Do you really have to exhale while ascending?

[scubatexastony]

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?

Actually, that's wrong since the tank is a solid container, it will still have 80 cu. ft. at any depth. Look at Boyles Law, P1V1=P2V2 or PV= K (constant)

As mentioned, the lungs are flexible, we need to fill them to the same volume, but at 33 fsw (2ata) you need twice as much surface pressure to fill them. That would mean you would go through your 80 cu. ft. in half the time you would at the surface, because you use twice the volume.

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

DIAMETER, no, as the volume of cylinders or balls isn't directly proportional to the diameter. But the SIZE (aka. volume) of a flexible object would be 1/2 if filled at the surface and taken to 2 ATA or 33fsw. Reverse the process from 33 fsw with the filled baloon, it will double in size (volume) at it reaches the surface, if it doesn't blow apart! Now do the math for 99 fsw to 66 fsw.

Good example Eval.

 

[devilfish]

Actually, that's wrong since the tank is a solid container, it will still have 80 cu. ft. at any depth. Look at Boyles Law, P1V1=P2V2 or PV= K (constant)

As mentioned, the lungs are flexible, we need to fill them to the same volume, but at 33 fsw (2ata) you need twice as much surface pressure to fill them. That would mean you would go through your 80 cu. ft. in half the time you would at the surface, because you use twice the volume.



DIAMETER, no, as the volume of cylinders or balls isn't directly proportional to the diameter. But the SIZE (aka. volume) of a flexible object would be 1/2 if filled at the surface and taken to 2 ATA or 33fsw. Reverse the process from 33 fsw with the filled baloon, it will double in size (volume) at it reaches the surface, if it doesn't blow apart! Now do the math for 99 fsw to 66 fsw.

Good example Eval.

Check this out. http://www.scuba-doc.com/physics.htm

 

[Butch103]

I have to ask why the original question was asked????

NJMike you quoted an article detailing an injury because they held their breath in the last 6 ft!!!!


Why ask the obvious?? It suggeste to me that you dont' believe your instructor, your certifying agency and the article written.

I have been on this board (and many others) and I am constantly amazed at newly certified divers and dive students questioning safety issues.

I appreciate this might be a harsh response, but cripes people think before you post!

 

[Rick Murchison]

I have been on this board (and many others) and I am constantly amazed at newly certified divers and dive students questioning safety issues.

The NAUI advanced diver manual used to (may still for that matter) describe proper weighting as the amount of weight that would make a diver neutrally buoyant with an empty BC and 500 psi at the safety stop of 15 FSW.
That's wrong, and unsafe because it means the diver would have no way of being anything but positively buoyant during the final 15' of ascent.
I hadn't really thought about it - and as an instructor I was teaching it - (as were most instructors in 2001) until a newly certified diver questioned it here on Scubaboard, and I suddenly realized what we were teaching was BS.
So... new divers, keep on questioning those procedures when they don't make sense to you. There's a chance it may not make sense at all, and we just don't know it yet
Rick

 

[MXGratefulDiver]

Sounds like you might believe that a 12in diameter baloon will compress to be 6in diameter at 2ATA. In actuality it would take 8 ATA to do that. Right?

Ah, no ... I believe that your examples are irrelevent to what's actually being discussed. But on further review of this thread I see that others have been trying to explain to you why that is.

You'll either get it or you won't ... but Boyle's Law doesn't concern itself with the diameter of a balloon ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

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.

Check this out. http://www.scuba-doc.com/physics.htm

... and your point is?

I get the impression you're parroting things without understanding what they truly mean ... or how they relate to pressure changes while scuba diving ... or why that's important to your safety.

... Bob (Grateful Diver)

 

[Missdirected]

The NAUI advanced diver manual used to (may still for that matter) describe proper weighting as the amount of weight that would make a diver neutrally buoyant with an empty BC and 500 psi at the safety stop of 15 FSW.
That's wrong, and unsafe because it means the diver would have no way of being anything but positively buoyant during the final 15' of ascent.
I hadn't really thought about it - and as an instructor I was teaching it - (as were most instructors in 2001) until a newly certified diver questioned it here on Scubaboard, and I suddenly realized what we were teaching was BS.
So... new divers, keep on questioning those procedures when they don't make sense to you. There's a chance it may not make sense at all, and we just don't know it yet
Rick

Holy canolies - so I am not a lunatic! Thank you, Rick!!!!!

 

[TSandM]

Devilfish DOES have a valid point. Since the volume of a sphere is proportional to the radius CUBED, it would take 8x the pressure to reduce the radius by one-half. In other words, volume decreases very quickly as the radius of a sphere decreases. So, if you ascend six feet to the surface, the change in radius of your alveoli would not be very great at all. However, not all structures within the lung are spherical -- Large volumes are in tubular airways, some of which are relatively rigid, so the volume expansion there has to be absorbed by the more flexible smaller airspaces. It is simplistic to assume that alveoli cannot rupture with those pressure changes because their radius won't increase enough.

The actual mechanism of air embolism still interests me, though. If it is alveoli that rupture, they should be putting air only into capillaries, which are tiny. It takes a significant volume of air to cause a symptomatic air embolism. It's never made a great deal of sense to me how this volume of air gets into the bloodstream from pulmonary barotrauma. We have ARDS patients on ventilators who require very high inspiratory pressures to inflate their lungs. They often get pneumothoraces, but they don't get air embolism. It's always bothered me, this disparity.

 

[MXGratefulDiver]

Devilfish DOES have a valid point. Since the volume of a sphere is proportional to the radius CUBED, it would take 8x the pressure to reduce the radius by one-half. In other words, volume decreases very quickly as the radius of a sphere decreases. So, if you ascend six feet to the surface, the change in radius of your alveoli would not be very great at all. However, not all structures within the lung are spherical -- Large volumes are in tubular airways, some of which are relatively rigid, so the volume expansion there has to be absorbed by the more flexible smaller airspaces. It is simplistic to assume that alveoli cannot rupture with those pressure changes because their radius won't increase enough.

The actual mechanism of air embolism still interests me, though. If it is alveoli that rupture, they should be putting air only into capillaries, which are tiny. It takes a significant volume of air to cause a symptomatic air embolism. It's never made a great deal of sense to me how this volume of air gets into the bloodstream from pulmonary barotrauma. We have ARDS patients on ventilators who require very high inspiratory pressures to inflate their lungs. They often get pneumothoraces, but they don't get air embolism. It's always bothered me, this disparity.

I get all that, Lynne ... but what his first post inferred was that the degree of expansion will be the same for a given distance regardless of depth. That's not the case.

... Bob (Grateful Diver)

 

[scubatexastony]

I get all that, Lynne ... but what his first post inferred was that the degree of expansion will be the same for a given distance regardless of depth. That's not the case.

... Bob (Grateful Diver)

Some people just don't want to get their brain in gear, or the calculator out and check the math!