How much air do you need?

Bob001 · 8th April 2012, 08:15 AM

The return of "Titanic" has me wondering this: As the boat sank passengers were pulled underwater, but their lifejackets brought them back to the surface. Some survived the experience (like Kate and Leo), but others drowned. Question: If you could seal a plastic bag around your head before you went under (always travel with duct tape), what volume of air would you need to survive for, say, two minutes? Would a standard supermarket grocery bag be big enough? Even if you were just trying to hold your breath, the bag would help keep water out of your nose and mouth. Or would this just be an exercise in futility? (No, I do not care to conduct an experiment...).

NewtonTrino · 8th April 2012, 08:28 AM

I would think the average person could probably hold their breath for 2 minutes in decent conditions. Ice cold water might make it harder. A plastic bag would probably be counter-productive as it would suck into your mouth if you tried to breath.

Ray Brady · 8th April 2012, 08:30 AM

All of it. I need all the air. Stop using it. It's mine.

Beerina · 8th April 2012, 08:32 AM

Tape over your mouth and nose would not kill you in a mere 2 minutes. I doubt even an out of shape person under exertion would pass out that fast, but don't quote me on it.

And a plastic bag taped around your throat would act as a very good flotation device.

I was under the understanding people in the water died from drowning from passing out from going numb in the near-freezing water. I didn't see the movie so I didn't know they portrayed people being sucked down by the sinking ship. Mythbusters busted that myth, but they did use a boat that was about 1/20th the size of the actual titanic.

NewtonTrino · 8th April 2012, 08:53 AM

As a side note as long as you hold your breath you can get sucked down pretty far without dying. A couple hundred feet down is perfectly fine for example.

Denver · 8th April 2012, 08:58 AM

We had a pool in the backyard what I was little. And we kids found we could put a small plastic bucket over our heads and, with some effort, hold it there while we sat on the bottom of the pool, breathing happily (and blissfully ignorant of the risks). If I recall, the main thing that drove us back to the surface was not the shortage of air, but the effort of holding the bucket under water.

So I imagine a plastic bag would be rather helpful, assuming the length of time was not too great, and ignoring the cold and probable hyperventilation.

TjW · 8th April 2012, 09:03 AM

Yep. Wikipedia says the record depth for no-limits apnea is 214 meters

NewtonTrino · 8th April 2012, 10:36 AM

Originally Posted by Denver

We had a pool in the backyard what I was little. And we kids found we could put a small plastic bucket over our heads and, with some effort, hold it there while we sat on the bottom of the pool, breathing happily (and blissfully ignorant of the risks). If I recall, the main thing that drove us back to the surface was not the shortage of air, but the effort of holding the bucket under water.

So I imagine a plastic bag would be rather helpful, assuming the length of time was not too great, and ignoring the cold and probable hyperventilation.

The only real risk in that situation would be going to the bottom of the pool, taking in a lungful of now compressed air and then holding your breath all the way back to the surface.

Bikewer · 8th April 2012, 11:09 AM

They had a fellow on Science Friday a while back talking about the science of drowning. He said that what would have killed many of these folks was what he called the "cold water respiratory response".
Essentially, suddenly being chucked into cold water causes an instant, involuntary gasp, which means that if you are already underwater, a big intake of water. If you fight this response and succeed in holding your breath, you can actually survive considerably longer.

He was primarily researching survival of naval accidents/warfare incidents.

casebro · 8th April 2012, 11:45 AM

The plastic bag taped over your head would at least prevent the intake of water. But in near drowning, I'd bet you'll tear it off and fill your lungs with water. I stayed down too long once when snorkling. Really hard to NOT inhale just short of the surface.

But if somebody says getting sucked down by a sinking ship is an un-true myth, I would believe them. The were thousands of survivors of ships sunk in WWII, and must have had first hand knowledge of watching their buds close to sinking ships. Interviews must have been done.

However, exiting a ship after it heads for the bottom might be a whole different problem. The hydro-dynamics could cause all kinds of swirling, and increase water pressure would push a victim INTO the boat. And the myth might have been started by a near-victim, who barely escaped- back in the wooden boat days.

akama1 · 8th April 2012, 10:26 PM

Originally Posted by casebro

But if somebody says getting sucked down by a sinking ship is an un-true myth, I would believe them. The were thousands of survivors of ships sunk in WWII, and must have had first hand knowledge of watching their buds close to sinking ships. Interviews must have been done.

