How do I control my bouyancy and trim?

[Don Burke]

If you go down when you stop you are _very_ unlikely to be too light. That AL80 holds about 5.9 pounds of air, about 5 of that is useable. You should be starting the dive heavy by the amount of useable gas in your tanks and ending the dive neutral.

My guess is you are carrying a few pounds too many.

 

[James connell]

deep_ocean once bubbled...
OK, I'm 6'4, 230lb. I dive in one piece 6.5 mm full body suit with a 80 AL tank, jacket bcd (Sherwood Silhouette) and last time wore 26lb. I may need another 2-4 pounds and will try it next time.

i'm the same size - i wear 24 lbs with the same set up but a steel bottle - i need 28 with an Al80.

keep practicing! it'll come. you aren't nuetral when you're swimming, you're just compensating for the error by swimming up or down! bad trim.
to be honest even when nuetral you move up and down as you breath. the secret is to know before you start to move that you will be and be ready to exhale/inhale as soon as you start to move. if you let yourself get going y'll have to use the inflator to catch yourself and y'll be chasing it. use your lungs to "micro" control bouyancy Before you need to use the bc. when you are always in the 'top' of your lungs let a litle air into the BC, if you can't take 2/3 a lung full with out flaoting up let a little out - at 6-4 you have large lungs Use them.
and again it'll get easier each dive. it takes time.

 

[MikeFerrara]

There's two issues to consider. One is buoyancy control and it is a skill. Make course corrections with the bc and fine adjustments with lung volume. The other is trim and that has two components. One is balance which is purly mechanical and is a function of where you center of gravity and center of buoyancy is. If they are in the ame place you can stay in any position without effort. If they are not in the same place, as you decend and your suit compresses and you add air to your bc the weight on you wast
(being offset from the bc) can pull your lower half down. If these two components are ffset in a different direction the effect can be different but the point is that we're trying to get them in the same place so the apposing forces don't torque us one way or the other. The second component to trim is body position and this is a skill. Changing body position will effect where those centers are and can be used to compensate for slight offsets in weight and buoyancy as well as to change position on purpose. Icture a beam balanced on a point. If you lengthen one end the beam tips one way. If you lengthen the other end the beam tips that way.

How does trim effect buoyancy control? If you are head up and foot down when swimming kicking will tend to push you up as well as foreward. In order to not go up, you let air out of the bc. What happens when you stop kicking? you sink. If you were head down and foot up the problem would be the opposit.

Attitude in the water does effect buoyancy in that if you raise your top half (the end with the bc) the air in the bc will expand , and so will the wet suit, making you more buoyant. The change in buoyancy over a few inches may not be much but it will cause movement. The more you move the more the volume of air changes and buoyancy. Once mastered this is a tool that can be used to gain extremely fine control over position. Also in the vertical position you present less resistance to movement in up and down than in the horizontal position so if you're not perfectly neutral going vertical will tend to increase the resultant movement.

The short version...

Get balanced and positioned so you can maintain a horizontal position without effort and stay that way. Having a little extra weight will be way less of a pain if it's in the right place and body position is correct. Of course with a full tank or at depth when your suit is compressed you'll always have a little extra weight and maybe a lot. A full thickness suit can take a bunch of lead to sink. Once at depth it may have very little buoyancy but you're still wearing the lead.

 

[Tropics Diver]

deep_ocean once bubbled...
The problem was vertically standing still. Like I mentioned, as long as I was moving forward, I had no problems staying off the bottom (it was a shallow dive and we were close to the bottom). As soon as I stopped I either had to touch the bottom, or I'd start going up. I couldn't seem to find the equilibrium

Don't forget that when you breath in you become more buoyant, and vice versa. It's normal to move up and down a bit as you breath in and out.

When you are vertical, the up and down movement with your breathing is exaggerated sicne you don't have as much resistance to up and down movement as when you are horizontal,

 

[perpet1]

roakey once bubbled...

gadget,

There's holding your breath and then there's holding your breath. If you hold your breath by closing the back of your throat (“glottis”?) you're right, you can suffer a lung over expansion injury.

If you hold your breath by keeping your throat open, but by pulling your diaphragm down and keeping it there, there’s no danger of an over expansion injury since gas can flow freely in and out of your lungs.

That said, except in rare cases I stay with what’s taught in OW and “keep breathing” but what I’ll do is breathe shallowly at the top or at the bottom of my lung volume if I have to ascend or descend, respectively. This is only for a short period of time to “get things moving” and then I can resume normal breathing. If I was correctly neutral, I’ll continue in the direction (up or down) that I started.

So at some level I have to disagree with you, you do use your lungs as a BC, but within some constraints.

Roak

I agree with Roak here. the fact that you inhale and expel air in your lungs they are infact a large factor in controlling your buoyency. It is how you use that force that matters. You can hurt yourself if you use it wrong and it can really improove your skills when done right.

