Sudden and uncontrolled acsent.

[pants!]

My guess is that while doing flips, these people started the same way I see everyone who does flips start. You're at ~ 45 degree angle, you put your head down and start finning and you essentially do a loop. Problem is that you go up a few feet before you start going back down, and rising a few feet without realizing it means you start rising even more.

 

[H2Andy]

Since he was able to halt his ascent at 10 feet while flirting with 500 PSI suggest to me his weight was not under.

yup, well put

i have managed to get into runaway ascents because of lack of attention
(hey, i'm a brand new drysuit user), while i know i am perfectly weighted.

same rig, exact same dive site, an hour later, and my bouyancy is good
throughout the dive because i was paying attention

 

[H2Andy]

2 pounds underweighted means you're 2 pounds positive at the end of the dive, no matter whether your tank is made of steel, aluminum, or popsicle sticks.

yessir...yessir.. but say you start your dive at 10 under, you will end up at 8 under

but if you start at 4 under, you'll end at 2 under, which won't be enough to hold
you if you need, say, 6 under to stay neutral or negative

then to complicate matters, if you start with 10 and end up with 8 (steel)
but start with 4 and end up with +2 (aluminum), your relative weight shift
has been much greater

it's relative because of the lower starting weight and the disproportinate
amount of bouyancy al tanks gain when empty ... which means you might end up
with enough weight (with steel) or not (with aluminum) because of the
relative amout of bouyancy they gain respectively (though both will
gain some)

 

[Soggy]

None of that made any sense, Andy. Sorry...

 

[pants!]

yessir...yessir.. but say you start your dive at 10 under, you will end up at 8 under

but if you start at 4 under, you'll end at 2 under, which won't be enough to hold
you if you need, say, 6 under to stay

then to complicate matters, if you start with 10 and end up with 8 (steel)
but start with 4 and end up with +2 (aluminum), your relative weight shift
has been much greater

it's relative because of the starting weight and the disproportinate
amount of bouyancy al tanks gain when empty ... which means you might end up
with enough weight (with steel) or not (with aluminum) because of the
relative amout they lose respectively

None of this makes any sense. If you have an HP80 on your back and start 10 pounds overweighted, you will end up 4 pounds overweighted because you consumed ~ 80cf of gas.

If you have an Al80 on your back and start 10 pounds overweighted, you will end up.. guess what.. 4 pounds overweighted because you consumed ~ 80cf of gas.

Relative weight shift is determined solely by the amount of gas you consume, nothing more, nothing less. It makes no difference at all what kind of vessel the gas was in. If you remove 80cf of air from a tank of any kind, it will be 6 pounds lighter.

Strapping a 4 pounds weight to an Al80 doesn't defy the laws of physics, it makes it behave exactly like an HP80. Strapping a 4 pound weight to a diver with an Al80 on his back doesn't defy the laws of physics, it makes him behave like a diver with an HP80 on his back.

 

[ChrisA]

... If you weight yourself based on a full AL80 at the beginning of the dive you will likely be fighting to stay down as the tank gets lighter and wants to float.

The above is correct. But it is also correct for steel tanks. We all were taught to do the weight check with a tank holding 500 PSI So We'd be able to do a safey stop at 15 ft at the end of the dive.

But to answer the question "what happened?" it's a classic run-away acent. I admit it has happened to me, something distracts you and you let bouency control get away from you.

What happen with the flip (I'm guessing of course) is that you loose eye contact with visual referenced that you use un-knowingly to maintain control of depth. Try this experimant. Swimm with eyes closed and see if you maintain the same depth. It takes some effort to learn to "feel" if you are going up or down. A flip might be like closing your eyes.

 

[Green_Manelishi]

you should not be doing acrobatics underwater...

Amen to that Brother Soggy.

Especially if/when you are NOT the only diver on a hang bar/deco line.

G_M

 

[H2Andy]

None of that made any sense, Andy. Sorry...

not surprisingly, since i'm typing and working at the same time

ok... take the starting weight of a full tank. the AL tank will weight less
than an equivalent ST tank.

so... the ST provides "built in" extra weight as compared to the AL tank

now.. the AL tank will gain MORE bouyancy at the end, compared to the
ST tank

so... with the ST, you go in weighting more and come out weighting more
than with the respective AL tank

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

i think... if this doesn't make sense, blame pants!

 

[pants!]

not surprisingly, since i'm typing and working at the same time

ok... take the starting weight of a full tank. the AL tank will weight less
than an equivalent ST tank.

Usually.

so... the ST provides "built in" extra weight as compared to the AL tank

now.. the AL tank will gain MORE bouyancy at the end, compared to the
ST tank

No, it will not. This is probably where you're messing up. An Al80 will gain less buoyancy than an E7-100, and the SAME amount as an HP-80.

 

[JeffG]

not surprisingly, since i'm typing and working at the same time

ok... take the starting weight of a full tank. the AL tank will weight less
than an equivalent ST tank.

so... the ST provides "built in" extra weight as compared to the AL tank

now.. the AL tank will gain MORE bouyancy at the end, compared to the
ST tank

so... with the ST, you go in weighting more and come out weighting more
than with the respective AL tank

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

i think... if this doesn't make sense, blame pants!

Get a refund from your instructor.