The one I was peeved with was that mythbusters did not consult was the sinking of the HMS Hood duing WW2. Only 3 survivers, and one of the survivers directly states he was pulled down with the ship, till what they believe escaping air from the ship broke the suction http://en.wikipedia.org/wiki/Ted_Briggs , thou it was the worse case senario, massive mass of ship sinking extreamly quickly. A lot harder to recreate at scale.

rjh01 · 9th April 2012, 01:57 AM

Originally Posted by NewtonTrino

The only real risk in that situation would be going to the bottom of the pool, taking in a lungful of now compressed air and then holding your breath all the way back to the surface.

Another risk would be staying there too long and then having no strength to get back to the surface. This could happen if you used up too much of the oxygen without a massive build up of CO2.

Yuri Nalyssus · 9th April 2012, 02:07 AM

Originally Posted by Beerina

... they portrayed people being sucked down by the sinking ship. Mythbusters busted that myth, but they did use a boat that was about 1/20th the size of the actual titanic.

I thought mythbusters confirmed this as an genuine phenomenon.

I remember the reconstruction they did in a pool with a big model ship; the effect of being 'pulled down' by the ship was as a result of the air bubbles being released from inside the ship as it went down reducing the bouyancy of the water above the ship so that the human body sinks instead of floats.

You get the same phenomenon when kayaking, if you get stuck in a 'stopper' like at the bottom of a wier or waterfall where the water is circulating around and tends to pull the boat back towards the cascade rather than allowing it to float downstream. In that circumstance it's difficult or impossible to paddle away from the danger as the water is so full of air bubbles that you can't get any purchase with the paddle.

Yuri

Yuri Nalyssus · 9th April 2012, 02:27 AM

Originally Posted by Yuri Nalyssus

I thought mythbusters confirmed this as an genuine phenomenon...

Yuri

Ah well, wrong again - http://dsc.discovery.com/videos/myth...-minimyth.html ... sigh

The test in the pool involved a very heavy, cube-shaped block which sank rapidly and actually sucked a diver under in a very dramatic fashion. Obviously I went to put the kettle on for a cup of tea at that point in the original programme, because when they used a 4 tonne ship sinking in a harbour in the second part of the test, no suction was observed. They did mention obviously that the Titanic was considerably bigger than 4 tonnes and scale might make a difference.

But a friend of my grandfather's hairdresser heard from their cousin's brother's chiropracter that a live octopus hatched out of her stomach - that's definitely true

.

Yuri

DavidS · 9th April 2012, 10:15 AM

Originally Posted by Yuri Nalyssus

I thought mythbusters confirmed this as an genuine phenomenon.

As to MythBusters trials in general: It should be obvious that MythBusters' "confirmed" is almost always more definitive than their "busted".

If they establish X and observe Y, that's enough to prove that Y can happen with X. It just did: DONE. It doesn't prove X necessary, sufficient, nor even related to Y. That's a different question, and explicitly avoiding it is one good reason their trials try to replicate conditions proposed by the myth.

If they establish X but do not observe Y, they've only shown that Y didn't happen for that particular X that particular time. Alone, that's not enough to prove that Y never happens with anything X-like. Even when it's possible to do, truly eliminating all such possibilities is often too tedious for fun TV programming. Where convenient and entertaining, they'll try to stretch conditions X or relax tests Y to further constrain those possibilities... but generally not eliminate them entirely.

As to the "sucked down" phenomenon: I'm truly amazed how many people believe there's no such thing.

Maybe my incredulity at their incredulity arises from my familiarity with fluid mechanics that makes the mechanism obvious to me. When a solid body (e.g. ship) moves through a fluid, the fluid will have to flow in behind the body to fill the space it leaves behind. When that body isn't streamlined (e.g. sinking ship), that inflow won't follow neatly along the boundary of the body. The boundary streamlines separate (which is what we mean by "not streamlined"), and the void behind gets filled by vortices. Anything in the fluid that's small enough will get dragged along in the vortices; if it happened to be in the right place, it'll follow the moving body for a bit. If that body's moving down (a sinking ship) and the vortex flow imposes drag forces on the particle (sailor) larger than its buoyancy, the particle gets dragged underwater.

Maybe it's pedantry about the word "sucked", because what's actually happening is that water in the vortices is pushing downward. Of course, it's really such pedantry that sucks, because such unbalanced push is really what that other "suck" means.

Float just about anything not-too-streamlined. Pull it underwater by applying some force to overcome its buoyancy. A sinking ship usually does that by putting water where it used to have air, but for clarity just hang a weight on a ship-shaped solid block of wood. Marvel at the bubbles of air dragged underwater by the vortices behind the sinking ship; if the ship didn't have any air to release, those bubbles must have got dragged down from the atmosphere by hydrodynamic forces.

Float some test particles first (say, sawdust for a bathtub model, sailors for a ship) and watch some of them go down, too. Try again with only a few test particles, and see how hard it is to put them where they'll get "sucked" down.

