Sudden and uncontrolled acsent.

[H2Andy]

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?

assuming they are both 8 lbs negative at 80 feet... and assuming a dry suit
with minimal compression (or none) to affect bouyancy as they ascend,
and assuming both tanks gain around 6 lbs of bouyancy, as we have
previously determined, they will both have 2lbs negative bouyancy
(i think... i suck at math)

but what if they both start with 5 lb weights? then they won't both
be 8 lbs negative at 80 feet, which is the scenario i keep mentioning

assuming *equal weights* other than the tanks is my scenario:

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?

 

[pants!]

assuming they are both 8 lbs negative at 80 feet... and assuming a dry suit
with minimal compression (or none) to affect bouyancy as they ascend,
and assuming both tanks gain around 6 lbs of bouyancy, as we have
previously determined, they will both have 2lbs negative bouyancy
(i think... i suck at math)

Correct

but what if they both start with 5 lb weights? then they won't both
be 8 lbs negative at 80 feet, which is the scenario i keep mentioning

assuming *equal weights* other than the tanks is my scenario:

That would be very stupid. If they both start with 5 lb weights, the diver in the Al80 starts out 3 pounds negative and ends up 3 pounds positive.

 

[Soggy]

but what if they both start with 5 lb weights? then they won't both
be 8 lbs negative at 80 feet, which is the scenario i keep mentioning

assuming *equal weights* other than the tanks is my scenario:

Then either one is underweighted or the other is overweighted. Weight is weight. It doesn't matter if it comes in lead or tank material.

Obviously, if a diver needs x amount of weight to dive with a steel tank, and uses that same amount of weight with an aluminum tank, he'll be underweighted.

I'm still not sure what your point is....

 

[H2Andy]

I'm still not sure what your point is....

what it's been all along.. it's easier to be underweighted at the end
of the dive if you are using aluminum than if you are using steel tanks

wearing steel tanks will give you a cushion of weight that the
aluminum tanks simply don't

way back in post 18:

thus, with a ST tank, you could probably be underweighted (independent of
the tank) and be ok, since the tank is providing you with negative bouyancy.

however, with the same weight but using an AL tank, you won't have that
"padding" of extra negative bouyancy, so the same weight won't be able to
hold an runaway ascent

 

[pants!]

what it's been all along.. it's easier to be underweighted at the end
of the dive if you are using aluminum than if you are using steel tanks

wearing steel tanks will give you a cushion of weight that the
aluminum tanks simply don't

That makes no sense either.

 

[Soggy]

That's kind of a weird way of looking at it. It's sort of like saying wearing enough weight gives you a cushion that being underweighted doesn't....

 

[H2Andy]

makes perfect sense to me

if you don't have your bouyancy dialed right, you might get away with
underweight with steel, but it won't be so easy with aluminum

anyway... i think we've beat this horse to death. i did learn something
very important, which is that aluminum and steel tanks gain bouyancy
pretty similarly, at around 6 lbs for 80 cf tanks

 

[JeffG]

makes perfect sense to me

if you don't have your bouyancy dialed right, you might get away with
underweight with steel, but it won't be so easy with aluminum

<snicker>

 

[H2Andy]

clear your minds, guys... say no to dogma

learn to look at the world anew

 

[Blackwood]

well... i think i'm misunderstanding your statement. most AL80's will end up
4 to 5 lbs positive when near-empty, whereas a steel 85 or 95 will be from
neutral to several pounds negative

He's talking about weight. You are talking about the relationship between weight and buoyancy.