Buoyancy and Breathing

[elan]

Try to breath with your diaphragm (yoga/martial arts style). Generally divers to try to expand their chest with "deep" breathing leading to big fluctuation in buoyancy. When using the diaphragm (breathing with the stomach) the lungs “expand” into the abdominal cavity, resulting in more controlled breathing and better buoyancy. I hope this explanation made any sense.

In fact it does not No matter how you breath (diaphragm or chest) the volume of the body and thus your buoyancy will change the same way.

 

[Akimbo]

… My SAC is still quite high (0.85ish) so I'd rather not sacrifice that. Should I wear more weight and keep more air in my BC? …

I am not sure I understand this part. Regardless of how you achieve neutral buoyancy, the buoyancy variation from your breathing cycle will have a virtually equal effect.

For example: For discussion, assume that you are neutral in a pool with lungs deflated to a half your normal exhalation cycle. Also assume your normal total inhalation is l liter = 1 KG, = 2.2 Lbs. Weighting yourself 10 Lbs negative with a full tank so you will be 3 Lbs negative with an empty thank would mean that you would need to ad 7 Lbs of buoyancy in your BC to reach neutral buoyancy, at the middle of your breathing cycle, at the beginning of the dive (full tank).

Now increase your belt by -10 Lbs and your BC by +10 Lbs. You are still neutral and you are still cycling by ½ a liter, ½ a kilo, or 1.1 Lbs above and below neutral. Therefore, the distance you move vertically is a function of time rather than the amount of air in your BC to reach neutral buoyancy. There is no mass effect because you are neutral and the increased hydrodynamic resistance is so small it would be difficult to measure.

Are you breathing fewer and deeper cycles now? Are you doing a better job at adjusting your buoyancy to neutral on the bottom? Are you more aware of your environment now? Your SAC (Surface Air Consumption Rate) is less of a factor than your variation in volume between breaths.

The only way to achieve neutral buoyancy thorough the breathing cycle is to use a counter lung like on a closed circuit rebreather Scuba (as opposed to a surface supplied closed circuit system).

 

[aquaregia]

Akimbo, I completely agree with you. I'll explain my reasoning, although with the note that I now see my original reasoning as ridiculous. I was considering my breath to be (say) 5# and my BC to be 20#, therefore my breath made up 20% of my buoyancy. With 30# in my BC, my breath would only be 15%. However, as you point out, my float/sink rate is dependent on the buoyancy change in real pounds. Thanks for the detailed rebuttal; when I read it I felt pretty dumb.

And yeah, I'm breathing less and deeper and not getting thrown into rocks as much as I was 20 dives ago. I've still got a long way to go though.

 

[stakanak]

As a new diver I have recently found how well the air exchange in your lungs "helps" with buoyancy issues. I have reduced my sac by using my BC inflator less and my lungs a little more (for buoyancy control). That is not to say I breath irregularly. Actually, like Saspotato states, it's fine tuning, but can be used to rise or drop quite a bit, especially if you slow your breathing pattern, and use good timing over irregular bottoms.
Every little bit of knowledge helps in this spectacularly wide ranged and wonderful sport. When I learn something new, it makes me smile all the more!
Good Diving.

 

[TSandM]

Just a couple of points, as this has evolved:

1. An optimal gas consumption rate requires a quiet, rhythmic breathing pattern. Rapid, shallow breathing, as many new divers do out of stress, is inefficient and results in high gas consumption. But the advice to use "slow, deep breaths" can be misconstrued; completely filling and emptying the lungs with each breath DOES result in bigger buoyancy issues, and doesn't reduce gas consumption. A normal breathing volume, done in a relaxed, rhythmic way around a normal center point for lung volume, is your goal. As somebody has already said, if you time it right, you'll exhale just before you begin to go up, and your excursions from target depth will be in inches, rather than in feet.


2. Your gas consumption will not be significantly related to the amount of air you put in your BC or dry suit, unless you are adding and dumping constantly. This is likely to occur if you are overweighted, but with proper weighting, you should basically add gas until you reach the deepest portion of the dive, and then gradually vent it as you ascend and as you empty your tank (and it therefore becomes less negative).

3. The only buoyancy issue that is really helped by adding weight is being underweighted -- and the symptoms of that are good buoyancy control at the beginning of the dive, and poor control at the end. Since the end of the dive also coincides with the ascent, and divers who aren't good at anticipating the need to vent their BC or suit tend to have problems them, it's common for people to diagnose that they are underweighted, when in fact it's just a technique issue. Adding weight won't help in that circumstance. This is the true value of a good weight check, done at the end of the dive, with 500 psi in the tank.

