Being able to sit on the bottom for a decent length of time while burning minimal oxygen and energy can be a big help when chasing a whole bunch of different species.

I usually do it by grabbing onto a rock or a bit of weed.

But sometimes the weed just isn't strong enough to keep you down and there isn't a rock to grab on to (e.g. nothing but sand underneath you).

To stay down in such situations I'll breathe out a little to reduce my buoyancy.

I do however set my weight belt up so that even fully breathed out I'm still positively buoyant at the surface (just), so that should I ever black out underwater, I'll still float back to the surface and have a chance of survival (as long as I'm above about 9 metres - below this I start to go negatively buoyant, so when I'm diving deeper waters I'll take off a weight, but this setup covers me for most situations).

What I'm wondering is just how much does this release of air from the lungs reduce my maximum dive time?

Just how much does the air in the lungs contribute to your maximum dive time versus the oxygen already in your blood?

Had a bit of a google on this topic and turned up this thread (http://www.thenakedscientists.com/HTML/ ... w/1000268/) - the impression I get from it is that the rate of oxygen being absorbed into the blood will increase a little with additional depth (i.e. additional pressure), but makes no mention of just how much oxygen is in the air in your lungs compared to how much is in your blood, or how much of the oxygen in that air is potentially available to you.

It does mention though that with decreasing depth (especially the last 10 meters), the reduction in pressure will actually pull oxygen out of your bloodstream and put it back into the air in your lungs.

This sudden decrease in blood oxygen levels can cause you to blackout.

According to wiki (http://en.wikipedia.org/wiki/Deep_water_blackout) this kind of blackout (precipitated by depressurisation on ascent from depth) is called a deep-water blackout, and usally happens in the last 3 metres after having dived to a depth of 10 metres or more.

This is not to be confused with shallow-water blackout which is due to hyperventilating -> see here(http://en.wikipedia.org/wiki/Shallow_water_blackout), although hyperventilation can often contribute to a deep-water black-out along with voluntary suppression of breathing.

Scary stuff eh? -> you can be on the bottom at sub 10 metres, feeling like you're good, start heading up again feeling like you've got plenty in reserve and will keep feeling that way all the way up until all of a sudden in the last 3 metres the oxygen suddenly jumps back into your lung air and boom - you're in the poo.

I wonder if there's a way around this? A way to protect yourself from this happening? -> what if, on the way back up, you exhaled most of the air in your lungs at the 4 metre mark? -> kinda hard for the oxygen to jump out of your blood and into lung air if there's no longer any air in there for it to jump into?

Okay, so you'd lose some buoyancy, but as you're already heading back to the surface from 10+ metres down, you've got probably some decent momentum towards the surface behind you. So if (like me) you're set up to be positively buoyant fully exhaled at the surface, even if you blackout in the last 3 metres (and even with no momentum towards the surface behind you), you'll still float up those last 3 metres. Is that right?

Hmmm..... but to get back to the original topic - does anyone here know how much the air in your lungs contributes to your maximum dive time?

Cheers.

Do a (dry) home experiment. Max breath hold with full lungs, max breath hold with empty lungs. I would expect full lungs to play a big role in your overall breath hold not to mention having volume for equalising.

I don't believe exhaling on your way to the surface would make any difference to deep water blackout. A gas is able to dissolve into a liquid in higher quantities under pressure. As this pressure is reduced the gas will leave the blood through the easiest pathway, ie the empty space in the lungs. Even if you exhale gas will still leave the blood and enter the empty space in your lungs.

I'm basing this on high school science from back in the day, if anyone has more accurate/referenced data I certainly won't be arguing my point

Letting air out of your lungs on the bottom? You're asking for a blackout. Do a freediving course instead of reading about it from unreliable sources on the internet.

Weight yourself for where you intend on diving. If your diving in 5m of water there's no point in being positively buoyant at 5m, its better to be neutral at 5m. If your diving in 10m, yes you want to be buoyant at 5m. Hanging onto weeds and letting air out of your lungs is asking for trouble and bad advice for anybody to be reading. The most important thing is to know your limits and stay within them.

I've never heard of anybody exhaling on ascent to prevent SWB. I've heard the opposite where on ascent, experienced divers suck back the air from their mask that they had put into it on the way down.

