Overweighted at beginning of dive but underweighted at end in shallows

[BRT]

The gas exchange happens in a very small space inside individual alveoli. Turbulence helps, but is also dampened by those alveoli. I would suggest that for the average diver, learning to stop moving so much would do more to alleviate CO₂ buildup. They'll do that by learning how to adjust their depth with only their breathing rather than kicking. The less you move, the less CO₂ you produce. The less CO₂ you produce, the less you need to transpire that CO₂, so the less you'll need to breathe.

Look at your respiration while you're sitting in a chair. Your chest moves so little because you're producing very little CO₂. Walk and your respiration will probably increase in frequency, rather than volume. Running will produce a rather large increase in both frequency and tidal volume. Just try to pause that breathing to maximise efficiency after a good run, wink, wink. Normally, it doesn't take much to fill and empty those tiny alveoli, even though there are hundreds of millions of them. So, keep your activity down. Don't scull. Adjust your depth with your breathing. Avoid currents like the plague. Relax and let the ocean do your work for you whenever possible. Be a Zen diver, but avoid that silly buddha pose!

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All good, but the longer you hold the air the more O2 goes to your body and the more CO2 leaves your body. If you doubt me try holding your breath for varying lengths of time and exhale into an O2 sensor.

 

[crofrog]

All good, but the longer you hold the air the more O2 goes to your body and the more CO2 leaves your body. If you doubt me try holding your breath for varying lengths of time and exhale into an O2 sensor.

During a breat hold, oxygen is continuously consumed by metabolic processes, and carbon dioxide is produced as a byproduct. The exchange of these gases relies on continuous ventilation to maintain the necessary balance. When you hold your breath, CO2 accumulates in the blood because it is not being expelled from the lungs, while O2 levels steadily decrease as it is used by the body without being replenished.

The process of gas exchange in the lungs depends on the partial pressures of O2 and CO2. For CO2 to be effectively expelled from the blood, the partial pressure of CO2 in the blood must be higher than in the lungs. Holding your breath leads to a continuous increase in the partial pressure of CO2 in your blood, which eventually reduces the gradient needed for CO2 to wash out. Simultaneously, as O2 is used up and its partial pressure drops in the lungs, the gradient needed for O2 to enter the blood diminishes, reducing oxygen uptake.

During a prolonged breath-hold, O2 levels drop, and CO2 levels rise in your blood, which is why the urge to breathe intensifies—the body responds to increasing CO2 (hypercapnia) and decreasing O2 (hypoxia). If you were to measure your exhaled breath with an O2 sensor after holding your breath, the sensor would show a lower O2 concentration than if you were breathing normally. Similarly, a CO2 sensor would indicate higher levels of CO2 due to the build-up during the breath-hold.

That’s my understanding as a lay person at least. If you want the lowest possible levels of co2 hyperventilation is the way to accomplish that. Obviously that would increase air consumption.

I supported your stance on triangle breathing because that works on your parasympathetic nervous system to lower heart rate and promotes calmness, and often when people get stressed out there breathing rate increases significantly which leads to respiratory alkalosis and more anxiety and it becomes a horrible positive feedback loop.

 

[BRT]

During a breat hold, oxygen is continuously consumed by metabolic processes, and carbon dioxide is produced as a byproduct. The exchange of these gases relies on continuous ventilation to maintain the necessary balance. When you hold your breath, CO2 accumulates in the blood because it is not being expelled from the lungs, while O2 levels steadily decrease as it is used by the body without being replenished.

The process of gas exchange in the lungs depends on the partial pressures of O2 and CO2. For CO2 to be effectively expelled from the blood, the partial pressure of CO2 in the blood must be higher than in the lungs. Holding your breath leads to a continuous increase in the partial pressure of CO2 in your blood, which eventually reduces the gradient needed for CO2 to wash out. Simultaneously, as O2 is used up and its partial pressure drops in the lungs, the gradient needed for O2 to enter the blood diminishes, reducing oxygen uptake.

During a prolonged breath-hold, O2 levels drop, and CO2 levels rise in your blood, which is why the urge to breathe intensifies—the body responds to increasing CO2 (hypercapnia) and decreasing O2 (hypoxia). If you were to measure your exhaled breath with an O2 sensor after holding your breath, the sensor would show a lower O2 concentration than if you were breathing normally. Similarly, a CO2 sensor would indicate higher levels of CO2 due to the build-up during the breath-hold.