CORed · 10th April 2012, 11:28 AM

Originally Posted by NewtonTrino

The only real risk in that situation would be going to the bottom of the pool, taking in a lungful of now compressed air and then holding your breath all the way back to the surface.

Probably not fatal, but a ruptured eardrum or two would be likely. Ouch.

phunk · 10th April 2012, 11:36 AM

Originally Posted by CORed

Probably not fatal, but a ruptured eardrum or two would be likely. Ouch.

Actually, it can be quite fatal, even in just a couple feet of water. It's not the eardrums that are a problem (although popping them will suck), it's the air embolism caused when the air expands in your lungs while you're holding your breath. It doesn't take much overpressure to force bubbles into your bloodstream.

RussDill · 10th April 2012, 07:32 PM

Originally Posted by Denver

We had a pool in the backyard what I was little. And we kids found we could put a small plastic bucket over our heads and, with some effort, hold it there while we sat on the bottom of the pool, breathing happily (and blissfully ignorant of the risks). If I recall, the main thing that drove us back to the surface was not the shortage of air, but the effort of holding the bucket under water.

So I imagine a plastic bag would be rather helpful, assuming the length of time was not too great, and ignoring the cold and probable hyperventilation.

We fixed your problem with a fairly large suction cup, some rope, and a ~70 quart bucket.

Mark6 · 11th April 2012, 07:55 AM

Originally Posted by Denver

We had a pool in the backyard what I was little. And we kids found we could put a small plastic bucket over our heads and, with some effort, hold it there while we sat on the bottom of the pool, breathing happily (and blissfully ignorant of the risks). If I recall, the main thing that drove us back to the surface was not the shortage of air, but the effort of holding the bucket under water.

Holy... crap.

You are very lucky to be alive.

The most dangerous part of ascent from any dive where you breath ambient pressure air, is last 8-10 feet. If you hold breath while ascending, your lungs expand, but how much they expand depends on relative pressure difference, not on absolute difference. The difference between 10' depth and surface is 1.3 (1 atm and 1.3 atm). Lung expansion by a factor of 0.3 is quite sufficient to rupture them, and to cause air embolism. And I suspect your average suburban parents do not have pure oxygen tanks handy, or know the first aid procedure for air embolism. Well, I have and know, and would not be surprised if Phunk does, but he and I would never allow such activity to begin with.

NewtonTrino · 11th April 2012, 08:21 AM

Originally Posted by phunk

Actually, it can be quite fatal, even in just a couple feet of water. It's not the eardrums that are a problem (although popping them will suck), it's the air embolism caused when the air expands in your lungs while you're holding your breath. It doesn't take much overpressure to force bubbles into your bloodstream.

Not to mention tearing your lungs and filling your chest cavity with compressed air.

Modified · 11th April 2012, 10:13 AM

Originally Posted by Mark6

Lung expansion by a factor of 0.3 is quite sufficient to rupture them, and to cause air embolism.

Even with a normal breath though (lungs filled to around half capacity)? That is, would there be a danger ascending quickly with breath held from 10 feet deep with half filled lungs?

Besides that, most home pools are 8 feet deep, so sitting on the bottom, Denver's lungs were probably centered about 6 1/2 feet deep. I would think that unless you held a very forced deep breath, a 20% expansion isn't going to cause a problem.

phunk · 11th April 2012, 11:17 AM

Originally Posted by Modified

Even with a normal breath though (lungs filled to around half capacity)? That is, would there be a danger ascending quickly with breath held from 10 feet deep with half filled lungs?

Besides that, most home pools are 8 feet deep, so sitting on the bottom, Denver's lungs were probably centered about 6 1/2 feet deep. I would think that unless you held a very forced deep breath, a 20% expansion isn't going to cause a problem.

It can happen in a couple feet if you take a deep breath and hold it before ascending.

With half full lungs, you'd be fine at pool depths. The problem is when someone instinctively takes a deep breath before taking their head out of the bucket, then holds it while ascending.

Mark6 · 12th April 2012, 10:33 AM

Anecdote from my own life:

Over 12 years of diving I panicked under water exactly once. Not in an ink-black lake, not inside a wreck, not when tangled in a flag line. No, I panicked in a 12' deep brightly lit pool, during my Divemaster training.

