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

[filmguy123]

Freediving relies on breath holding... Scuba divers are taught to "never hold their breath"

As I understand, the world record in free diving is something like 700 feet (insane...)

How do freedivers hold their breath with this sort of depth and not experience problems? Why must Scuba divers not hold their breath if freedivers rely on that?

Thanks for helping me understand!

 

[ResortDiver850]

Freedivers aren't breathing compressed air, and they aren't at depth for very long.

 

[CptTightPants21]

The Answer--Boyle's Law

When a free diver takes a breath at the surface the air at the given volume is at surface pressure (14.7 psi). As the free diver descends, the pressure exerted by the water will compress their lungs and the air inside them. When a freediver is at 200+ ft, their lungs are so small and compressed they couldn't even take a breath from a regulator if their life depended on it. When a freediver returns to the surface, the air returns to the pressure/volume that it started with, which is safe for the freediver's lung capacity.

A scuba diver breathes compressed air at depth which means that the air is denser/more air. If we were to descend while holding our breath the same thing as what happens to the freedivers would occur. If however we ascend, the compressed air in our lungs begins to expand as their is less pressure being exerted to compress it. If we hold our breath and create a closed system, the expanding air will cause our lungs to pop like a balloon (mild exaggeration) if it doesn't find a way to escape.

As long as you don't close your throat and/or breathe naturally, the air in your lungs will continue to off gas/equalize and you won't have anything to worry about.

 

[fmerkel]

Cuz free divers can't breath underwater.....

 

[tbone1004]

basically what tight pants said.

rough easy math. Let's say a free diver takes a breath of 4l *very big btw* at the surface and descends to 100ft, and comes back. That 4l of air goes to 1l, and then back to 1l so the net change is 0. If however a scuba diver takes a breath at 100ft, because they are under 4ata of pressure, that 4l of tidal volume contains 4x the mass of air at the surface. So that diver takes a breath at 100ft of 4l, but if they hold their breath they will have a total volume of 16l of air in their lungs at the surface. If this volume exceeds the total capacity of your lungs, then they will break. Your epiglottis is much stronger than your lung tissue. People have literally died via lung overexpansion injury from taking too big of a breath in 3ft of water on scuba, holding their breath, and coming up to the surface

 

[Phantom Menace]

^^^ What tbone and tight pants said.

FYI - the records for Freediving are currently:
Constant Weight with Fin: 128m (420 ft. held by Russian Alexey Molchanov)
Constant Weight no Fins: 101m (331 ft. held by New Zealander William Trubridge)
Free Immersion: 124m (407 ft. held by New Zealander William Trubridge)

"Constant Weight with Fin" (CWT) means the diver swims down with fins (or actually a monofin as they perform better) and returns to the surface with everything the went down with i.e. they cannot hang on to a hunk of lead to assist with the trip down then let it go before they swim back.

Constant Weight No Fins (CNF) is the same as above except no fins are allowed.

Free Immersion (FIM) is when the diver can pull themselves down a rope and then back up.

The "No Limits" record is huge - but also suicidal in my opinion. AIDA (the main body that recognises freediving records) is not actively encouraging No Limits dives in competition now. For No Limits (NLT) the diver will descend on a weighted sled and the inflate a lift bag to get back to the surface.

 

[red_instead]

 

[filmguy123]

So two related questions:

In Snuba (breathing air from the surface, at 1 ATA). If a diver went down to 100 feet (4 ATA) breathing this surface air, uncompressed - would that change anything? Why or why not?

Tightpants mentioned that at 200 feet, the free diver's lungs were so small that he would not be able to take a breath from a regulator if he tried... why not? How come a SCUBA diver can breathe from a regulator at 200 feet under via a regulator under the same pressure? Is it because of the constant breathing that occurred and stopped the lungs from compressing so much...? How does that work?

Thanks!

 

[CptTightPants21]

1. Snuba--Person could not breathe the air from the surface "uncompressed". The air might be 1 ATA at the surface, but in order to make it down to the person at 100ft it would need to be compressed by at least 4ATA. Ever see the movies where the "pumps" are supply air to the guy walking around in a big metal suit? A regulator naturally compensates by supplying air that is ~140psi + ambient pressure (aka pressure created by the depth)

2. Yes, the constant breathing that occurs acts to constantly "equalize" the lungs. It doesn't need to be constant breathing, but it can't be from 0 to 200ft. I don't want you think that if you hold your breath from 0 to 20 ft you are going to die.

 

[SCRedWolf]

So two related questions:

In Snuba (breathing air from the surface, at 1 ATA). If a diver went down to 100 feet (4 ATA) breathing this surface air, uncompressed - would that change anything? Why or why not?

Thanks!

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. Beyond that depth you have to breathe compressed air which helps overcome the outside pressure. In SCUBA our regulators are used to regulate the air pressure from our tanks to approximately match the pressure outside our bodies. So at ~33 feet they provide air pressure to us at 2 ATA, ~66 feet @ 3 ATA, ~99 feet @ 4 ATA (approximate numbers). That makes it easy to breathe naturally as we get deeper and we don't pop like balloons breathing 3000 psi compressed air straight from our tanks.

And to drive it home (I hope) if you realize that at ~100 feet our lungs are full of air at 4 ATA which matches the outside pressure at that depth. If we hold our breath as we ascend to ~66 feet our lungs still have 4 ATA of pressure but the outside pressure is only 3 ATA. They're going to expand a lot. And it's going to hurt badly. A freediver never has more than 1 ATA in their lungs since they're breathing normal, uncompressed sea-level air. Their lungs shrink considerably at depth and return to normal size when they surface.

Hope this helped.