Lungs underwater

[JackD342]

What's sauce for the goose, is sauce for the gander, I guess.
Still...if an uppity student did that to me, I think we'd have a serious talk about his taking away his safety net.
And unless it was the very last dive of the course, I think he'd have to finish with another instructor. I wouldn't fail him, but I don't think I really want someone presumptuous enough to test me in the middle of class. Still, bad on the instructor for not being able to control his reflex, lol!
Sorry, Angelo!

You may need to add the context of what course you are teaching. I doubt that any of the mainstream Open Water Diver courses include ripping off a student's mask with no warning on a training dive. That is a different level of instruction, and will have a very different level of expectations for completion.

 

[Angelo Farina]

You may need to add the context of what course you are teaching. I doubt that any of the mainstream Open Water Diver courses include ripping off a student's mask with no warning on a training dive. That is a different level of instruction, and will have a very different level of expectations for completion.

Exactly. That was not a course. I was already a 2 stars CMAS instructor, and I was teaching since 5 years. I did already complete the course for 3 Stars, passing the Italian exam. That further exam was just for converting the Italian certification (FIPS) to international (CMAS). So the examiners were just evaluating the skillness of candidates, there was really nothing to teach us...
Of course my reaction was indeed very stupid. And it was effectively the very last dive, after two hard days on the lake, with just a 5mm wet suit, making exercises, swimming tests, recovery simulations, etc.
The examiners were acting as students, doing stupid things, and we, the instructors, had to take care of them.
Hence in my case the examiner was simulating being an unexperienced student, and I was the instructor in charge. It can happen that a student, moving badly, rips off the mask of the instructor. And the instructor should not rip off his mask in return. Which instead was what I did...
So definitely there were good reasons for firing me.
Instead, strangely, I passed the exam.
I always had a doubt that it was for my wife, who was also being examinated together with me. She was more brilliant than me, she made no error, and I suppose that she had some role in avoiding me to be rejected...

 

[EireDiver606]

This is extremely bad advice. Do this with a full breath and, if the rock is big enough or you are shallow enough, you will damage your lungs.

Nobody said anything about using your BC buttons. The proper way to do this is inhale to initiate the rise and then, as you are exhaling, use your fins to maintain forward progress.

I semi agree with your first point, why are you arguing?
No you wont damage anything. You only hold your breath to initiate a small ascent, if you keep holding it, well then you're mr. stupid... and obviously you are using your fins to "maintain forward progress" lol. Its diving, you do fin...

Sure there is. The only question is whether you call it "special". Your consumption rate is simply the number of breaths you take per minute x the volume of each breath. If you want to decrease consumption, one of those has to decrease. Decreasing the volume of your breaths is a bad idea, it leads to increased CO2 levels and if shallow enough, decreased O2 levels. So the only option left is to decrease the number of breaths per minute. The one advantage to breathing compressed air is you have enough O2 molecules per breath to safely slow down your breathing rate at the same work level compared to your surface rate. But it's not an intuitive thing to most people. Many divers gradually adopt a slower breathing rate over the course of dozens or hundreds of dives. But many don't. There's no reason to doom people to poor consumption rates by keeping this information from them. It's something you can work on if you are aware of it.

But with a decreased amount of breaths, they will be longer breaths, therefore they are forced and not natural. You will be subconciously stressing a tiny bit from forcing yourself to take less breaths. Just dive, dont listen to people talking BS about breath techniques. All it is is comfort, how you dive, equipment, gas and fitness which determines your surface consumption rate.

 

[lowwall]

I semi agree with your first point, why are you arguing?
No you wont damage anything. You only hold your breath to initiate a small ascent, if you keep holding it, well then you're mr. stupid...

I completely agree with this, but what you originally said was "hold your breath for 10 secs as you ascend over a rock". Either you meant something different from what you wrote or I'm having a major comprehension issue. Maybe it was supposed to be 1 second?

But with a decreased amount of breaths, they will be longer breaths, therefore they are forced and not natural. You will be subconciously stressing a tiny bit from forcing yourself to take less breaths. Just dive, dont listen to people talking BS about breath techniques. All it is is comfort, how you dive, equipment, gas and fitness which determines your surface consumption rate.

Pretty much everything we do underwater is not natural. For example, our normal breathing pattern is exhale, pause, inhale. Oh and we hardly ever fly in air, even with fins. The point is that we learn a lot of skills to be comfortable and safe underwater. Further consciously shifting our already unnatural breathing pattern might as well be one of them. It's only a conscious effort for a few dives, it rapidly becomes habit. Apparently we both agree that most people will get there eventually, but I think it's worth a bit of work to get there after tens of dives rather than 100s.

 

[EireDiver606]

Just dive. Gas consumption will decrease naturally.

Only after sorted everything else out first.

 

[howardzxq]

Now I'm reading a book about scuba diving and this book has the following paragraph:
"Let's say a freediver has 4 l of air in the lungs. At 10 meters the pressure doubles and the volume of air in the lungs squeezed by water is 2 l. At 30 m it is 1 l."

Also, I googled it and found a lot of pictures like that
https://i.pinimg.com/originals/9f/90/3a/9f903a464526b710814c12dd0f725b92.jpg
that show that the volume of lungs is 5 times less at 40 m.

And, I don't understand why it is true. This law is true for BCD or balloons. But my lungs are not a balloon, they are inside my body, and water presses my rib cage. I can agree that the volume can decrease twice or something like that but not 5 times. (Let's say the volume of my lungs becomes 1 l, but the volume inside my rib cage is almost the same, so something should fill the difference (gas or liquid), what is it?)

Could someone explain why I'm wrong?