One of the tests you need to pass to become a PADI Divemaster is "equipment exchange while buddy-breathing". The instructor and the trainee take off their masks, exchange them, and put them on. Then do same thing with their bouyancy vests (to which tank and regulator are attached). While doing all this, both must breath from the same regulator -- the one initially instructor's. This is much harder than it sounds, and is incredibly stressful. My heart rate and oxygen demand went way up -- as expected. What I did not expect was the instructor having problems. As I struggled to put his mask on, and to keep track of one regulator that was giving me air, he signaled "up". At which point I panicked. Not "OMG, I am going to die!" panic, more like "OMG, I am going to fail the class!" -- but the result was the same. My awareness narrowed drastically, and as the instructor went up taking the air source with him, I completely forgot I had a perfectly good regulator hanging off me. What I should have done was take my own regulator into mouth, calm down, then surface. Instead I went up with my lungs essentially empty, yet actually felt pressure inside my chest as what little air there was expanded. At which point I remembered "hum as you ascend" rule, which keeps your airway clear for air to escape, and surfaced without injury. Then I found out the instructor aborted the exercise because he was having trouble, not because of anything I did. We rested, and repeated without incidents. But this was the closest I ever came to a serious injury while scuba diving, and the only time I panicked. In a YMCA pool.

akama1 · 12th April 2012, 09:01 PM

Originally Posted by Mark6

One of the tests you need to pass to become a PADI Divemaster is "equipment exchange while buddy-breathing". The instructor and the trainee take off their masks, exchange them, and put them on. Then do same thing with their bouyancy vests (to which tank and regulator are attached). While doing all this, both must breath from the same regulator -- the one initially instructor's. This is much harder than it sounds, and is incredibly stressful. My heart rate and oxygen demand went way up -- as expected. What I did not expect was the instructor having problems. As I struggled to put his mask on, and to keep track of one regulator that was giving me air, he signaled "up". At which point I panicked. Not "OMG, I am going to die!" panic, more like "OMG, I am going to fail the class!" -- but the result was the same. My awareness narrowed drastically, and as the instructor went up taking the air source with him, I completely forgot I had a perfectly good regulator hanging off me. What I should have done was take my own regulator into mouth, calm down, then surface. Instead I went up with my lungs essentially empty, yet actually felt pressure inside my chest as what little air there was expanded. At which point I remembered "hum as you ascend" rule, which keeps your airway clear for air to escape, and surfaced without injury. Then I found out the instructor aborted the exercise because he was having trouble, not because of anything I did. We rested, and repeated without incidents. But this was the closest I ever came to a serious injury while scuba diving, and the only time I panicked. In a YMCA pool.

Damn. need to remember that. I still wish to get my Basic dive licence. each year I keep putting it off.

SezMe · 12th April 2012, 11:55 PM

Originally Posted by Mark6

Anecdote from my own life:

We had to do a similar exercise before going down for a week in Hydrolab (scroll down to about the middle). While exchanging with my buddy, for some reason I just could not get enough air. The facility manager saw I was having trouble so kept the exercise going (he later told us). I finally had to swim over to a small "bubble" that was kept in the area in order to recover. I have no idea what happened to cause the gulping-for-air problem. But we passed the test and went on with the mission because we stayed down. Going up after getting saturated was, of course, impossible. so not panicking and staying down was the key in this test exercise.

8th April 2012, 08:15 AM	#1
Bob001 Muse Join Date: Dec 2006 Posts: 884	How much air do you need? The return of "Titanic" has me wondering this: As the boat sank passengers were pulled underwater, but their lifejackets brought them back to the surface. Some survived the experience (like Kate and Leo), but others drowned. Question: If you could seal a plastic bag around your head before you went under (always travel with duct tape), what volume of air would you need to survive for, say, two minutes? Would a standard supermarket grocery bag be big enough? Even if you were just trying to hold your breath, the bag would help keep water out of your nose and mouth. Or would this just be an exercise in futility? (No, I do not care to conduct an experiment...).
Bob001 Muse Join Date: Dec 2006 Posts: 884

8th April 2012, 08:30 AM	#3
Ray Brady Muse Join Date: Jan 2011 Posts: 579	All of it. I need all the air. Stop using it. It's mine.
Ray Brady Muse Join Date: Jan 2011 Posts: 579

8th April 2012, 08:32 AM	#4
Beerina Sarcastic Conqueror of Notions Join Date: Mar 2004 Location: A floating island above the clouds Posts: 23,835	Tape over your mouth and nose would not kill you in a mere 2 minutes. I doubt even an out of shape person under exertion would pass out that fast, but don't quote me on it. And a plastic bag taped around your throat would act as a very good flotation device. I was under the understanding people in the water died from drowning from passing out from going numb in the near-freezing water. I didn't see the movie so I didn't know they portrayed people being sucked down by the sinking ship. Mythbusters busted that myth, but they did use a boat that was about 1/20th the size of the actual titanic.
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8th April 2012, 08:58 AM	#6
Denver Philosopher Join Date: Sep 2007 Location: Denver, Colorado Posts: 6,606	We had a pool in the backyard what I was little. And we kids found we could put a small plastic bucket over our heads and, with some effort, hold it there while we sat on the bottom of the pool, breathing happily (and blissfully ignorant of the risks). If I recall, the main thing that drove us back to the surface was not the shortage of air, but the effort of holding the bucket under water. So I imagine a plastic bag would be rather helpful, assuming the length of time was not too great, and ignoring the cold and probable hyperventilation.
	__________________ Dreams inevitably lead to hideous implosions -- Invader Zim

