of non-tank weight?

 

[Green_Manelishi]

the tank gets lighter as it gets emptier, thus needing to displace less water

The TANK weighs the same, full or empty. The weight of the content of the tank is the variable.

 

[H2Andy]

can you realistically separate the two?

what i mean is that the contents and the tank are one for bouyancy purposes

the whole package weights less, though, yes, it's the air that weights less; the
tank itself doesn't change weight...and yet... it does weigh less

 

[H2Andy]

btw, thank you guys for a very instructive thread (for me, at least)

 

[pants!]

Andy, think of these two divers. They are physically identical (twins, if you will).

One is wearing an Al80 and 10 pounds on a belt.

One is wearing an HP80 and 5 pounds on a belt.

Both tanks are full, and both divers are 8 pounds negative at 80 feet.

At the end of the dive (let's assume they both breathe their tanks dry at 80 feet), what type (positive or negative) and amount (lbs) of buoyancy will each diver have?