Hi there,

I actually made an account because I wanted to share the explanation that I've been given that made most sense to me. I had the same question as you for the longest time. Here it is:

At depth, the water pressure pushes on our tympanic membranes (and also our cheeks and other soft, easily compressible parts of our head/neck). When we equalise our ears, we are shifting air from our lungs into our middle ear in order to push back against the water pressure pushing inward. The deeper we go, the higher the pressure pushing in on our tympanic membranes. As a result, we equalise further by pumping even more air from our lungs into our middle ear. When we pump air from our lungs into our middle ear, our lungs are reducing in size (like exhaling, except into our ears). This results in the overall observation that our lungs are "squeezing down".

Edit for clarification: the volume that we are equalising is not just the space of our middle ear. The middle ear is a continuous chamber with the Eustachian tube, and thus the pharynx, and thus the lungs. We need to pressurise this entire space which has a total volume of [space of lungs + air spaces of head/neck]. It would be impossible to pressurise the middle ear alone (which @inquisit astutely pointed out is tiny in volume) without pressurising the entire rest of the chamber because high pressure air from the middle ear would just flow out the Eustachian tube to a lower pressure zone.

Edit continued: When we descend 30m underwater to an ambient pressure of 4 Bar, we need to pressurise our middle ear to 4x what it was at the surface. To do this we must also pressurise the entire chamber space continuous with our lungs to 4x the air density of what it was at the surface. Hence, the lungs reduce in size to just under a quarter of its volume (specifically, one quarter of its volume minus the volume needed to pressurise the head/neck air spaces). So what they say about our lungs reducing to a quarter of their size when we dive 30m is 100% correct, but the explanation that it's because our ribcage and lungs are being squeezed like a balloon is in my opinion imperfect.

//

Naturally, the deeper we go the more air pressure we need in order to equalise. This is why we need to breathe denser air as we dive deeper. This is why someone had to invent the modern stage 1 regulator which brings the pressure in our cylinder to a pressure that matches ambient pressure (actually slightly higher than ambient pressure). As we dive deeper our regulator gives us denser air, and we end up going through our air supply quicker. I saw all this in some divemaster exam prep video on YouTube. I know it might seem like a bit of a tangent but I find it to be a cool fact that ties things in nicely.

I've personally seen lots of internet answers talking about Boyle's law and whatnot, but I think Boyle's law can only contribute a tiny amount (just the degree to which your ribcage can be squeezed). The rest I believe is due to us equalising.

I hope this makes sense to someone I'm a medical doctor in Australia, previous HSC physics state ranker and of course a keen diver who loves education. I absolutely have no intention of spreading misinformation. This is the explanation which personally makes the most sense to me. If you disagree please let me know so we can learn together. I'll absolutely delete my reply if I discover I am wrong.

 

[inquis]

Anyone know the volume of the inner ear at the surface? Qualitatively, it's pretty small, so the gas routed to it to equalize is also fairly small. (Five times a very small number is still pretty small.) The balloon effect still dominates the lung size in my view.

 

[howardzxq]

Anyone know the volume of the inner ear at the surface? Qualitatively, it's pretty small, so the gas routed to it to equalize is also fairly small. (Five times a very small number is still pretty small.) The balloon effect still dominates the lung size in my view.

Hi

No doubt the volume of the middle ear is very small (at only around 1ml), and five times this is still tiny. The volume of air we need to pressurise this space is more than just the space of the middle ear though. Because the middle ear is not a perfectly sealed air chamber, to pressurise this space we would have to pressurise all the spaces along the way. If we didn't pressurise all the spaces along the way then the air would just escape from the middle ear (from high pressure zone) out of the Eustachian tube to a lower pressure zone.

In this sense, the total space we are pressurising is not just the middle ear, but the entire chamber in our head/neck that connects to it. This would include pressurising the length of the Eustachian tube, the space in our nasopharynx/around the nasal concha, the space in our mouth, the space of our larynx, but most importantly the remaining space in our lungs!

The total added volume from the middle ear down to the pharynx (I did some additions based on google searches lol) is a measly ~100ml. It isn't this tiny volume that matters, it's the fact that this chamber lies in continuation with our lungs that matters. Because the chamber lies in continuation with our lungs, the space we need to pressurise is [space of lungs + approx 100ml).

When we descend 30m underwater to an ambient pressure of 4 Bar, we need to pressurise our middle ear to 4x what it was at the surface. To do this we must also pressurise the entire chamber space down to our lungs to 4x the air density of what it was at the surface. Hence, the lungs reduce in size to just under a quarter of its volume (specifically, one quarter of its volume minus the 300ml needed to pressurise the head/neck air spaces).

The lungs are observed to shrink down to a quarter of its size and gets explained by popular media as the water pressure physically "squeezing" our thorax.

This is my view.

Does this sound plausible in your opinion?

 

[inquis]

Isn't this issue obviated when someone takes another breath when on scuba and lungs go back to their normal size? Yes, pressure is 4x, but so is the quantity of gas (the "n" in the ideal gas law: PV/n is constant). (If this were not the case, there would be no issue holding your breath on ascent.)

@howardzxq, if you're talking about free diving, have you considered the effect of blood shift that @Angelo Farina cited?

 

[lowwall]

A couple of quick comments.

The middle ear is close enough to sealed given the short time frames we are discussing. That's why we need to equalize in the first place.

Lung volume doesn't decrease to 1/# of atmospheres, the air in the lungs does. A lot of the lung is incompressible fluid or tissues. Also, lungs are already set up to handle a decrease in the amount of air contained in them. It happens every time we exhale. You have to get somewhere in the 35m range before you run out of natural elasticity. After that you are depending on expansion of the diaphragm into the chest cavity. Freedivers who go deeper than that do special stretches and are careful about their posture to allow the diaphragm to compensate.