8th April 2012, 09:03 AM	#7
TjW Penultimate Amazing Join Date: Mar 2005 Location: up in the air Posts: 10,004	Yep. Wikipedia says the record depth for no-limits apnea is 214 meters
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8th April 2012, 08:28 AM	#2
NewtonTrino Illuminator Join Date: Jul 2007 Location: Bothell, WA Posts: 3,782	I would think the average person could probably hold their breath for 2 minutes in decent conditions. Ice cold water might make it harder. A plastic bag would probably be counter-productive as it would suck into your mouth if you tried to breath.

8th April 2012, 08:53 AM	#5
NewtonTrino Illuminator Join Date: Jul 2007 Location: Bothell, WA Posts: 3,782	As a side note as long as you hold your breath you can get sucked down pretty far without dying. A couple hundred feet down is perfectly fine for example.

8th April 2012, 10:36 AM	#8
NewtonTrino Illuminator Join Date: Jul 2007 Location: Bothell, WA Posts: 3,782	Originally Posted by Denver We had a pool in the backyard what I was little. And we kids found we could put a small plastic bucket over our heads and, with some effort, hold it there while we sat on the bottom of the pool, breathing happily (and blissfully ignorant of the risks). If I recall, the main thing that drove us back to the surface was not the shortage of air, but the effort of holding the bucket under water. So I imagine a plastic bag would be rather helpful, assuming the length of time was not too great, and ignoring the cold and probable hyperventilation. The only real risk in that situation would be going to the bottom of the pool, taking in a lungful of now compressed air and then holding your breath all the way back to the surface.

8th April 2012, 11:09 AM	#9
Bikewer Philosopher Join Date: Sep 2003 Location: St. Louis, Mo. Posts: 9,540	They had a fellow on Science Friday a while back talking about the science of drowning. He said that what would have killed many of these folks was what he called the "cold water respiratory response". Essentially, suddenly being chucked into cold water causes an instant, involuntary gasp, which means that if you are already underwater, a big intake of water. If you fight this response and succeed in holding your breath, you can actually survive considerably longer. He was primarily researching survival of naval accidents/warfare incidents.

8th April 2012, 11:45 AM	#10
casebro Philosopher Join Date: Jun 2005 Posts: 6,802	The plastic bag taped over your head would at least prevent the intake of water. But in near drowning, I'd bet you'll tear it off and fill your lungs with water. I stayed down too long once when snorkling. Really hard to NOT inhale just short of the surface. But if somebody says getting sucked down by a sinking ship is an un-true myth, I would believe them. The were thousands of survivors of ships sunk in WWII, and must have had first hand knowledge of watching their buds close to sinking ships. Interviews must have been done. However, exiting a ship after it heads for the bottom might be a whole different problem. The hydro-dynamics could cause all kinds of swirling, and increase water pressure would push a victim INTO the boat. And the myth might have been started by a near-victim, who barely escaped- back in the wooden boat days.
casebro Philosopher Join Date: Jun 2005 Posts: 6,802	__________________ Please pardon me for having ideas, not facts. Some have called me cynical, but I don't believe them. It's not how many breaths you take. It's how many times you have been breathless that counts.

