Uncontrolled descent

bigsnowdog · Sep 23, 2009

I got into a conversation recently with someone about descending, and rate of descent. The remark was made that as soon as you are down a ways you should begin putting air into the BCD to control the rate of descent. It was also suggested that you could develop such a rate of descent that you might go too deep.

I admit to a lack of experience, but as I think about it, assuming you are properly weighted [not too heavy] and with a full tank, it would not be possible to develop an out of control descent rate from which you could not recover. I would think the friction between you and the water would serve to slow the descent rate, and you would finally stop descending.

Thoughts on this?

Thalassamania · Sep 23, 2009

As you descend things compress and you get more negative, it you do not add air to offset the negative buoyancy you will accelerate to terminal velocity in water, just like a skydiver does in another fluid: air. So in a sense your friend's remark has an element of truth in it. The problem lies, however, in the concept of terminal velocity, which is when the acceleration of gravity is offset the friction of the object falling. In air that's rather fast, somewhere between 100 and 200 miles per hour depending on how the body presents itself. In water, that is more on the order of 100 to 200 feet per minute. Anyway, it's not a bad idea to add some air, relieve squeeze in your drysuit and slow your descent.

Sirto · Sep 23, 2009

bigsnowdog:
I got into a conversation recently with someone about descending, and rate of descent. The remark was made that as soon as you are down a ways you should begin putting air into the BCD to control the rate of descent. It was also suggested that you could develop such a rate of descent that you might go too deep.

I admit to a lack of experience, but as I think about it, assuming you are properly weighted [not too heavy] and with a full tank, it would not be possible to develop an out of control descent rate from which you could not recover. I would think the friction between you and the water would serve to slow the descent rate, and you would finally stop descending.

Thoughts on this?

Damn, Thalassamania posted his reply while I was typing mine. We basically said the same thing. Here was my post:

You will eventually stop descending ... when you hit bottom. Seriously though, you won't just keep speeding up but rather will eventually hit a terminal velocity just like in sky diving when the drag (upward force) equals your negative bouyancy(net downward force). Since the amount of negative bouyancy is small (assuming you aren't grossly overweighted) the speed needed to balance it is small, so, you won't drop too fast.

In order to stop dropping you need to add upward force. This is done by adding some air to your bc, adding air to your lungs, finning or any combination of these.

fisheater · Sep 23, 2009

As you descend, the increased water pressure compresses your exposure protection and reduces its buoyancy. If you were weighted to be negative at the surface (so you could begin your dive), you would be significantly more negative at depth wearing thick exposure protection. Whether you could counter that with swimming alone depends upon your weighting, depth, exposure protection and conditioning, but it would be possible to be more negative than you could swim against and would need your BC or equivalent.

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bamamedic · Sep 23, 2009

bigsnowdog:
I got into a conversation recently with someone about descending, and rate of descent. The remark was made that as soon as you are down a ways you should begin putting air into the BCD to control the rate of descent. It was also suggested that you could develop such a rate of descent that you might go too deep.

I admit to a lack of experience, but as I think about it, assuming you are properly weighted [not too heavy] and with a full tank, it would not be possible to develop an out of control descent rate from which you could not recover. I would think the friction between you and the water would serve to slow the descent rate, and you would finally stop descending.

Thoughts on this?

On a slightly different note, it's also really important to control your rate of descent to avoid ear/sinus problems. If you're descending too quickly, and start experiencing ear pain, by the time you're able to arrest your descent, you stand a good chance of causing ear damage.

Plus, it can be harder to keep track of dive buddies during a quick descent.

The deeper you go, the more your wetsuit compresses, and the less buoyant you become, thus making you descend even quicker.

redacted · Sep 23, 2009

It is not so much an uncontrolled descent as a descent that may not be halted gracefully if you are not paying attention. I have had a few close calls with some of the deeper sandy bottoms in Cozumel. Makes you appreciate a good power inflator.

DaleC · Sep 23, 2009

There's a video on youtube that shows what could happen. Just google Yuri Lipski.

I used to vent all my air to descend until I read about the old school divers kicking down to overcome pos bouyancy at the surface. Now I keep the air in and jack knife to kick down. I also tap the power inflator to slow down long before I hit the bottom. Not only do I look "graceful" (cough cough) I also avoid a prolonged blast on the LPI that might cause it to freeze open.

I am, of course, a cold water diver.

mpgunner · Sep 23, 2009

I recently dove with a new, larger tank (130) but with the same weight. I was amazed at that "sinking feeling" I had the whole dive. As we went down past 100 feet I still needed a bunch of air and work working the whole time. Made me think you can sink if your not careful.

vinegarbiscuit · Sep 23, 2009

I've never seen it myself - and I hope I never will - but uncontrolled descents do happen and can have calamitous results. I believe the nomenclature is "dirt dart". The increased water pressure and resultant compression will squeeze one's exposure suit into a second skin, so even the buoyancy of a thick suit offers little protection against what's happening. The increased pressure can also result in freeflowing regulators, burst sinuses/bloodvessels/eardrums, and induced vomiting and vertigo. Hopefully by then, one will have encountered the bottom and be in a physical and mental condition to extriacate oneself from the mess...but the odds are great.

Cave Diver · Sep 23, 2009

And then, there are those people that empty their wing and jump off the back of a boat with the trigger on a scooter pressed all the way down.

Uncontrolled descent

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bigsnowdog

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Thalassamania

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Sirto

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fisheater

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bamamedic

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redacted

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DaleC

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vinegarbiscuit

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Cave Diver

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