buoyancy test..... HUH ?

[donacheson]

I have read a few posts that mention buoyancy tests, and to be honnest - this is the first I hear of it.
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Is there any advice on how to get my bouyancy right ?

Some of the advice you've gotten is sound; some could be improved.

Your objective while diving should be to be able to maintain neutral buoyancy throughout a dive. The minimum amount of lead weight you need to do that is that amount required to keep you neutral just below the surface with an empty BC and near-empty tank while breathing normally. This ensures that you can always do a comfortable safety stop and ascend to the surface under control at the end of a dive.

Overweighting means you always have air in your BC and this air expands with decreasing depth, causing you to accelerate toward the surface, and compresses with increasing depth, causing you to accelerate even deeper. While being a pound or two overweight isn't difficult to handle, being 10 pounds overweight can make it difficult to maintain a desired depth or rate of ascent or descent, especially during the ascent. For a diver neutral at 15 feet with 10 pounds of lead in excess of the minimum amount, descending 10 feet means the diver is at least 1.7 pounds heavy while ascending 10 feet means the diver is at least 2.6 pounds light. These amounts mean adding at least 1.7 or dumping at least 2.6 pints of air, respectively, in order to remain neutral. By way of contrast, a minimally weighted diver in tropical gear (no neoprene) neutral at 15 feet with an empty BC and near-empty tank doesn't need to add nor remove air to/from his BC under the same circumstances.

Once a minimal weight for you and your gear is established, then you can experiment with trim (moving weights about) so you can easily swim, and even drift for a short period of time without moving hands or feet, in a horizontal position. As others have pointed out, swimming horizontally is the most efficient way.

 

[jbd]

For whatever it’s worth, I disagree that being over weighted makes a significant impact on bouyancy control.

MikeS,
I have to strongly disagree with this statement, repsectfully of course.

Over weighting will in fact have very significant effect on bouyancy control.
The specific reason is the compressibility of air or whatever gas is being put into the BC bladder. Bouyancy control is all about a divers density in relation to the water he or she is in. Density is directly related to the volume of an object. The easily compressed gas in the BC bladder means that the volume of the diver is easily changed. An overweighted diver has a large volume of air in the BC bladder that can be changed quickly to a small volume just by and increase in external pressure from the water.

What starts out as a slow drift downward will rapidly snowball into an express ride to the bottom as the volume of air in the BC is compressed by the increasing water pressure.

For truly excellent bouyancy control one needs to have the least amount of air in the BC possible to achieve the desired affect. This means that a diver needs to use the least amount possible for the gear worn and the water being dived.

I do agree with your comments relating to trim and bouyancy control.

 

[scubafool]

One thing that I have seen mentioned here is horizontal trim. However, no one has mentioned that having horizontal trim is a big help with bouyancy control. You present a much larger surface area to resist up and down movements, and so up or down forces are much slower acting, giving you more time to respond, and the normal up and down movements created by your breathing cycle tend to cancel each other out. Back when I was struggling to get a handle on my bouyancy, it was amazing to me how much easier things became when I got my weight distributed correctly.

 

[jk47]

I think all, or at least most, new divers will have an issue similar to yours. Try the suggestions outlined here for a few dives. If you still don't feel like you've got your weighting, trim, etc. dialed in, see if your lds can set up a buoyancy focused dive for you with an instructor (I don't mean a full specialty, just a one time individual session). I did that and it was immensely helpful. Turned out I was slightly overweighted, but mostly that my buoyancy control wasn't actually as bad as I thought, I just need to relax a little and practice some buoyancy skills.

 

[nolatom]

Great advice above.

Remember also, salt water's about 2 percent denser than fresh, so you need a little more weight compared to fresh water. Cold water's a bit denser than warm, too.

When descending, I try to think of my BC inflator as being like the air brakes on a train (or on the New Orleans streetcars, if they ever get them back running): you activate the brake a little, then wait for the train to slow down, then add a little more brakes, wait some more, then you've 'stopped the train' just before the bottom.