8th April 2012, 10:26 PM	#11
akama1 Critical Thinker Join Date: Jun 2009 Posts: 283	Originally Posted by casebro But if somebody says getting sucked down by a sinking ship is an un-true myth, I would believe them. The were thousands of survivors of ships sunk in WWII, and must have had first hand knowledge of watching their buds close to sinking ships. Interviews must have been done. The one I was peeved with was that mythbusters did not consult was the sinking of the HMS Hood duing WW2. Only 3 survivers, and one of the survivers directly states he was pulled down with the ship, till what they believe escaping air from the ship broke the suction http://en.wikipedia.org/wiki/Ted_Briggs , thou it was the worse case senario, massive mass of ship sinking extreamly quickly. A lot harder to recreate at scale.
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9th April 2012, 01:57 AM	#12
rjh01 Gentleman of leisure Tagger Join Date: May 2005 Location: Planet Earth Posts: 17,201	Originally Posted by NewtonTrino The only real risk in that situation would be going to the bottom of the pool, taking in a lungful of now compressed air and then holding your breath all the way back to the surface. Another risk would be staying there too long and then having no strength to get back to the surface. This could happen if you used up too much of the oxygen without a massive build up of CO2.
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9th April 2012, 02:07 AM	#13
Yuri Nalyssus Graduate Poster Join Date: Jun 2004 Location: The Ancient Isle of Avignuon Posts: 1,074	Originally Posted by Beerina ... they portrayed people being sucked down by the sinking ship. Mythbusters busted that myth, but they did use a boat that was about 1/20th the size of the actual titanic. I thought mythbusters confirmed this as an genuine phenomenon. I remember the reconstruction they did in a pool with a big model ship; the effect of being 'pulled down' by the ship was as a result of the air bubbles being released from inside the ship as it went down reducing the bouyancy of the water above the ship so that the human body sinks instead of floats. You get the same phenomenon when kayaking, if you get stuck in a 'stopper' like at the bottom of a wier or waterfall where the water is circulating around and tends to pull the boat back towards the cascade rather than allowing it to float downstream. In that circumstance it's difficult or impossible to paddle away from the danger as the water is so full of air bubbles that you can't get any purchase with the paddle. Yuri
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9th April 2012, 02:27 AM	#14
Yuri Nalyssus Graduate Poster Join Date: Jun 2004 Location: The Ancient Isle of Avignuon Posts: 1,074	Originally Posted by Yuri Nalyssus I thought mythbusters confirmed this as an genuine phenomenon... Yuri Ah well, wrong again - http://dsc.discovery.com/videos/myth...-minimyth.html ... sigh The test in the pool involved a very heavy, cube-shaped block which sank rapidly and actually sucked a diver under in a very dramatic fashion. Obviously I went to put the kettle on for a cup of tea at that point in the original programme, because when they used a 4 tonne ship sinking in a harbour in the second part of the test, no suction was observed. They did mention obviously that the Titanic was considerably bigger than 4 tonnes and scale might make a difference. But a friend of my grandfather's hairdresser heard from their cousin's brother's chiropracter that a live octopus hatched out of her stomach - that's definitely true . Yuri
	__________________ "The test of democracy is freedom of criticism." -David Ben-Gurion Peasant: Now we see the violence inherent in the system. King: Shut up! Peasant: Come and see the violence inherent in the system, help, help! I’m being repressed! King: Bloody peasant! Peasant: Ooh, what a giveaway, did you hear that... that’s what I’m on about, d’you see him repressing me? You saw it didn’t you... - Monty Python and The Holy Grail

9th April 2012, 10:15 AM	#15
DavidS Muse Join Date: Dec 2006 Posts: 519	Originally Posted by Yuri Nalyssus I thought mythbusters confirmed this as an genuine phenomenon. As to MythBusters trials in general: It should be obvious that MythBusters' "confirmed" is almost always more definitive than their "busted". If they establish X and observe Y, that's enough to prove that Y can happen with X. It just did: DONE. It doesn't prove X necessary, sufficient, nor even related to Y. That's a different question, and explicitly avoiding it is one good reason their trials try to replicate conditions proposed by the myth. If they establish X but do not observe Y, they've only shown that Y didn't happen for that particular X that particular time. Alone, that's not enough to prove that Y never happens with anything X-like. Even when it's possible to do, truly eliminating all such possibilities is often too tedious for fun TV programming. Where convenient and entertaining, they'll try to stretch conditions X or relax tests Y to further constrain those possibilities... but generally not eliminate them entirely. As to the "sucked down" phenomenon: I'm truly amazed how many people believe there's no such thing. Maybe my incredulity at their incredulity arises from my familiarity with fluid mechanics that makes the mechanism obvious to me. When a solid body (e.g. ship) moves through a fluid, the fluid will have to flow in behind the body to fill the space it leaves behind. When that body isn't streamlined (e.g. sinking ship), that inflow won't follow neatly along the boundary of the body. The boundary streamlines separate (which is what we mean by "not streamlined"), and the void behind gets filled by vortices. Anything in the fluid that's small enough will get dragged along in the vortices; if it happened to be in the right place, it'll follow the moving body for a bit. If that body's moving down (a sinking ship) and the vortex flow imposes drag forces on the particle (sailor) larger than its buoyancy, the particle gets dragged underwater. Maybe it's pedantry about the word "sucked", because what's actually happening is that water in the vortices is pushing downward. Of course, it's really such pedantry that sucks, because such unbalanced push is really what that other "suck" means. Float just about anything not-too-streamlined. Pull it underwater by applying some force to overcome its buoyancy. A sinking ship usually does that by putting water where it used to have air, but for clarity just hang a weight on a ship-shaped solid block of wood. Marvel at the bubbles of air dragged underwater by the vortices behind the sinking ship; if the ship didn't have any air to release, those bubbles must have got dragged down from the atmosphere by hydrodynamic forces. Float some test particles first (say, sawdust for a bathtub model, sailors for a ship) and watch some of them go down, too. Try again with only a few test particles, and see how hard it is to put them where they'll get "sucked" down.
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10th April 2012, 11:28 AM	#16
CORed Illuminator Join Date: Dec 2008 Posts: 4,295	Originally Posted by NewtonTrino The only real risk in that situation would be going to the bottom of the pool, taking in a lungful of now compressed air and then holding your breath all the way back to the surface. Probably not fatal, but a ruptured eardrum or two would be likely. Ouch.
CORed Illuminator Join Date: Dec 2008 Posts: 4,295