That’s my understanding as a lay person at least. If you want the lowest possible levels of co2 hyperventilation is the way to accomplish that. Obviously that would increase air consumption.

I supported your stance on triangle breathing because that works on your parasympathetic nervous system to lower heart rate and promotes calmness, and often when people get stressed out there breathing rate increases significantly which leads to respiratory alkalosis and more anxiety and it becomes a horrible positive feedback loop.

I think I agree. but we are not talking about breath hold diving. We are talking about hesitating breathing with fuller lungs vs hesitating with emptier lungs. Taking a breath and immediately exhaling it and then waiting will not get as much O2 out of the air as doing it the other way.

 

[crofrog]

I think I agree. but we are not talking about breath hold diving. We are talking about hesitating breathing with fuller lungs vs hesitating with emptier lungs. Taking a breath and immediately exhaling it and then waiting will not get as much O2 out of the air as doing it the other way.

Go hyperventilate and report back.

Any “holding” is going to increase co2 to some extent.

 

[JW59]

Hi all,

Recently went to Curacao and I'm not sure if it's just a matter of figuring out my new gear or what but I figured someone here might have some insight. Many of the dives there seem to have to the later 1/3 of the dive in the shallows where we ran the tanks relatively low at the end of our hour given we were only a few meters from the surface. Using AL80's and I was using a 5mm wetsuit but I found at the beginning of the dive I would be overweighted and have to put a fair amount of air in my BCD to level out at ~20-25m. That was relatively fine as I prefer to be a bit overweighted for initial decent. This issue is that for the end of the dive, with a bit of waves, it was a real struggle to maintain that 4-6m of depth and not just float to the surface. I drained all the air in my BC from every valve and even tried rotating my body around to get any last bits of air but no matter what I did I would just start floating up to the surface on my inhale.

Not sure if this is just one of those thing where there's nothing you can do but wondered if anyone had come across this and if they had any solutions.

For reference I am ~185 lbs and was using 14lbs of lead

Thanks in advance.

NN

Hi Nick. You should be concerned about your buoyancy, as it is a safety issue. Secondarily, being incorrectly weighted will cause you to use more air for a given dive. Here is my recommendation for getting the right amount of weight:

1. With a full tank and your usual diving configuration of wetsuit and accessories, burn your tank down to 700 psi at the safety stop, or if in an open area like a reef, be at the 15' level.

2. Now that you're at 15' with 700 psi, dump all the air out of your BCD.

3. If correctly weighted, you should be neutrally buoyant at this point. If negatively buoyant, remove weight. If positively buoyant, add weight. The goal here is to get to the amount of weight that makes you neutrally buoyant at your safety stop with 700 psi. Then, when you're finished with your safety stop, you should be getting back on the boat at about 500 psi.

Using this method, you will be slightly negative at the beginning of your dive and only slightly positive when down to 500 psi. By being only slightly negative at the beginning of your dive, you will be able to get to the surface should you have a total BCD failure. Divers who add weight "so I can get down to the bottom" are ignoring the safety issue.

Hope this helps!

Jamie

 

[scubadada]

...you will be slightly negative at the beginning of your dive...

Your negative buoyancy at the start of the dive will be the weight of the gas that you will be consuming during the dive. For an AL80, that will be a little over 5 lbs, more for larger cylinders. Trouble descending is generally due to air in the BC, wetsuit, and/or lungs.

At the end of the dive, you want to be able to hold your safety stop or last deco stop and make a slow, controlled ascent to the surface. Your GF blossoms over that last 10-15 feet.

 

[JW59]

Yes, the weight of the gas in your tank contributes to buoyancy. However, too much lead is the problem that must be solved. Air (nitrox, helium) weighs what it weighs!

 

[The Chairman]

All good, but the longer you hold the air the more O2 goes to your body and the more CO2 leaves your body. If you doubt me try holding your breath for varying lengths of time and exhale into an O2 sensor.

As the air in your lungs increases in CO₂, the perfusion of CO₂ slows. It's a function of partial pressure tension. This is the problem with skip breathing. As you try to conserve air by breathing less, your CO₂ levels rise. As your CO₂ level rises, your need to breathe increases. CO₂ increases also can cause needless anxiety and headaches. This is the crux of gas physics and our physiology.

