Controlling Buoyancy with your breath: Why?

[boulderjohn]

I hope people realize that the reason breathing in makes you more buoyant is NOT because you are now lighter. You are, in fact, now heavier because you have added a certain amount of air to your body. When there is more air in a tank early in a dive, after all, it is heaver than when it loses air. The difference between having more air in the tank and having more air in your lungs is that the tank stays the same size, but your body (the chest area specifically) gets bigger with the air intake.

Archimedes' principle tells us that we are neutrally buoyant when our body (with all the equipment) weighs exactly the same as the equivalent volume of the water we are in. If we are neutrally buoyant and then expand our lungs by adding air, we are now larger, and because that additional air has little weight, we are now lighter than that larger volume of water. If we exhale, our chest area shrinks, we are now smaller, and we become heavier than the same volume of water.

 

[Storker]

Breathing deeply underwater will waste your gas, and if your are properly weighted, render you somewhat out of control.

Shallow breathing will retain CO2 and can lead to hypercapnia.

I prefer long, deep breaths. Just get into the rhythm and feel and control the up/down movement imposed by your breath. It takes a little practice, but it isn't very difficult. Shallow breathing can easily be bad for you.

 

[Bob DBF]

I hope people realize that the reason breathing in makes you more buoyant is NOT because you are now lighter. You are, in fact, now heavier because you have added a certain amount of air to your body. When there is more air in a tank early in a dive, after all, it is heaver than when it loses air. The difference between having more air in the tank and having more air in your lungs is that the tank stays the same size, but your body (the chest area specifically) gets bigger with the air intake.

Archimedes' principle tells us that we are neutrally buoyant when our body (with all the equipment) weighs exactly the same as the equivalent volume of the water we are in. If we are neutrally buoyant and then expand our lungs by adding air, we are now larger, and because that additional air has little weight, we are now lighter than that larger volume of water. If we exhale, our chest area shrinks, we are now smaller, and we become heavier than the same volume of water.

I love a smartass. Buoyancy and weight are sometimes related, but not as directly as some may want it to be.


Bob

 

[RainPilot]

Guilty as charged, I also missed the "Why?" part of the OP question.

When I dive rebreather, I cant use my lungs to control my buoyancy so I have to use the air spaces in the equipment (wing and counter lungs)
If I need to go up by a foot or 3, especially shallow dives, then it becomes a ballet of add a squirt to the wing, start to rise, gas expands, now dump to stay at new level, once over the rock or whatever then I need to dump a little more to descend again and then the wing compresses so add a little more to level off.

It is astounding to see how much gas I use for that in a up-down dive on a shallow reef, to the extent that now I go around stuff (not over) to minimize the PITA.

With OC, once you have the breath control dialed in then it is MUCH quicker, more responsive to dive with an almost empty wing and use breathing to adjust in the water column, and since it is breathing the gas wastage is minimal.

If you have your weighting dialed in correctly (empty-ish tank at the surface) then the ascent phase becomes so much simpler. The old dump/inflate/dump/inflate of the wing is so much harder to get correct than a simple intuitive breath control.

I also find that in the pool it is a lot more tricky due to the percentage ATA shift with small changes. Once you get below 10m it is a LOT easier. Hang in there and practice practice practice .

 

[billt4sf]

Seemed to me that enough weight to allow me to concentrate on good breathing was a bigger +, than absolute minimum weight and sloppy buoyancy when breathing.

Sure. nothing wrong with being overweighted a pound or two.

I think you are overthinking this. If you take normal size breaths, but maybe a but more slowly, kind of "sipping" the air but not taking huge breaths, you'll find that's it's fun and easier diving than being overweighted (by like more than a couple pounds) and taking long breaths.

If you are correctly weighted then using your breath to control your buoyancy in the water is easy and fun. If you are overweighted, then you'll have lots of air in your BC and it will expand like hell when you take long breaths. I am speaking from experience.

-Bill

 

[boulderjohn]

I love a smartass. Buoyancy and weight are sometimes related, but not as directly as some may want it to be.

Was I being a smart ass? There were posts earlier that indicated people did not understand this, and I know from instructing advanced students that many people do not understand this.

It is the relationship of weight and volume that makes the difference. An ocean liner weights considerably more than a sinking diver, but it floats because its volume is so much greater.

 

[Bob DBF]

Not intentionally.

I do agree that a lot of divers don't understand it, quite possibly instructors as well, which is probably why so many divers are overweighted for no reason.


Bob

 

[Shotmaster]

I learned to dive before BCDs. You had to get your rig balanced or your workload increased too much and then it wasn't fun. You learned breath control. I see people with too much weight a lot. I also see them pumping and dumping air. Good training is the answer. I work at keeping track of my trim weight, going from swim trunks, 3mm, and 5mm and if I am just sight seeing or spearfishing, what any additional gear does to my load balance. I keep track of all weight. On deeper dives I usually add a puff or two to make up for suit compression. New divers won't be aware unless instructed properly. With the reduced class times, I don't know how they can get this across effectively.

 

[The Chairman]

I learned to dive before BCDs. You had to get your rig balanced

I also learned to dive without a BC, but no one talked about a balanced rig. In fact, the biggest feature when I bought my first pair of Jets in 1969 was that I could kick the crap out of the reef and not hurt the fins. The environment was of little concern to us, and time at the surface was always a struggle. The first I heard about a balanced rig was from the DIR crowd around the turn of the century.

 

[Bob DBF]

The first I heard about a balanced rig was from the DIR crowd around the turn of the century.

I didn't hear that expression until I was on Scubaboard however, I was taught to weight myself so that I could float on the surface with a near empty or empty tank. This was not a noble quest, but a way to conserve lead and gear that would have to be dropped so you could stay on the surface.

Later I got a Mae West, which could be inflated orally through a modified tire valve (PITA), but the CO2 cartridges were not cheap so I stuck with the weight that left me slightly positive on the surface. With the advent of the Horsecollar and jacket BC, weight was no longer tied as directly to buoyancy.


Bob