Do you really have to exhale while ascending?

[devilfish]

What I THOUGHT was....say you have an 80 cu. ft. tank on the surface. At 33' you would have half of that, since the pressure doubled...so you'd have 40 cu. ft. of air.

At 66' you'd have 1/3, or approx. 26 cu. ft. of air. At 99', 1/4 or 20 cu. ft.

So from 99' to 66', a change of 33', the volume of air would change by only 6 cu. ft.

The same change, from 33' to 0', would change by 40 cu. ft., meaning that the volume of air expands much more nearer the surface.

Am I wrong?

I wasn't talking about volume, I was talking about physical size of a flexible object, diameter not volume displacement. And btw, scuba tanks don't compress.

 

[Quarrior]

What I THOUGHT was....say you have an 80 cu. ft. tank on the surface. At 33' you would have half of that, since the pressure doubled...so you'd have 40 cu. ft. of air.

At 66' you'd have 1/3, or approx. 26 cu. ft. of air. At 99', 1/4 or 20 cu. ft.

So from 99' to 66', a change of 33', the volume of air would change by only 6 cu. ft.

The same change, from 33' to 0', would change by 40 cu. ft., meaning that the volume of air expands much more nearer the surface.

Am I wrong?

You are correct. We need to keep in mind there are 2 pressures that must be considered. Internal pressure created by the volume itself and the external pressure which is the constraning element.

If the internal pressure increases as the volume increases and the external pressure decreases, thus allowing for the expansion of the container (lungs), that internal pressure will go somewhere and since the lungs are soft, pop.

Even a tank has a pressure relief valve called a burst disk.

 

[Quarrior]

I wasn't talking about volume, I was talking about physical size of a flexible object, diameter not volume displacement. And btw, scuba tanks don't compress.

Yes, but the discussion was about holding one's breath from 6ft to the surface.

However, if you were to take a tank down to 33ft and fill it to max capacity down there and attach a lift bag to it and send it to the surface, I'd be willing to be that the burst disk would go before it hits the surface.

 

[NJMike]

Maybe I messed everybody up by saying "pressure" instead of "volume"?

 

[Charlie99]

Six feet of pressure change is the same at any depth. On a full lung, I mean full lung if it takes 6ft ascent to cause a lung to rupture, it does not matter at what depth. From 6ft to the surface the change in pressure is 2.67psi, from 96ft to 90ft it's still 2.67psi. If that's what it takes to cause injury, it will happen at any depth.

That calculation would be relevant IF the lungs were rigid containers. They aren't, so what is more relevant is the percentage or volume of expansion needed in the lungs to accomodate the change in depth. When looked at it this way, Boyles law is the dominant term and indeed, as noted in real life, lung expansion injuries are more likely to occur in shallow depths.


Another simple way to look at it is to realize that very, very rarely does someone truly breath in to the full limit of their lungs. This reserve inspiration capacity will "absorb" the excess gas volume as you ascend with a closed airway. At shallower depths, the reserve inspiration capacity is filled up with a much smaller depth change.

 

[EvaL]

I think the issue is volume of air (air expansion) in the lungs as one ascends.
I hope I get this right from reading my Divemaster workbook last week: there is 1 atmosphere of pressure change for each 33 feet of seawater.
So 1 gallon of air at the surface (1 atm) is compressed to: 1/2 gallon at 33ft (2 atm), 1/3 gallon at 66ft (3 atm), 1/4 gallon at 99ft (4 atm), 1/5 gallon at 132ft (5 atm) and so forth.
Coming up from 132ft depth to surface, the increase in volume
from 132ft to 99ft is (1/4)/(1/5) = 5/4 or 125%,
from 99ft to 66ft is (1/3)/(1/4) = 4/3 or 133.33%,
from 66ft to 33ft is (1/2)/(1/3) = 1.5 or 150%,
from 33ft to the surface is 1/(1/2) = 2 or 200%.
Hence the air in your lungs expand the most (while ascending) the closer you are to the surface.
(Can someone let me know if my calculations are incorrect?)
Definitely, do not hold your breath coming up, particularly the last few feet.
Dive safely.

 

[Charlie99]

In OW certification classes students are taught variants of "always keep breathing" and "never hold your breath". Obviously, if these rules are followed, you will avoid lung overexpansion injury.

My personal rule is a bit more liberal ---- "Always keep the airway open". Of course, I will inadvertantly violate that rule anytime I swallow or cough, but even then there isn't a problem if not ascending.

I found that during OW cert class it was difficult to do a 20' CESA because I was humming or blowing too many bubbbles. A couple years later I decided to learn how to do a CESA properly. I turns out that, by simply keeping the AIRWAY OPEN and letting the air bubble out on its own, that it was very easy to do a CESA from 90'.

Just as with normal diving, how full you let your lungs get affects your buoyancy. As you hold your lung volume at any position, any excess air from expansion will just flow naturally out of your mouth -- the same as if you were breathing out, even though you are holding your lung volume (via the chest muscles) at the same point. It is a very strange feeling to leave the bottom with normal lung volume, have air bubble out of your mouth for over a minute, and arrive at the surface (acutally I stopped at 10' in my practice CESA) with your lungs still having the same amount of air in them.

Don't try this unless you have good voluntary control of the airway closure at the back of your throat -- glottis, epiglottis, or whatevere that doohickey that closes right at the beginning of a cough is called.

Charlie Allen

 

[diver_pirate]

The initial comment was to control assent rate by holding ones breath. You are in a physical medium that has mass, inertial and resistance.

If hold your breath either full or empty it takes far longer to significantly modify the assent rate than adjusting your BC. If your lungs can add or subtract a liter of volume by you holding your breath you can add or subtract 2 pounds of lift/weight. When you practice a fin pivot notice how slow that change takes place.

Its silliy to practice such a activity that has such an enormous ability to put you in risk.
--DP

 

[Meng_Tze]

..at the back of your throat -- glottis, epiglottis, or whatevere that doohickey that closes right at the beginning of a cough is called.

Charlie Allen

That is funny

 

[NJMike]

So if we are talking about volume, then maybe we're all on the same page. What do you say, Devilfish?