Again, do a 100m sprint, and try holding your breath to facilitate CO₂ removal. Nope. Your lungs know instinctively that the faster you breathe, the quicker you'll get all that CO₂ out of your system. There are chemoreceptors in the carotid and aortic arteries that monitor ph and CO₂ levels. When they increase, so does your heart rate and respiration. Again, lazy divers create less CO₂. I just don't move needlessly underwater. My hands are still, and while my legs do a frog kick, I glide in between those kicks. My trim facilitates the glide. Many who dive with me, often comment that I always look the same in the dive.

However, too much lead is the problem that must be solved.

Too much weight is often a function of bad trim. If the feet are angling down, then kicking produces both forward and upward thrust. Weight is often added to combat that upward thrust. Trim should be natural. IOW, if you stop near the bottom, 6 inches or so, your feet should not touch the bottom. Close your eyes, stay completely still, and you should be right in the same place 60 seconds later. Adjust weight backwards and forwards to allow you to do this. Once you get a minute, go for two, then three. I can manage five minutes.

 

[BRT]

As the air in your lungs increases in CO₂, the perfusion of CO₂ slows. It's a function of partial pressure tension. This is the problem with skip breathing. As you try to conserve air by breathing less, your CO₂ levels rise. As your CO₂ level rises, your need to breathe increases. CO₂ increases also can cause needless anxiety and headaches. This is the crux of gas physics and our physiology.

Again, do a 100m sprint, and try holding your breath to facilitate CO₂ removal. Nope. Your lungs know instinctively that the faster you breathe, the quicker you'll get all that CO₂ out of your system. There are chemoreceptors in the carotid and aortic arteries that monitor ph and CO₂ levels. When they increase, so does your heart rate and respiration. Again, lazy divers create less CO₂. I just don't move needlessly underwater. My hands are still, and while my legs do a frog kick, I glide in between those kicks. My trim facilitates the glide. Many who dive with me, often comment that I always look the same in the dive.

Too much weight is often a function of bad trim. If the feet are angling down, then kicking produces both forward and upward thrust. Weight is often added to combat that upward thrust. Trim should be natural. IOW, if you stop near the bottom, 6 inches or so, your feet should not touch the bottom. Close your eyes, stay completely still, and you should be right in the same place 60 seconds later. Adjust weight backwards and forwards to allow you to do this. Once you get a minute, go for two, then three. I can manage five minutes.

So you think it improves the CO2 buildup to hesitate breathing with your lungs on the empty side vs on the full side?

 

[johndiver999]

Once again people are recommending significant modifications in breathing in order to address buoyancy/weighting issues.

Other than for temporary and relatively infrequent adjustments in buoyancy, I think it is far superior to use scuba gear (as it was intended) - especially the BC to make these adjustments.

If you are floating around and not moving during a dive, barely using your fins, then it is not so critical exactly how you breathe because the respiratory demands are not that significant. However, if/when you need to actually move forcefully and with some speed, the respiratory situation changes a good bit and it is important to ventilate as efficiently as you can. The importance of efficiency increases with depth and workload - that is exactly why people want to recommend "good" regulators for deep, demanding dives. Why the hell buy a sports car and never push the gas pedal down?

I think other people have described the optimal manner well when they say to use a slightly deeper inhalation and exhalation cycle and a slow rate when resting and an accelerated rate when working. I'm not so sure about the pausing myself, I just do what feels right.

Once you start moving a greater tidal volume of air back and forth, the concomitant effect on buoyancy comes into play (simply due to physics). So when a diver is under a work load, an attempt to artificial manipulate dead air space in the lungs becomes less effective and less desirable.

If/when you begin to be stressed, it is far better to allow yourself to breathe fully in and out, rather than try to play games and manipulate buoyancy, by for example, not inhaling fully (if you are underweighted) or not exhaling completely (if you are overweighted).

In my experience, it is far better to dump a bit of air in the BC than to risk CO2 headache or worse. Put simply, when you start working hard, you start having your lungs more full (on average), so you will start to float more.

Take enough lead to stop your ascent at ANY depth and with zero air in the BC - and in reality, if you want to be able to effective deploy an SMB (and breathe comfortably and safely) you want ANOTHER 3 -5 lbs to allow yourself to hang and rest on the SMB string and keep the smb standing, more or less vertically.

Who wants to be perfectly neutral at 15 feet with 500 psi and then in an emergency with 150 psi, float up uncontrollably into an oncoming boat propellor from a depth of 6 feet? If you are diving in a full 7 mm wetsuit, there is a big swing in buoyancy between a safety stop depth and the surface - if you are wearing a dive skin, there is zero effect, so conditions matter a lot in this case.