Also remember, your lungs are like a mini-BC. The 'swing' between a full breath in, full breath out, is (I think) about 5 pounds for the typical person. So from half-full lungs, if you inhale, you've lost 2 pounds, exhale, you've gained 2 pounds. Once you're reasonably neutral for a given depth, you can "breathe" your way over and under the terrain if you time it right.

Anybody have better info about the 5 pounds? Takes me about 4-plus breaths to inflate a 23-lb lift BC, so that's what I'm figuring.

 

[WKenny]

What is the price of your booklet regarding bouyancy control and where do you sell it?


kennywek@aol.com

 

[Ber Rabbit]

I definitely recommend jbd's buoyancy book. It's small but packed with tips and actual exercises (including illustrations) you can do to fine tune your buoyancy.
Ber :lilbunny:

 

[MikeS]

MikeS,
Over weighting will in fact have very significant effect on bouyancy control.

I guess we will have to agree to disagree.:mooner: I understand that being over weighted increases the change in buoyancy resulting from the pressure change caused by changing depth; I just don’t see it as a significant factor compared to the lift created by swimming through the water at an angle or moving your hands and feet around.

All divers should start the dive heavy to compensate for the weight of the air that will be gone when it is time to do the safety stop. If I remember correctly there’s about 5 pounds in an aluminum 80.

 

[jbd]

I guess we will have to agree to disagree.:mooner: I understand that being over weighted increases the change in buoyancy resulting from the pressure change caused by changing depth; I just don’t see it as a significant factor compared to the lift created by swimming through the water at an angle or moving your hands and feet around.

All divers should start the dive heavy to compensate for the weight of the air that will be gone when it is time to do the safety stop. If I remember correctly there’s about 5 pounds in an aluminum 80.

Mike,
I guess we will have to agree to disagree(respectfully of course ) Actually I don't see that we are all that far apart on the issues--mostly just the degree of significance each point has regarding bouyancy control.

Regarding you last commnet about the extra 5 lbs to compenstae for tank swing; I don't consider that as being overweighted. Thats part of being properly weighted. When I talk of being overweighted I'm talking about people being 8 to 12 lbs or even more beyond being properly weighted.

 

[Yazrick]

I am back from the big blue waters and I must thank you one and all for the advice that you gave me.

On my first dive back in the ocean, I firstly checked to see if I was weighted correctly, buy completely defaltting my BC and exhaling (I dropped about 4Kgs from my belt before we started). I dropped to just under the surface of the water and stayed there for a while. Everyone around me descended rather fast, finning their way to the bottom, but I took my time.

I noticed that I was slowly starting to drift downwards. I tipped myself over (facing down) and gently glidded down to the bottom, without finning at all. The descent was slow and gave me enough time to equalize.

Once I reached the bottom I signalled my buddy and the DM that I need to get my boyancy sorted. I tried a few fin pivot tests (unsuccessfully), and then started inflatting my BC. Slow bursts, waiting a minute or so to see if there was any effect, then a little more. The group did become a little restless with me not following them, but I stuck to my guns to get my boyancy sorted. Luckily for me the DM waited until I was floating horizontally

I few things that I noticed on this dive was, although I could easily float horizontaly, using my breath to control up and down movement was not working. When I started accending, I could feel myself accelerate to the top, and would have to deflate my BC again. Again only in small amounts as the thing I did want was to plummet to the bottom again.

The second dive went allot better.
Again, a gently float down to the bottom, inflatted my BC just enough to get me off the ground (this was all done on a sand patch btw), and then float as we go. Some of the more experienced divers asumed (what I would call) a "seated position" (looks like you are sitting on a chair), and just floated with the current. When I tried this, I dropped and had to level my self in horizontal position again to regain control of my boyancy.

By my 4th dive, I could comfortably float horizontaly and use my breath to lift my up and down. But when I remain in one spot, I still tend to sink ever so slightly.

None the less - my weightless experience was achieved and it's all due to your advise.

I will keep practicing every time I dive I think

Thanks everyone.

BTW:
If you coulds send me the details of the booklet - I would greatly appreciate it.

Thanks