10th April 2012, 11:36 AM	#17
phunk Graduate Poster Join Date: Aug 2007 Posts: 1,915	Originally Posted by CORed Probably not fatal, but a ruptured eardrum or two would be likely. Ouch. Actually, it can be quite fatal, even in just a couple feet of water. It's not the eardrums that are a problem (although popping them will suck), it's the air embolism caused when the air expands in your lungs while you're holding your breath. It doesn't take much overpressure to force bubbles into your bloodstream.
phunk Graduate Poster Join Date: Aug 2007 Posts: 1,915

10th April 2012, 07:32 PM	#18
RussDill Illuminator Join Date: Oct 2003 Location: Phoenix, AZ Posts: 3,328	Originally Posted by Denver We had a pool in the backyard what I was little. And we kids found we could put a small plastic bucket over our heads and, with some effort, hold it there while we sat on the bottom of the pool, breathing happily (and blissfully ignorant of the risks). If I recall, the main thing that drove us back to the surface was not the shortage of air, but the effort of holding the bucket under water. So I imagine a plastic bag would be rather helpful, assuming the length of time was not too great, and ignoring the cold and probable hyperventilation. We fixed your problem with a fairly large suction cup, some rope, and a ~70 quart bucket.
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11th April 2012, 07:55 AM	#19
Mark6 Illuminator Join Date: Mar 2008 Posts: 4,178	Originally Posted by Denver We had a pool in the backyard what I was little. And we kids found we could put a small plastic bucket over our heads and, with some effort, hold it there while we sat on the bottom of the pool, breathing happily (and blissfully ignorant of the risks). If I recall, the main thing that drove us back to the surface was not the shortage of air, but the effort of holding the bucket under water. Holy... crap. You are very lucky to be alive. The most dangerous part of ascent from any dive where you breath ambient pressure air, is last 8-10 feet. If you hold breath while ascending, your lungs expand, but how much they expand depends on relative pressure difference, not on absolute difference. The difference between 10' depth and surface is 1.3 (1 atm and 1.3 atm). Lung expansion by a factor of 0.3 is quite sufficient to rupture them, and to cause air embolism. And I suspect your average suburban parents do not have pure oxygen tanks handy, or know the first aid procedure for air embolism. Well, I have and know, and would not be surprised if Phunk does, but he and I would never allow such activity to begin with.
Mark6 Illuminator Join Date: Mar 2008 Posts: 4,178	__________________ Gamemaster: "A horde of rotting zombies is shambling toward you. The sign over the door says 'Accounting'" Last edited by Mark6; 11th April 2012 at 07:57 AM.

11th April 2012, 08:21 AM	#20
NewtonTrino Illuminator Join Date: Jul 2007 Location: Bothell, WA Posts: 3,782	Originally Posted by phunk Actually, it can be quite fatal, even in just a couple feet of water. It's not the eardrums that are a problem (although popping them will suck), it's the air embolism caused when the air expands in your lungs while you're holding your breath. It doesn't take much overpressure to force bubbles into your bloodstream. Not to mention tearing your lungs and filling your chest cavity with compressed air.

11th April 2012, 10:13 AM	#21
Modified Illuminator Join Date: Sep 2006 Location: SW Florida Posts: 4,062	Originally Posted by Mark6 Lung expansion by a factor of 0.3 is quite sufficient to rupture them, and to cause air embolism. Even with a normal breath though (lungs filled to around half capacity)? That is, would there be a danger ascending quickly with breath held from 10 feet deep with half filled lungs? Besides that, most home pools are 8 feet deep, so sitting on the bottom, Denver's lungs were probably centered about 6 1/2 feet deep. I would think that unless you held a very forced deep breath, a 20% expansion isn't going to cause a problem.

