Sudden and uncontrolled acsent.

[Evric]

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

 

[Damselfish]

It's quite likely you went up a little while you were flipping, which would make the air in your BC and suit expand, which would make you ascend a little faster, which would make the air expand more...

You may have also been holding more air in your lungs while you were playing.

 

[akscubainst]

Not knowing gear configurations, experience levels, etc.

I would guess improper weighting, near empty Aluminum tanks, and a little bit of poor buoyancy control.

Maybe you can give us more info.

Dave

 

[Charlie99]

I would guess improper weighting, near empty Aluminum tanks, and a little bit of poor buoyancy control.

I understand the improper weighting, but what does the material of the tank have to do with it?

An near empty tank of xx cubic feet will be x amount lighter, whether it is steel or aluminum.

------

Expanding on Damselfish's comments ..... even if properly weighted, when wearing lots of wetsuit it will compress at 55' and you will have to add signficant amount of air to get neutral. As you go up, this air expands and you go up faster. If you are diving overweighted, then the amount of air in your BCD to be neutral at 55' is even greater, making a runaway ascent even more likely if you inadvertantly ascend while doing your flips.

 

[akscubainst]

An AL 80 is about 4lb positive when empty. Most steel tanks are either neutral or a few pounds negative. 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.

 

[pants!]

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.

This applies equally to every tank ever created.

 

[H2Andy]

An near empty tank of xx cubic feet will be x amount lighter, whether it is steel or aluminum.

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

that's a big difference, and so the steel tanks will be more
"forgiving" if you are accidentally under-weighted


but yes, you weigh yourself for the end of the dive, with a near-empty tank,
to be able to hold a hover right below the surface or right at the surface

as to runaway ascents, if you get behind the ball and the gas in your
BC starts expanding as you ascend, it will get away from you.

as others have said, the key is to anticipate the ascent and start getting
more negative as you ascend to make up for the increased bouyancy of
the air in your BC/drysuit

 

[spectrum]

An AL 80 is about 4lb positive when empty. Most steel tanks are either neutral or a few pounds negative. 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.

Assuming an exposure suit the net bouyancy of the gear will become positive regardless of cylinder type.

As said some inadverdent rise in the water column got things expanding (suit and BC displacement) and physics took over. Being that low on air (weight) exacerbated the situation.

Since he was able to halt his ascent at 10 feet while flirting with 500 PSI suggest to me his weight was not under. Not sure about the friends they may be under wieight or just vented their BCs poorly.

Now I'm not quite sure about his gas planning, planning to stay at 55' before heading to the surface at 500 PSI?

Are you 3 certified divers?

Pete

 

[pants!]

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

that's a big difference, and so the steel tanks will be more
"forgiving" if you are accidentally under-weighted


but yes, you weigh yourself for the end of the dive, with a near-empty tank,
to be able to hold a hover right below the surface or right at the surface

as to runaway ascents, if you get behind the ball and the gas in your
BC starts expanding as you ascend, it will get away from you.

as others have said, the key is to anticipate the ascent and start getting
more negative as you ascend to make up for the increased bouyancy of
the air in your BC/drysuit

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.

 

[Soggy]

ACK...

First let's correct all this crap about tanks and material...
An aluminum 80 is about 4 (four) lbs positive when empty and about 2 lbs negative when full. Air weighs about 1 lb per 12.5 cft. Tank material or service pressure has no effect on this. 80 cft of gas will always weigh ~6 lbs.

This means, that, regardless of what the tank is made of, to be neutral with an empty tank, one needs to be 6 lbs OVERWEIGHTED at the beginning of the dive. Now, diving a steel tank means that you will need less weight than an aluminum tank, but the buoyancy SWING is unchanged.

While I'm sure you were excited, especially as a new diver, you should not be doing acrobatics underwater...this, combined with poor situational awareness and not great buoyancy control is what caused your uncontrolled ascent.