Why must scuba divers "never hold their breath" while freedivers can?

[Kevrumbo]

Assume you're at the surface with a tube in your mouth (a snorkel) the air you're breathing is at 1 ATA and for all intents and purposes the pressure outside of you is still 1 ATM since you're floating at the surface. Easy to breathe when the pressure outside and inside the body is roughly the same. Now try (don't really!) to take a 100 foot tube to create an insane snorkel and sink to 100 feet. The pressure outside your body is now at 4 ATA (weight of air pressure and water pressure). The air you're attempting to breathe in is still at 1 ATA. You are going to have to create some serious suction power on that tube to overcome the pressure outside your body. At some point the pressure outside your body is so great you can't physically overcome that pressure to take in a breath. I honestly don't know what that depth would be, probably at lot shallower than you think. . .

Hope this helped.

Q & A: snorkeling limits | Department of Physics | University of Illinois at Urbana-Champaign

The pressure of the inhaled breath must balance the surrounding or ambient pressure to allow inflation of the lungs. It becomes virtually impossible to breathe air at normal atmospheric pressure through only a simple long snorkel tube to the surface, even at only three feet under the water.

 

[tursiops]

This is a really scary thread! The OP professes (in another thread he started) to have taken scuba training.....so should NOT need to ask this question! The importance of the issue is that holding your breath as a scuba diver is life -threatening. Did the OP not learn that, and why it is so?.

 

[CptTightPants21]

This is a really scary thread! The OP professes (in another thread he started) to have taken scuba training.....so should NOT need to ask this question! The importance of the issue is that holding your breath as a scuba diver is life -threatening. Did the OP not learn that, and why it is so?.

I wouldn't fault the OP for not understanding some of the nuances and key fundamentals of what he is asking. In basic OW, students are taught not to hold their breath and I am sure some of boyle's law is mentioned, but there is a difference between mentioning boyle's law and completely understanding the concept so that you can apply it in a variety of circumstances. I wouldn't expect a student who hasn't completed a nitrox course to understand the questions he is asking and even then it might take some more experience.

 

[tursiops]

I wouldn't fault the OP for not understanding some of the nuances and key fundamentals of what he is asking. In basic OW, students are taught not to hold their breath and I am sure some of boyle's law is mentioned, but there is a difference between mentioning boyle's law and completely understanding the concept so that you can apply it in a variety of circumstances. I wouldn't expect a student who hasn't completed a nitrox course to understand the questions he is asking and even then it might take some more experience.

Actually, I WOULD expect Boyle's law to be understood in a basic class. i would not pass someone who did not understand, in fact. The plastic bottle crushing, the balloon getting bigger....these are pretty easy concepts. No fancy math needed.

 

[CptTightPants21]

Actually, I WOULD expect Boyle's law to be understood in a basic class. i would not pass someone who did not understand, in fact. The plastic bottle crushing, the balloon getting bigger....these are pretty easy concepts. No fancy math needed.

Right, but what is taught about boyle's law? Air from surface at 33 ft is 1/2, air at 66ft is 1/3, air taken from 66ft is 3x on the surface, etc. Very basic concept. Now use that to understand why freedivers can hold their breath going down 200ft, but not scuba divers? Why can't a person breath from a snorkel that is 6ft long?

A person may be able to add, subtract, multiple, and divide, but it still takes a little bit "extra" to be able to do algebra.

An OW student who could answer these questions would have a VERY high understanding of boyle's law AND its application in the real world. Can you say with 100% certainty that all your OW students could answer these questions?

 

[tursiops]

Well, breathing from a snorkel 6 ft long is not a Boyle's Law issue (it is a lung issue), but I suspect I could take a balloon that is full at the surface down and show it gets smaller, then bring it back up without it popping, Then fill a balloon to the same size but at depth, and bring it up and watch it pop. Free divers vs Lungs.

Can I say with 100% certainty all my OW students could understand this? No, I can't even say with 100% certainty that all the people reading this will understand it!

 

[SCRedWolf]

This is a really scary thread! The OP professes (in another thread he started) to have taken scuba training.....so should NOT need to ask this question! The importance of the issue is that holding your breath as a scuba diver is life -threatening. Did the OP not learn that, and why it is so?.

Well there's being told it's a bad thing vs. being explained and understanding why it's a bad thing. Sounds like the OP was simply told.

 

[Steve_C]

With snuba compressed air is forced down to the helmet which is open at the bottom. The fact that water does not enter the helmet means that the air pressure remaining in the helmet is the same as the water pressure. But they only go about 20 ft down. You rise slowly coming up a ladder so same issues as a shallow scuba dive.

 

[SCRedWolf]

Well, breathing from a snorkel 6 ft long is not a Boyle's Law issue (it is a lung issue), but I suspect I could take a balloon that is full at the surface down and show it gets smaller, then bring it back up without it popping, Then fill a balloon to the same size but at depth, and bring it up and watch it pop. Free divers vs Lungs.

Yeah, that. How come I can usually use 1000 words to explain something then someone else comes along and does it in 50?

 

[Kevrumbo]

Well, breathing from a snorkel 6 ft long is not a Boyle's Law issue (it is a lung issue), but I suspect I could take a balloon that is full at the surface down and show it gets smaller, then bring it back up without it popping, Then fill a balloon to the same size but at depth, and bring it up and watch it pop. Free divers vs Lungs.

Can I say with 100% certainty all my OW students could understand this? No, I can't even say with 100% certainty that all the people reading this will understand it!

While understanding the relationship of pressure and volume is both vital and obvious to all divers -it explains this most important rule of scuba diving of never holding your breath while ascending- fewer understand why density is just as important. Like pressure and volume, though, understanding density is also a matter of common sense.

To begin, let's squeeze a balloon. As you saw know, this reduces its internal volume. So, if the volume is reduced, the molecules within the balloon have to come closer together. (Think of a conductor pushing a crowd of people aboard a subway car.) In other words, the air gets "thicker," but the more appropriate term is denser. There is one important difference between pressure/volume and pressure/density relationships, however. Note that the former relationship is inversely proportional-if one factor goes up, the other goes down. In the latter, the relationship is directly proportional-if one factor goes up, so does the other.

Luckily, the change in the density of a gas is as predictable as the change in its volume. If, for example, you take a flexible container to 2 ATA ( 33 feet/10 meters) the volume will decrease by 1/2 but the density will double. In turn, at 3 ATA the volume will be one-third and the density will triple; at 4 ATA the volume is now one-fourth and the density quadrupled and so on. This relationship is just as important because it explains why a diver consumes his air more rapidly the deeper he goes. . .

In free diving (breath hold), as a diver descends, his lungs decrease in size according to Boyle's Law. But this isn't the case when using scuba because a diver fills his lungs completely with every breath: To fill the lungs, the density of the air inhaled must increase to equal the ambient (surrounding) pressure, which is the regulator's function with a full tank cylinder supply pressure. We are not physically able to sustain the filling of our lungs for long only with a snorkel to the surface even at an ambient pressure as shallow as 1.1 ATA (3 feet deep). . .