11th April 2012, 11:17 AM	#22
phunk Graduate Poster Join Date: Aug 2007 Posts: 1,915	Originally Posted by Modified Even with a normal breath though (lungs filled to around half capacity)? That is, would there be a danger ascending quickly with breath held from 10 feet deep with half filled lungs? Besides that, most home pools are 8 feet deep, so sitting on the bottom, Denver's lungs were probably centered about 6 1/2 feet deep. I would think that unless you held a very forced deep breath, a 20% expansion isn't going to cause a problem. It can happen in a couple feet if you take a deep breath and hold it before ascending. With half full lungs, you'd be fine at pool depths. The problem is when someone instinctively takes a deep breath before taking their head out of the bucket, then holds it while ascending.
phunk Graduate Poster Join Date: Aug 2007 Posts: 1,915

12th April 2012, 10:33 AM	#23
Mark6 Illuminator Join Date: Mar 2008 Posts: 4,178	Anecdote from my own life: Over 12 years of diving I panicked under water exactly once. Not in an ink-black lake, not inside a wreck, not when tangled in a flag line. No, I panicked in a 12' deep brightly lit pool, during my Divemaster training. One of the tests you need to pass to become a PADI Divemaster is "equipment exchange while buddy-breathing". The instructor and the trainee take off their masks, exchange them, and put them on. Then do same thing with their bouyancy vests (to which tank and regulator are attached). While doing all this, both must breath from the same regulator -- the one initially instructor's. This is much harder than it sounds, and is incredibly stressful. My heart rate and oxygen demand went way up -- as expected. What I did not expect was the instructor having problems. As I struggled to put his mask on, and to keep track of one regulator that was giving me air, he signaled "up". At which point I panicked. Not "OMG, I am going to die!" panic, more like "OMG, I am going to fail the class!" -- but the result was the same. My awareness narrowed drastically, and as the instructor went up taking the air source with him, I completely forgot I had a perfectly good regulator hanging off me. What I should have done was take my own regulator into mouth, calm down, then surface. Instead I went up with my lungs essentially empty, yet actually felt pressure inside my chest as what little air there was expanded. At which point I remembered "hum as you ascend" rule, which keeps your airway clear for air to escape, and surfaced without injury. Then I found out the instructor aborted the exercise because he was having trouble, not because of anything I did. We rested, and repeated without incidents. But this was the closest I ever came to a serious injury while scuba diving, and the only time I panicked. In a YMCA pool.
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12th April 2012, 09:01 PM	#24
akama1 Critical Thinker Join Date: Jun 2009 Posts: 283	Originally Posted by Mark6 One of the tests you need to pass to become a PADI Divemaster is "equipment exchange while buddy-breathing". The instructor and the trainee take off their masks, exchange them, and put them on. Then do same thing with their bouyancy vests (to which tank and regulator are attached). While doing all this, both must breath from the same regulator -- the one initially instructor's. This is much harder than it sounds, and is incredibly stressful. My heart rate and oxygen demand went way up -- as expected. What I did not expect was the instructor having problems. As I struggled to put his mask on, and to keep track of one regulator that was giving me air, he signaled "up". At which point I panicked. Not "OMG, I am going to die!" panic, more like "OMG, I am going to fail the class!" -- but the result was the same. My awareness narrowed drastically, and as the instructor went up taking the air source with him, I completely forgot I had a perfectly good regulator hanging off me. What I should have done was take my own regulator into mouth, calm down, then surface. Instead I went up with my lungs essentially empty, yet actually felt pressure inside my chest as what little air there was expanded. At which point I remembered "hum as you ascend" rule, which keeps your airway clear for air to escape, and surfaced without injury. Then I found out the instructor aborted the exercise because he was having trouble, not because of anything I did. We rested, and repeated without incidents. But this was the closest I ever came to a serious injury while scuba diving, and the only time I panicked. In a YMCA pool. Damn. need to remember that. I still wish to get my Basic dive licence. each year I keep putting it off.
akama1 Critical Thinker Join Date: Jun 2009 Posts: 283

12th April 2012, 11:55 PM	#25
SezMe post-pre-born Join Date: Dec 2003 Location: Santa Barbara, CA Posts: 16,374	Originally Posted by Mark6 Anecdote from my own life: We had to do a similar exercise before going down for a week in Hydrolab (scroll down to about the middle). While exchanging with my buddy, for some reason I just could not get enough air. The facility manager saw I was having trouble so kept the exercise going (he later told us). I finally had to swim over to a small "bubble" that was kept in the area in order to recover. I have no idea what happened to cause the gulping-for-air problem. But we passed the test and went on with the mission because we stayed down. Going up after getting saturated was, of course, impossible. so not panicking and staying down was the key in this test exercise.