I don't understand: breathing deeply and using breathing for buoyancy control seem to contradict each other

[johndiver999]

In my experience, the importance of deep breathing begins at around 30 m and is no longer negligible at 50 m.

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Yes for sure. For all the people who float around in a motionless state with their arms extended forward and their hands holding nothing in 80 or 100 ft, who cares how you breathe?

The test is when you are deep and working, like say spearfishing, where you might suspend breathing for a while (in order to not scare the fish) and then make the shot and sprint after the fish and wrestle with it (in depths past recreational). In those types of situations, the way you breathe can make a big difference in your performance and recovery.

For me, it is critical to avoid panting and to not get so far "behind" in my respiration, that I can not take the time to fully exhale. The full (or at least comfortably complete) exhalation is what drives out the Co2 and promotes recovery.

With a partial pressure of oxygen being 5 or 8 times higher than on land, the challenge is moving CO2 not oxygen. When I am deep and stressed aerobically, I can feel the panic trying to set in and then when I feel the urge to begin an inhalation before I have fully exhaled, then I know for sure that this is my limit and I need to try to stop motion (with big muscles anyway) and focus on the full exhalations for quite a few cycles.

 

[Minisub]

If you want to get the most O2 out of your air and get rid of the most CO2 when you exhale, you will pause after the inhale, not after the exhale.

Do you have any Walter Kidde compressors

 

[BHWookiee]

I actually had the same question a while ago, as I’m trying to improve my SAC rate, I asked AI and got this response, which I felt was actually pretty good., pasting it here if you find it helpful:

That’s a brilliant question—and you’ve pinpointed one of the most misunderstood subtleties in scuba diving.

Here’s the deal:

The Core Idea

Yes, you’re absolutely right

• Inhale = slight rise
• Exhale = slight fall

And also:

• Slow, deep breathing = lower gas consumption

So how do you reconcile the two?

The Secret: Buoyancy Control Isn’t in the Breath — It’s in the Breath Control.

Let’s break it down:

1. Get Neutrally Buoyant at Your Average Lung Volume

• At rest, breathe in and out normally.
• Add or vent air from your BCD until you neither rise nor sink while gently finning or hovering.
• This is your neutral buoyancy baseline—not at full inhale or full exhale, but somewhere in the middle of your breathing cycle.

2. Use Breath Modulation, Not Extremes

• Once you’re neutral at that average lung volume, you can make tiny buoyancy adjustments using your breath:
  • A slightly deeper inhale than normal gives you a gentle rise.
  • A slightly longer exhale than normal gives you a slow sink.
• You’re not trying to float up on a full breath and plummet down on an empty one. That’s inefficient and makes you seesaw.
• Think of breath like fine-tuning, not a substitute for your BCD.

3. Long, Slow Breaths Still Work

• When you breathe deep and slow, you’ll gently rise a little as you inhale, and settle a little as you exhale.
• But if your buoyancy is properly trimmed, these changes are tiny and smooth, not disruptive.
• Think of it like hovering with a gentle pulse, not bouncing.

Mental Cue

Picture yourself as a jellyfish:

• Rise and fall rhythmically and gracefully, in a narrow vertical range.
• The movement is almost meditative. It’s not meant to take you up or down 2 meters—just to gently ride your own breath.

Tips to Practice

• On your next dive, hover mid-water near a visual reference (rock, coral head).
• Focus on minimizing how much you move vertically as you breathe.
• Try to slow down your breath without increasing the rise/fall amplitude. That’s when you’ll start to master it.

 

[EdMcNeill09]

There are important differences between breathing on land (1 bar) and underwater (e.g. 5 bar).
Let's look for the influence of pPO2 on tolerated pPCO2.
The following is translated from German: Karl's Kreislauftauchgeräte Seite

The Influence of Oxygen Partial Pressure on Breath Holding Time

Unfortunately, some diving books still state that breath holding time is independent of oxygen partial pressure. However, this is not the case when it comes to increased ppO2!For example, if one breathes pure oxygen at the surface instead of air, one will notice a significant increase in the possible breath holding time. This effect is due to a shift in the threshold of the body's ppCO2 sensors that regulate breathing.

Divers whose normal breath-holding time (after breathing air) is 3 minutes will be surprised to notice that they can hold their breath for about 6 minutes after breathing pure oxygen beforehand (this test was conducted at the surface).
A graphic: 'Blood-ppO2 & -ppCO2 during apnea after oxygen breathing' will be added later.

If our respiratory control system, which is adapted to 1 bar, regulates higher pPCO2 values at higher ambient pressures, the question is what can be done about it.
Since the respiratory stimulus threshold is raised, we can only keep the starting value (lowest pPCO2) of a respiratory cycle low in order to keep the average pPCO2 as low as possible. The only way to do this is to exhale deeply in order to get rid of as much CO2 as possible. Then inhale deeply to dilute the remaining amount as much as possible.
Unfortunately, I cannot cite any scientific sources that describe this topic qualitatively or, even better, quantitatively.
In my experience, the importance of deep breathing begins at around 30 m and is no longer negligible at 50 m.

