Uncontrolled descent

Uncontrolled descent. It hasn't happened to me and I hope it never happens to me or to anyone else.
My questions is as follows: What are possible causes for uncontrolled descent? How do you prevent uncontrolled descent (and I don't mean when you hit the bottom of the ocean floor)?

Hi Avic

I would say the most common reasons are being overweighted and having a lack of air in your buoyancy device when entering the water.
Another problem can be that people can mistakenly dump all of their air when starting their descent, which ends up being faster than they initially planned.

HTH
Sf

i can understand overweighted, but don't people usually put out air a little bit at a time? Isn't it common sense to slowly descend by letting air out a little bit at a time?

If common sense was so common, everybody would have it...

I dump all the air in my bc, but I'm weighted so that when I do that and then exhale I'll go down pretty slow. If you were to have too much weight and do that you drop like a rock. I see it a lot. I don't think people even mistakenly do that, I think as relatively inexperienced divers were trained to let the air out of their bc when descending and haven't yet acquired good control of their bc, their weights and their lungs to know any better.

You probably see this happen most frequently among novice divers who are overweighted and/or are wearing a very thick wet suit.

The diver requires more weight to overcome the inherent buoyancy of the thick wet suit. As the diver starts descending the neoprene compresses. The neoprene compression undergoes its most severe buoyancy loss at about 15 feet (5 m) or so, depending upon the suit (this is one reason many divers have difficulty maintaining depth at the safety stop).

When the wetsuit compression kicks in and the extra buoyancy is lost, the additional weight required to overcome the inherent buoyancy results in a very rapid descent. The diver is generally caught unawares of this buoyancy change and doesn't anticipate it with incremental additions of air into the B/C. The result is a very quick drop until the diver can get control of the descent with his/her B/C.

That said, what generally happens next is the onset of an unexpected/unwanted rapid ascent due to the fact that, in order to stop the descent, the diver has put too much air in the B/C too quickly and has become positively buoyant.

I know a few stories about this.

One sounds like an urban legend to me..... It was a story I read in a British dive rag about a diver who made a descent and didn't start to compensate until he was very deep (40 or 50 metres). At that point his descent was so fast that his BCD apparently couldn't fill fast enough to stop it. In another version of this story it was a heavily laden tek diver who didn't start compensating his descent until 300ft.

Another story is one I saw myself. It was a diver who got so tangled up in monofilament thta he couldn't reach his inflator. He was diving along a wall and sank. Someone had to go get him or he would have gone all the way to the bottom.

A third was an incident from a dive boat in British Columbia whereby a grossly overweighted diver made a free fall to something like 60 metres losing consciousness in the process and dying. It's unclear to me why the diver was unable to arrest her descent but there were a number of complicating factors, including inexperience, unknown gear configurations, too much weight, cold turbid water (she learned to dive in Mexico) and making a deep water entry with no checkout.

To me it seems impossible to get a run away descent unless you either (a) carry more weight than your BCD can support (b) go very deep very quickly before you start to compensate for your descent or (c) have a major equipment malfunction or some other issue like the monofilament guy did. The first two problems are things you have 100% control over.

R..

There are also a few areas, for example Barracuda in Cozumel, where if the currents are running just right, you can get a down current. Just move in close to the wall to get out of it.

I don't know if you are posting this question because of a comment I made yesterday in the thread "tips for newbies" when I commented that I once had to grab a diver on a wall dive as he was going rapidly downwards past 40m.
Basically it started as Kraken commented, thick wetsuit which rapidly compressed, BC maybe a little small for the amount of lead carried and in the case I quoted a slow power inflator. The diver had passed 30m without putting any air into the BC.
Result was a fast descent rate with the little air in the BC being compressed due to increasing depth at practically the same rate that the inflator was feeding air into the BC.
He was so concentrating on the BC inflation that he forgot to fin upwards to slow his descent.

Diver0001:

..snip..
One sounds like an urban legend to me..... It was a story I read in a British dive rag about a diver who made a descent and didn't start to compensate until he was very deep (40 or 50 metres). At that point his descent was so fast that his BCD apparently couldn't fill fast enough to stop it. In another version of this story it was a heavily laden tek diver who didn't start compensating his descent until 300ft.
..snip..

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We posted practically in parallel. This is basically what happened in the case I witnessed which didn't happen in Europe. Some BC brands have very slow power inflators. This doesn't matter much most of the time - it actually makes the BC easier to control, but when you need inflation in a hurry it can be critical. It's one of the things I look for when buying a BC.

scubatoys:

There are also a few areas, for example Barracuda in Cozumel, where if the currents are running just right, you can get a down current. Just move in close to the wall to get out of it.

Click to expand...

Hmm interesting take, haven't been to Cozumel so can't comment on that particular spot. However, i do a lot of strong current diving (Palau, French Polynesia) these are either in passes into and out of atolls or on Corners where there is a flat topped pinnacle that juts out into the ocean. Therefore swimming close to the wall is not possible as the wall ends at a dropoff at 60 feet. The current takes you and spits you off the wall or out of the pass into the open water, once you are out there aways there is no wall to swim close to as it is impossible to swim back to it. Sometimes there is a strong downcurrent, sometimes no, all depends on the strength of the current on the reef. This is a regular part of the SOP in these places and yes divers at reputable dive ops are given sausages, signalling devices etc in case of seperation from the guide. Please everybody do not post in response that this is a silly way to dive etc etc. THe reason i am stating this is the fact that this is the most common situations i can see for an out of control descent.

What you need to do in that situation is...watch your bubbles and computer, feel what your ears are doing, add air to your BC and kick up, but be extremely ready to dump that air asap when the current kicks you out of the eddy as it usually does. Once the current has taken you far enough from the drop off the down current ends, and you can do a decent safety stop. SOmetimes this can scare the pants off people, that is why such locations are best left for experienced divers rather than newbies. THese things happen but not all the time so don't let this post scare anyone, very dependent on tides etc (bigger exchange bigger current) Dive guides in the areas are very aware of these things but Mother Nature being what it is things can change extremely rapidly so it can catch people off guard from time to time.

Anyhoo, two things to know you are in a downcurrent....sudden ear pain when you think you are goin up and bubbles staying around you....

Mike