There is a movie clip somewhere on this forum about SWB, watch it, very informative.

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I don't get sea sick, the sea gets sick of me!

Here it is

viewtopic.php?f=26&t=2278&hilit=shallow+water+blackout

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I don't get sea sick, the sea gets sick of me!

This is not what I learned.
It is true that depth increases the partial pressure of Oxygen, hence leading to better absorption, while the decrease of depth make the absorption of Oxygen worse, but it does not release oxygen into the lungs


Correct: any sudden drop of the partial pressure of Oxygen could lead to a blackout.


The Oxygen does not "jumps back", it is just absorbed less efficiently, but this fact -if you are just above the conscience threshold- can precipitate a black-out.


Look: if you exhale, you are lowering the partial pressure of Oxygen, leading to less Oxygen absorbed and a potential blackout. By the way, even a profound exhale would still leave plenty of air in your lungs.


...and the laryngospasm would probably not let the water in after you blacked-out... and then ? If your buddy is not able to put you on your back and awake you quickly, your laryngospam would eventually (say, 60 seconds ?) relent and water would start to seep in your mouth, making you lose your buoyancy while unconscious, and... no prizes for guessing what's next.

A piece of advice: please, do a freediving course before running into trouble.

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Best regards,
Luca.

A piece of advice: please, do a freediving course before running into trouble.[/quote]


^^^^

What he said

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"If people concentrated on the really important things in life, there'd be a shortage of spear guns"

Absolutely agree with doing a freediving course.
Personally I would stay away from SSI and PADI, was told by an instructor one or both were still teaching hyperventilation up til a couple of years ago.

I have done AIDA 2 and 3, and the stuff you learn about how your body works underwater and under pressure, and the mechanics of blood, oxygen, all that stuff is very valuable.
My depth, fishing, confidence and ability to help a mate if they get into trouble have all increased dramatically.
Hope this helps
Cam

Interesting topic. Exhale breath hold diving can cause issues equalising. I have seen it performed as a training tool to practice mouth fill equalising techniques.

Diving on partial lung volume also known as exhale diving or "functional residual capacity" aka frc diving is widely used by many competitive freedivers for training to simulate greater depths. FRC is just a fancy way of saying half full lungs, it's supposed to be a neutral point after a relaxed exhale. At rest an average person is only using around %20 - %30 of their Vital Capacity (difference between maximum exhale & maximum inhale). To give just a rough approximation for arguments sake on a full lung breathold %50 - %65 of your usable oxygen stores are in your lungs, the rest in your blood.

So on a half exhale you may have around 2/3 of oxygen vs a full inhale. If doing a dive to depth the work you do to get to the bottom is reduced as the change in bouyancy is far less. Also the amount of oxygen used on the bottom is likely reduced because the with increased effects of pressure on your body both Bradycardia (reduced heart rate) and Vasoconstriction (reduced blood flow to outer limbs) increases. This is otherwise known as the "dive reflex".

Some potential risks you may want to know about with this approach:-
You are much more likely to push closer to your limits as your urge to breathe will come far later compared to you theoretical max dive time. This is because CO2 is the main trigger for the urge to breathe and diving on FRC will generate far less CO2.

You definitely do not want to take more then a few slow deep breaths while doing this, strictly no hyperventilation even a conservative amount. Also make sure to use much less weight compared to inhale.

The only way to dive safely is to dive with a buddy trained in blackout rescue techniques. This is doubly true while trying out a new diving approach which you are not familiar with.

P.S. The whole sucking oxygen out of your lungs in the last 10m or so of ascent is a very outdated theory which has zero scientific facts to support it.

The rapid expansion of lungs before reaching the surface may cause an increased risk of blackout where someone is already very hypoxic and on the edge of blackout anyway. While performing statics in a pool you want to avoid standing up rapidly at the end as the change in blood pressure will definitely increase the chances of a blackout or samba. This is similar to people (particularly elderly) getting dizzy spells or fainting from standing up quickly after sitting down a long time.

So I think it's a far more plausible explanation that pressure changes from rapidly expanding lungs could cause blood pressure changes leading to an increased chance of blackout in someone that is already hypoxic.