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This is very helpful. It also explains an experience I had a couple of years ago. I typically log 8-10 dives a week on 32% Nitrox. A couple years ago I went from Honolulu to Maui to log dives on Avelo in preparation for an Avelo IDC. I breathed normally and after my second morning dive had a slight headache. I logged a night dive the same day and and again had the headache. Only after the CEO of Avelo laughingly told me I had a C02 headache did I realize he was right. I breathed normally like I do on NItrox but still had a C02 buildup. Fascinating.

Still I would advise people to breathe normally underwater and if they start to get a headache breath more deeply.

The reason for this advice is that breathing deeply from the diaphragm is not good advice for people with large lung capacity as it makes learning Buoyancy next to impossible. Breathing from the diaphragm is great advice for an average body type with a small tidal volume while breathing restfully. But large lung divers typically get overweighted during training which reduces their zone of stable buoyancy. Every pound extra shrinks the zone of stable buoyancy. In extreme cases it probably eliminates the possibility of stable buoyancy entirely.

 

[EdMcNeill09]

The importance of deep breathing begins at home: it is fairly well established by now that slow deep abdominal breathing promotes relaxation and reduces stress. In diving, relaxation is generally believed to improve buoyancy control and gas mileage. QED.

Yes and no. Slow deep breathing definitely promotes relaxation and reduces stress. Absolutely true.

For people with large lung capacity or larger than average tidal volumes a full breathe can make achieving neutral buoyancy very difficult. While this is most commonly found among big bodied divers with big lungs it is also a reality with both men and women who are smaller. I was teaching someone last week who was 5'7" and 155lbs. He has larger than average lung capacity and was having a very hard time getting neutral. learning to breathe more naturally was the key for him.

 

[dmaziuk]

Yes and no. Slow deep breathing definitely promotes relaxation and reduces stress. Absolutely true.

For people with large lung capacity or larger than average tidal volumes a full breathe can make achieving neutral buoyancy very difficult.

I'm 6'1 and I swam through formative years and then did martial arts on and off, most recently: taiji, i.e. a lot of diaphragm breathing. My lung size is such that when I do chest X-ray, half the time they take two shots: top and bottom(*). I normally take slow deep breaths when I dive and I have no problem staying neutral. At least in my normal vacation mode: I might need a little practice in a 7 mm suit as I haven't done that in a while.

Lung capacity is not a problem if you're a swimmer. Here's a simple test: if they can't float on their back in speedos in a regular swimming pool, they just can't relax in the water.

*) The other half would say "do you sing?" and go move the camera back and refocus the beam before they take the shot and realize the bottoms of my lungs are out of the frame.

 

[EdMcNeill09]

I'm 6'1 and I swam through formative years and then did martial arts on and off, most recently: taiji, i.e. a lot of diaphragm breathing. My lung size is such that when I do chest X-ray, half the time they take two shots: top and bottom(*). I normally take slow deep breaths when I dive and I have no problem staying neutral. At least in my normal vacation mode: I might need a little practice in a 7 mm suit as I haven't done that in a while.

Lung capacity is not a problem if you're a swimmer. Here's a simple test: if they can't float on their back in speedos in a regular swimming pool, they just can't relax in the water.

*) The other half would say "do you sing?" and go move the camera back and refocus the beam before they take the shot and realize the bottoms of my lungs are out of the frame.

Out of curiosity did you have any difficulty learning neutral buoyancy?

 

[johndiver999]

• Add or vent air from your BCD until you neither rise nor sink while gently finning or hovering.

I thought that AI description was really good.

However, a small detail, when checking your buoyancy, you want to completely stop swimming for a few seconds (not gently finning) unless there is some compelling reason why you can't stop kicking at the moment. A good check, requires a few moments of pause, if you want to fine tune things.

Another tip, that might be added .. is that you can often look at the particles in the water around you, in order to get a quick and accurate feedback on if you are rising or sinking, maybe be quicker and easier than looking at a computer, or the bottom, or your buddy. This only works, if the water is not too turbulent however.

 

[dmaziuk]

Out of curiosity did you have any difficulty learning neutral buoyancy?

Not that I recall, really. Find yourself a (n ex-) competitive swimmer student and see how they fare: I only have a sample of one me, it may be that one exception that reaffirms the rule. Or a freediver -- they should get it too.

 

[scubadada]

Not that I recall, really. Find yourself a (n ex-) competitive swimmer student and see how they fare: I only have a sample of one me, it may be that one exception that reaffirms the rule. Or a freediver -- they should get it too.

I was a competitive swimmer from age 4 through high school. I was certified at 16. My buoyancy and gas consumption have always been good