Roak, as for your earlier comment about proper trim and you should not be able to do a fin pivot if you are properly trimmed, well I have to dissagree with you there. Since you legs and feet are staticly buoyent (posative or negative for the most part and I am not going into drysuits here but even ther the goal is to maintain it as static as posible) and your upper body (lungs and BC) is veriable then you will NEVER establish a consistantly vertical trim as you go up and down. I agree you will come to a point of horizontal trim but you will not maintain this as you adjust your BC for posative and negative. The best you can hope is that your feet and lower legs are perfectly buoyent but that is not realistic. If they are in fact perfectly buoyeny and your upper body swings from posative to negatrive then your trim will naturally swing with that as your upper body will move faster since it is acted upon by the force of buoyency and your lower body will just follow. This will cause the proper fin pivot motion. If you were trimmed horizontalll then if you established neutral buoyency above the bottom I would expect that at some time your feet would follow but this would not be instantly and I would give money to see you do it in a plain as you added buoyency to your upper body.

To do what you are trying to explain you would need to add a proporational beuyancy over your entire body evenly head to toe. This is not the case unless you have invented a new type of buoyency device (and I am aware you could more or less do this with a Drysuit as it would have the ability to ad buoyency over your entire body but you should not be controlling beuyency with a dru suit)


Pete

 

[JohnCollins]

Zept once bubbled...


How do you tell the difference? I've read several comments like this, but I have no idea whether I'm holding my breath with my diaphragm or my throat (note: I am trying this at my desk, not in the water).

Z

Your glottis is at the back of your mouth, where you swallow. You can close it off against your soft palate so no air can pass through and then you can have big problems.

Try this. Take a deep breath and "hold" it, meaning keeping your lungs and diaphragm expanded. Now try puffing small amounts of air in/out with slight movements of your diaphragm. If you can, your glottis is open, if you can't, it's closed. If closed, you'll feel your diaphragm moving and you'll also feel right where the air is pressing in your throat at the very back of your mouth near the soft palate. You'll figure it out in about 1 and a half seconds, tops.

Keeping your glottis open means that your lungs and airway are "open" and if you rise a little bit when diving and keeping your lungs expanded, the expanding gas in your lungs will simply blow right through your second stage out the exhaust ports. It's pretty easy for you to get to feel this closed glottis condition. When you do, don't ever do it underwater because then, if you rise a little bit, the expanding gas in your lungs won't go out the exhaust port on your second stage, it will blow right through your lungs into your chest cavity, and that will ruin your whole damn day, guaranteed. As you say, practicing at your desk is better than practicing when diving .

Roakey, Pugg, et all, what a great thread! Enough here to keep anyone busy practicing for a while. Preesh!

 

[MikeFerrara]

perpet1 once bubbled...


Roak, as for your earlier comment about proper trim and you should not be able to do a fin pivot if you are properly trimmed, well I have to dissagree with you there. Since you legs and feet are staticly buoyent (posative or negative for the most part and I am not going into drysuits here but even ther the goal is to maintain it as static as posible) and your upper body (lungs and BC) is veriable then you will NEVER establish a consistantly vertical trim as you go up and down. I agree you will come to a point of horizontal trim but you will not maintain this as you adjust your BC for posative and negative. The best you can hope is that your feet and lower legs are perfectly buoyent but that is not realistic. If they are in fact perfectly buoyeny and your upper body swings from posative to negatrive then your trim will naturally swing with that as your upper body will move faster since it is acted upon by the force of buoyency and your lower body will just follow. This will cause the proper fin pivot motion. If you were trimmed horizontalll then if you established neutral buoyency above the bottom I would expect that at some time your feet would follow but this would not be instantly and I would give money to see you do it in a plain as you added buoyency to your upper body.

To do what you are trying to explain you would need to add a proporational beuyancy over your entire body evenly head to toe. This is not the case unless you have invented a new type of buoyency device (and I am aware you could more or less do this with a Drysuit as it would have the ability to ad buoyency over your entire body but you should not be controlling beuyency with a dru suit)


Pete

This isn't correct Pete. If your center of gravity is in the same place as your center of buoyancy there are no apposing forces so assuming the same body position you'll stay balanced even as overall buoyancy changes and your whole body will rise. If those centers are offset then it's as you describe but if that's the case you won't trimmed.

However it is still possible to do a fin pivot but you have to arch your back lika a son-of-a-gun to keep your feet on the bottom (like you would want to. When you do this you're essentially doing something similar to the DIRF COG drill but with your feet touching the bottom.

 

[roakey]

What Mike said. Thanks for saving me the typing

Roak

 

[eod]

MikeFerrara once bubbled...


This isn't correct Pete. If your center of gravity is in the same place as your center of buoyancy there are no apposing forces so assuming the same body position you'll stay balanced even as overall buoyancy changes and your whole body will rise. If those centers are offset then it's as you describe but if that's the case you won't trimmed.

However it is still possible to do a fin pivot but you have to arch your back lika a son-of-a-gun to keep your feet on the bottom (like you would want to. When you do this you're essentially doing something similar to the DIRF COG drill but with your feet touching the bottom.

If you breathe in you will change your center of bouyancy towards your head (assuming horizontal position). I can't say if it is relevant and you sure can adjust your center of gravity towards your head by arching your back or bending your legs.

 

[MikeFerrara]

eod once bubbled...


If you breathe in you will change your center of bouyancy towards your head (assuming horizontal position). I can't say if it is relevant and you sure can adjust your center of gravity towards your head by arching your back or bending your legs.

Yes, when you breath in your lungs get more buoyant and when you put air in your bc you add buoyancy to the same region. So...this area might be a good place to have some weight. OTOH, if all the weight are lower and you have to put a bunch of air in the bc it stands you up. Of course your build also effects things.