Bouyancy, breathing and SAC

[sillygrendel]

I'm fairly new to diving and am starting to reach the point where my bouyancy control is getting to be pretty good.

Sunday I was out practicing a bunch of stuff at the quarry. Anyway... While I practice backward kicks I like to hover as close to the platform as possible without touching so I can have a reference of exactly how far back I move with a given kick. I also take it as an opportunity to practice bouyancy and trim by keeping myself inches above the platform without touching it. I also try not to rise to far off of it.

Normally while diving and swimming around I take looonggg, slooowww breaths, timing them with kicking/gliding to maintain pretty fine bouyancy control. I find that if I do the same thing (without kicking to compensate) I'll rise up and sink down about 4' each breath cycle. I find when trying to maintain fine bouyancy control (within 1' or so, without kicking) I end up breathing more quickly to minimize the amount I move in a given cycle, also leading to a higher SAC.

I'm starting to think this is just a fact of life and that if I want to maintain depth that finely I'm going to have to stay slightly on the + side and do few fin flips to maintain depth. That or just deal with the higher SAC.

What do you all do?

 

[Uncle Pug]

sillygrendel once bubbled...
I find when trying to maintain fine bouyancy control (within 1' or so, without kicking) I end up breathing more quickly to minimize the amount I move in a given cycle, also leading to a higher SAC.

If you were not breathing as deeply your SAC wouldn't go up... and neither would you.

Try replacing *more quickly* with *less deeply*.

 

[Rick Murchison]

Lets look at the physics involved a bit, and perhaps you can retain your slow breathing without much of an excursion. I'm going to start out at an elementary level for the benefit of any who may need it, so if it's too basic please just bear with me...
Neutral buoyancy is sort of like balancing a ball on your head. It is theoretically possible to attain perfect balance, but any displacement will accelerate in the direction of displacement unless countered by some corrective force. In other words, neutral buoyancy is an unstable condition.
If I have managed to obtain perfect neutral buoyancy, any displacement in an upward direction will result in the expansion of gasses in my BC, my exposure suit, and my lungs, making me positively buoyant and causing my upward movement to continue and to accelerate. Likewise, any downward displacement from neutral will cause those same gasses to be compressed, making me negatively buoyant and the downward movement will continue and accelerate. The only corrective force I want to use to reduce or eliminate this movement is the size of my lungs. (I don't want to have to mess with my BC or the gas in my drysuit, and there's nothing I can do about any neoprene I'm wearing)
Once movement upwards or downwards has started I have a double problem. Not only are the gases causing my buoyancy to get further from neutral as I rise or descend, but in moving I have momentum that must be overcome with a force in a direction opposite the movement. So, if I am rising, and exhale only to the point that I am neutrally buoyant again, momentum will keep me rising and I'll become positively buoyant again, and just keep right on going up...
Likewise, if I exhale until I stop rising, I have only stopped because I have applied a downward force sufficient to stop the upward movement, and I am now negatively buoyant. If I do nothing at this point I will now begin to sink, and the gasses will compress and I'll just keep going down. If I think "I need to keep exhaling until I start down so I can get back to where I was," then I'll really be negative and sink with vigor!
So how do I time my breathing to keep from having this yo-yo effect?
If I am neutral, then as I inhale I know I will start to rise. The instant I detect this rising, I begin to exhale slowly until the rise stops. Realizing that I'm already negative by the time the rise stops, as I stop rising I begin to slowly inhale again, expecting to descend a bit before the effect of my inhalation takes effect. Likewise, as my descent comes to a stop on inhalation, I know I am already positively buoyant again and it is time to begin my exhalation even before I've started to rise. As I practice this I find that I may even be leading the stops a tad, for example beginning an inhalation even before a rise has completely stopped, and that my verticle displacement with each breath is in inches rather than feet. After a while and a lot of practice (I've been at it over 30 years and still working) I've found that I can breathe slowly and deeply and not move vertically any perceptable distance at all. But I spend literally hours practicing in the pool for every hour on real dives to maintain the skill.
Improved SCR is automatic with improved buoyancy control.
** This "breathing to control buoyancy" only applies to open circuit Scuba... CCR is a different matter entirely**
Rick

 

[devilfish]

Rick Murchison once bubbled...

So how do I time my breathing to keep from having this yo-yo effect?
If I am neutral, then as I inhale I know I will start to rise. The instant I detect this rising, I begin to exhale slowly until the rise stops. After a while and a lot of practice (I've been at it over 30
years and still working)


Rick

BRAVO!!! You are the first one in a long time that explained the concept of true breath control.
I have a video of my ex who had only basic certification, yeah she was my student, diving for 16 years and never had the interest in other "C" cards, anyway, hovering in mid water, perfect trim, breathing normally and not moving up or down an inch. That's true breath control. She could put most instructors to shame. I still use the video to show and tell.

 

[sillygrendel]

I'll have to get out and play with my breathing some more... Knowing it is possible with just breath control is a big thing.

Hopefully with practice I should get it!

 

[mudguppy]

sillygrendel once bubbled...
I end up breathing more quickly to minimize the amount I move in a given cycle, also leading to a higher SAC.

?

Hey! I am so glad you posted your question. I was going to ask about that very same thing.
Great info. I can't wait to go practice some more now!


Rick, Thanks for the physics lesson. Something that I can relate to

 

[DSJ]

You explained something my bouyancy intructor could not.

I too was under the impression one had to breath off the "top" or "bottom" to adjust bouyancy, but now I see it is a matter of adjusting the phase of breath to the phase of vertical movement.

I see by keeping the two "Sine" waves 180 degrees out of phase they will cancel and keep you near the desired depth. To rise, or sink a bit, shift a bit before returning.

It seems obvious after you explained it...

David

 

[WKenny]

That was a great explanation.

Thank you

 

[ShakaZulu]

Ok, I have to agree, never thought of this, maybe the old guys does have some experience us kids can learn from........haha. I will shift those two sine waves 180 next time, and see what happens. Thanks uncle Rick.

Now, what to do with women?

 

[DA Aquamaster]

Now, what to do with women?

It's funny you should ask that. Just a few days ago, a diver diving with Uncle Ricky demonstrated perfect bouyancy. Uncle Ricky was moved to the point of tears and granted the diver one wish. The diver thought about it a lot and decided that he wanted a bridge from the mainland to Hawaii because he was afraid to fly and got sick on boats, yet always wanted to dive in Hawaii.

Uncle Ricky frowned and pointed out how incredibily hard it would be to build a bridge that long and suggested he make an easier wish. The diver thought about it some more and said "Uncle Ricky, I want to understand women". Uncle Ricky responded, "Would you like two lanes on that bridge or four?"