A question about buoyancy control at shallow depth

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Kiwi Sean

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Hi all

I realise that it is harder to maintain neutral buoyancy in shallow water than it is in deeper water, so I have been trying to work on my buoyancy skills on recent shallow dives (<30ft max depth).

On a dive last weekend, I got myself to neutral, horizontally trimmed, and was quite comfortable staying at that depth 15 feet +/- 1 foot with breathing (it was cold and I was breathing deep!).

When I came out of horizontal trim to vertical trim, without changing anything else, I found myself ascending quite rapidly.

I was hoping that someone could venture an opinion as to why that might have happened?

Cheers

Sean
 
As you rotate to vertical the air in your BC is at a higher point and therefore expands. This is further compounded if you are wearing a thick wetsuit. That part of your wetsuit below the point of rotation is compressed and that above expands. At 15 feet the difference in these two can be quite significant.
 
Hi all
When I came out of horizontal trim to vertical trim, without changing anything else, I found myself ascending quite rapidly.

you have changed your air-spaces distribution along the water column :) If your rotating point is the crown of the head you would start sinking if it's the feet - ascending.
 
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As you rotate to vertical the air in your BC is at a higher point and therefore expands. This is further compounded if you are wearing a thick wetsuit. That part of your wetsuit below the point of rotation is compressed and that above expands. At 15 feet the difference in these two can be quite significant.

At that depth of less then 2 ATA expansion and compression of your wetsuit and air should not be that significant I would think if you are moving from a horizontal to vertical position but I am not sure.

It could be how the air moves in your BCD. Possibly moving to your shoulder area rapidly that could cause movement in the water column vertically.
 
In the horizontal position, your body is providing maximum resistance to any depth changes. As you breathed deeply, you cycled between buoyancy states but friction minimized your movement and the cycle kept you in about the same positio0n. When you went vertical, your body then offered minimum resistance to depth changes. The first deep inhale and you were on your way up. With minimum friction, you probably moved up too far in the water column before your breathing cycle could stop you and brijng you back down.
 
Proper weighting and trimming makes all the difference at shallow depth dives.
 
Changing positions had nothing to do with it other than maybe unconscious kicking. More likely it was that when you were horiziontal, you were kicking and that kept you down. Moving to verticle, plus the shallower water and your wetsuit starting to decompress requires dumping air. Just work on it, I'm sure you will do just fine with a little practice.
 
At that depth of less then 2 ATA expansion and compression of your wetsuit and air should not be that significant I would think if you are moving from a horizontal to vertical position but I am not sure.

It could be how the air moves in your BCD. Possibly moving to your shoulder area rapidly that could cause movement in the water column vertically.

Expansion and contraction is actually more significant in shallow water because a few feet change in depth is a much greater percent of the total depth. Sudden shift of expanding air to the top of the bc is likely causing the out-of-control rise in the water for the last few feet. To the OP: try exhaling or breathing shallowly "off the top" of your lungs for the last few feet.
 
Sudden shift of expanding air to the top of the bc is likely causing the out-of-control rise in the water for the last few feet.

I doubt the air position makes any difference, the expansion will. I would think that (provided the diver did not accsend) the position of the air is not realvent to their bouyancy, only their trim would be affected.
 
I'm with Dave on this one. I think in the horizontal one can use their fins to subtly correct any up or down movement before it turns into a runaway. In the vertical however, there is limited (if any) ability to push down and any leg movement tends to push up.
 

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