 

[elan]

There is no mass effect because you are neutral and the increased hydrodynamic resistance is so small it would be difficult to measure..

I think you are confusing the mass effect with the weight effect. You can be weightless but have great mass. The effect of inertia depends upon the mass. The more the mass is the more force required to move the object to the same distance in the same period of time. Thus if you have greater mass you will have less fluctuations in the water provided you create the same vectors of forces with your breathing.

 

[dkktsunami]

Can't begin to tell you how to do this but after you have done more diving you will begin to time your breathing (exhaling just before you go up, inhaling just before you go down). This isn't a very scientific explanation but eventually you will time your breathing, without thinking about it, and this will help to minimize the up and down effect as your lungs expand and contract. Like RJP said, stop thinking about breathing and improvement will follow.

 

[k ellis]

now that you got your sac rate where you want it practice breathing while in the water as others have said and breathe just enough in to get you to start to ascend then breathe out just enough to begin a descent. With time and patience youll get it. Thats one of the fine arts of scuba is when you finaly get it to where your breathing does all the work for you as far as ascents and descents

 

[Akimbo]

I think you are confusing the mass effect with the weight effect. You can be weightless but have great mass. The effect of inertia depends upon the mass. The more the mass is the more force required to move the object to the same distance in the same period of time. Thus if you have greater mass you will have less fluctuations in the water provided you create the same vectors of forces with your breathing.

Unlike something weightless in space, you must also factor media density. In practice, hydrodynamic drag dampens small variances around neutral buoyancy to the point it is difficult for a diver to detect. Inertial mass, active gravitational mass, and passive gravitational mass are virtually zero on a neutrally buoyant object. Yes there is a difference, but ambient water movement generates far greater forces in most conditions a diver would experience.

In deeper water (where gas expansion has less effect with depth changes) it is quite easy to achieve neutral buoyancy on a 500 Lb steel object on a lift bag and push it around (feet planted on the bottom). The same object in a small current is many times more difficult to control. From personal experience, two Scuba divers can swim a neutrally buoyant 32" diameter x 10' long metal capsule with a dry weight in the 3400 Lb ball park. It will stop (loose all forward inertia) when the divers stop kicking in about a foot.

 

[MauiScubaSteve]

I teach my students by telling them they should breath in normally, so in that respect I agree with RJP. Then I define a "normal" inhalation as ~2/3rd full lungs. The exhalation is a different animal; I tell my students it should be ~twice as long as the inhalation (2 sec in - 4 sec out, 3 sec in - 6 sec out, etc.). The best breathing petite women divers I have timed do ~5 sec in ~10 sec out (4 breaths per minute :shocked2, which I think causes brain damage, but most are blonde anyway.

If you are breathing in normally, when you exert more you breath in more, and if you double the time of inhalation you then exhale more (longer). I call it a kind of mantra and tell them that for me breathing this "mantra" helps relax me, like meditation. If my students have to be brainwashed it might as well be beneficial brainwashing.

The air in the BC is the rough buoyancy adjustment; the lungs are the fine adjustment (as stated already). The optimal adjustment is to change the position of your "range" of inhalation/exhalation. If you ascend ~5' from a "perfect" BC air amount, breathing from the "bottom" of the lungs gives you a more negative net buoyancy (completely empty to ~2/3rd full). If you descend ~5' from a "perfect" BC air amount, breathing from the "top" of the lungs gives you a more positive net buoyancy (completely full to 1/3rd full). In this hypothetical example, a perfect breathing cycle at the in between depth would be from 1/6th empty to 5/6th full and one could theoretically not adjust BC air content for up to 10' depth change.

Perfection is not in the realm of typical human beings, but understanding the "hypothetical perfect" can help with what actually happens in the real world.

Timing the inhalations/exhalations so that you get the required ~2/3rd in/out cycle with the least oscillation of depth is the key (as stated already). Since the time of inhale is different than the time of exhale, it is pretty hard to maintain a constant depth without sometimes exhaling / inhaling early, or exhaling / inhaling late. Now combine this with currents or surge around walls or big reefs and sometimes you have to aggressively exhale to keep from being lifted too much for your BC air content, or inhale aggressively to keep from sinking too much for your BC air content.

The above situations (current or surge at "walls") is where Horizontal trim "only" seems questionable. Not only do I exhale aggressively when being lifted by water, but I "swim down" as needed. "Swimming up" when water is moving down is also helpful, in combination with aggressive inhalation.

I am not claiming this is the only way to describe / teach breathing / buoyancy, but I have had a lot of success describing / teaching it this way.