Controlling Buoyancy with your breath: Why?

[frndrfoe]

I always found it very difficult to maintain buoyancy at shallow depths when over weighted. If you move a foot or two vertically the air expands/compresses more quickly at shallow depths and shoots you to the surface or sends you downward. Just like a wetsuit loses so much buoyancy in the first atmosphere underwater.

 

[gypsyjim]

Part of open water was learning to weight yourself correctly. It sounds like this did not happen well in your class.

I watch instructors in open water show new students how when they are correctly weighted, on the surface, with no air in your bladder, relaxed, inhaling and exhaling will raise your face slightly out of the water, and lower your face beneath the surface.

Then you add air to float, empty the bcd and exhale to begin descent.

Excess weight requires a diver to keep adding air, not just to compensate for depth, but to "carry" all that extra lead. You end up with a very large bubble on your back that you are dragging around the entire dive, using up more of you energy, and air, just to move through the water.

It also means you need quite a bit more air in the bcd just to keep that lead afloat on the surface.

It is a wondrous feeling to be so well balanced that a slow, gentle breath, deeper or shallower, will be all that is required to change your depth.

 

[boulderjohn]

For some reason we were thought to take long deep breaths when diving, not sure where this comes from but I have heard it from two different OW instructors.

It is the unfortunate wording used in most beginning scuba instruction. What you actually do is something along the the ideas of the ancient Greek concept of the Golden Mean--the ideal is usually halfway between the extremes. We don't want to take short, shallow breaths, but we don't what to be taking huge gulps of air, either. The long breaths they describe are the golden mean between those extremes.

When you are trying to control your buoyancy as depths change, you have to deal with the fact that your BCD and your thermal protection are reacting to those changes.You can control those reactions by constantly adding or removing air from your BCD, or you can do it through your breathing. If you do it through your breathing, the change in your total volume that comes from the breathing must overcome the total change of volume that comes from those other factors. The more air in the BCD, the more you have to overcome through the breathing. Every pound you are overweighted requires about a pint of otherwise unnecessary air in the BCD to compensate. If you are 8 pounds overweighted, you have to have a gallon of air in the BCD to compensate.

When I am in the pool with students while wearing a 3mm wet suit and an AL 80 tank, I am also overweighted a bit because that is something instructors do for reasons related to student control. I do a demonstration in which I go from the very bottom of the deep end of the pool (12 feet) to the very top and back down to the bottom, stopping and hovering at times along the way, using nothing but my lungs to make those changes in depth. It would be easier for me to do it if I were not overweighted a little, but I can do it. On the other hand, if I am in the pool working on technical diving and carrying steel doubles, I am very much overweighted. I can only control my depth this way for a few feet. If my depth changes more than that, the change in the BCD volume is too much, and I have to add or remove air from it.

 

[Lorenzoid]

Someone mentioned "balancing acts." Another balancing act is knowing when to use breath control to make fine adjustments in depth and when to use the BC to make major adjustments. In Fundies, my instructor noted I had developed a bad habit of trying to over-use breath control, and pointed out that if not used judiciously it can cause a diver to use more of his air supply than if he were to use breath control only for fine depth adjustments and the BC for more major depth adjustments.

 

[redacted]

Basic physics suggests that went you take a large neutrally buoyant mass, like a diver, and add a little gas it will become positively buoyant ad start to rise. I suspect you are used to being negatively buoyant and compensating by applying just a bit of upward propulsion as you exhale. Once you get comfortable with neutral buoyancy and relaxed breathing, the problem you percieve should almost disappear.

 

[The Chairman]

You do have to get weighted properly. That will affect the buoyancy swing as you ascend/descend as well as trim swing. When that bubble shifts, so does your trim. Reduce the bubble and reduce both swings.

 

[htatton]

The biggest problem I see with a big bubble is Boyles Law. You come up a little, bubble expands and the elavator starts up getting faster and faster as you go up. Same problem going down, You go down a little and the bubble contracts and the elavator starts down at an accelerating pace.

It was only when, Thanks to Net Doc and his Trim and Buoyancy Course, that I started concentrating on getting a very small bubble did control become easy. The second benefit is my SAC dropped (.15 cft/min) which greatly extending my time on the bottom. Nice steady relaxed breathing is the norm. Breathe with lungs mostly full to go up a little, Breathe with lungs mostly empty to go down a little.

What I'm practicing now is to get my weight so I can have almost NO bubble on my safety stop.

Still learning and having a blast. Can't let you frisky youngsters have all the fun.

 

[RyanT]

My OW students won't get certified by me if they aren't able to control their depth with their breath. In fact, they have to be able to descend to the bottom of the pool, ascend to the top and then descend to the bottom again using only their breathing. No control? No cert.[/QUOTE]

My hero!

 

[rongoodman]

If you are heavy and unable to control your depth with your breathing, then you have to resort to kicking and/or lots of small buoyancy adjustments. Using your breathing to make small adjustments to your depth is much easier than other methods.

I'm going to disagree with this a little. I find that the quickest and easiest adjustments can be made with finning, followed by breathing, with BC adjustments coming last.

 

[oly5050user]

In a situation like this I suppose you would swim up or drop your weights.

Out of air you can do a cesa or alt air ascent. Once on surface still need to oral inflate or drop lead. Why drop lead if you can oral inflate without any drama? I can see drop lead if necessary of course, but is it not best to have options?