A question about buoyancy control at shallow depth

[ivobj]

Hi all

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!).

One thing that should help you is to avoid breathing deep, as you mentioned, at this time you managed to achienve some balance, but in general you should not have your lungs completly full. What happens in this case is that when you lung is full of air you shoot up to the surface and as the expansion is quicker (as other friends mentioned, the % of pressure change is higher as you get close to surface) it is very dificult to control your ascent, more likely you will tend to bouce up and down as you quickly breath out and take out the air of the BC, and then inhale again and eventually have to inflate the BC again to compensate for the quick descent when you exhale, and the process start all over. Better to inhale half way and exhale long and continously, no pause between inhale and exhale.

Hi all

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

The other issue, about your position in the water was well explained by other friends, it is just a mater of the air expanding further when it moves higher to your shoulders.

There is though another idea that can help you, the diving position is horizontal, always, you should not go vertical to ascent, on the contrary, it is not so relevant in recreational dive, but would at least be a good habit. Ascent horizontaly, this gives you 2 advantages:

1 - By ascending horizontaly you have better control of your buoyancy, as you should have noticed, avoiding a quick (therefore dangerous) ascent, your ascent should always be controled, it is probably the most critical moment of the dive in terms of safety

2 - By staying horizontal, you keep your whole body basically on the same pressure, what favours the blood flow and elimination of dissolved gases thru your breathing. As I mentioned before, this is an important issue more for decompression, which is not the case here, but always a good habit, go vertical only when you feel your head reached the surface.

Hope it helps...

 

[bill-da-ho]

One thing that should help you is to avoid breathing deep, as you mentioned, at this time you managed to achienve some balance, but in general you should not have your lungs completly full. What happens in this case is that when you lung is full of air you shoot up to the surface and as the expansion is quicker (as other friends mentioned, the % of pressure change is higher as you get close to surface) it is very dificult to control your ascent, more likely you will tend to bouce up and down as you quickly breath out and take out the air of the BC, and then inhale again and eventually have to inflate the BC again to compensate for the quick descent when you exhale, and the process start all over. Better to inhale half way and exhale long and continously, no pause between inhale and exhale.








The other issue, about your position in the water was well explained by other friends, it is just a mater of the air expanding further when it moves higher to your shoulders.

There is though another idea that can help you, the diving position is horizontal, always, you should not go vertical to ascent, on the contrary, it is not so relevant in recreational dive, but would at least be a good habit. Ascent horizontaly, this gives you 2 advantages:

1 - By ascending horizontaly you have better control of your buoyancy, as you should have noticed, avoiding a quick (therefore dangerous) ascent, your ascent should always be controled, it is probably the most critical moment of the dive in terms of safety

2 - By staying horizontal, you keep your whole body basically on the same pressure, what favours the blood flow and elimination of dissolved gases thru your breathing. As I mentioned before, this is an important issue more for decompression, which is not the case here, but always a good habit, go vertical only when you feel your head reached the surface.

Hope it helps...

How would you ascend being horizontal? unless your using your Inflate to ascend, wich I understood you were suppose to avoid doing? Finning up in the horizontal position would seem difficult to me, unless you were traveling a fairly long distance of a swim, I'm thinking more of a deeper dive here tho too. Just curious.

 

[elan]

How would you ascend being horizontal? unless your using your Inflate to ascend, wich I understood you were suppose to avoid doing? Finning up in the horizontal position would seem difficult to me, unless you were traveling a fairly long distance of a swim, I'm thinking more of a deeper dive here tho too. Just curious.

You do not need to fin. Just use your lungs to make yourself slightly positive and you will go up w/o finning.

 

[ivobj]

You do not need to fin. Just use your lungs to make yourself slightly positive and you will go up w/o finning.

Elan said it all, finning is your tool to go forward, in few ocasions backwards (usually not possible or very dificult with fins that are too flexible), and to chage direction. BC or wings, in my case, is your tool to achive neutral buoyancy only, no up and down using BC / wing. Your tool to control up and down, once you are neutral, is your lungs, inhale just a little more (not full lungs) to go up, release air from BC, achive neutral buoancy again, restart the process, you'll go up horizontaly in a very well controlled and confortable manner.

 

[grf88]

Maybe the OP could clarify something. Were you practicing buoyancy in a stationary position, no finning, just hovering in one spot or did you have forward momentum. I had assumed you were practicing in a stationary position just trying to hold one spot.

 

[TSandM]

Although I suspect the culprit here is fin movement, I will also say that changing the distribution of negative and positive components to your buoyancy DOES change the net. I believe I have enough body control now to be sure that I am not finning when I don't want to be finning, and I have played with various positions underwater, and at the same depth reading on the gauge, different positions result in different net buoyancy.

As far as ascending horizontally, if you are cycling around neutral with you breathing (as you would be if you were hovering quietly at one depth) then simply breathing with your lungs almost full will cause you to begin to rise. As you rise, you vent suit, wing and/or lungs to control or arrest your progress upwards. It's a bit tricky to master, but very possible.

 

[Kiwi Sean]

Maybe the OP could clarify something. Were you practicing buoyancy in a stationary position, no finning, just hovering in one spot or did you have forward momentum. I had assumed you were practicing in a stationary position just trying to hold one spot.

thanks for all the informative replies. I don't recall kicking at the time, but went through the same exercise again last night (night diving is such good fun!) and may have been without thinking about it.

to answer grf88's question, I was (attempting) to attain neutral in a stationary position, hovering in one spot.

cheers and thanks again everyone
Sean

 

[gcbryan]

Horizontal ascending is the way I handle this but more than likely when you are going from horizontal to vertical you aren't just rotating around the same spot. When you go vertical you are probably also going up a foot or two and at shallower depths this will make you more positive. If you happened to already be a little light to begin with this would help enable the unwanted ascent.

 

[grf88]

to answer grf88's question, I was (attempting) to attain neutral in a stationary position, hovering in one spot.

Sean

This is how I read your original post. Rotating about your centre of gravity to a vertical position will raise the air spaces, lungs, air in BC and probably about 60-70% of the bubbles in your wetsuit to a higher point thus initiating an ascent even if everything else remains exactly the same. Any air in the BC will be the greatest contributor as it will rise to the top of the BC.

 

[Teamcasa]

Let me understand something. I said:

the position of the air is not realvent to their bouyancy, only their trim would be affected.

And then saw this:

This is how I read your original post. Rotating about your centre of gravity to a vertical position will raise the air spaces, lungs, air in BC and probably about 60-70% of the bubbles in your wetsuit to a higher point thus initiating an ascent even if everything else remains exactly the same. Any air in the BC will be the greatest contributor as it will rise to the top of the BC.

I still don't believe that changing your CG or the movement of any air spaces will have any effect your bouyancy.

Your ballast and the water volume displaced by air remains the same, no matter what position the air pocket(s) is in. It will effect your trim around but not change any bouyancy values.

This is right.

You do not need to fin. Just use your lungs to make yourself slightly positive and you will go up w/o finning.

And then.

..then simply breathing with your lungs almost full will cause you to begin to rise. As you rise, you vent suit, wing and/or lungs to control or arrest your progress upwards. It's a bit tricky to master, but very possible.