Lung Over Expansion Info

[raybo]

Last Saturday, I was at the local "hole" that divers around the southwest come to, especially the winter, to get their O/W dives completed. Since joining the board, my radar goes on a lot more around there than it used to. So, while waiting for the rest of my group to arrive, I strolled around and “observed” some of the activities.

On a typical weekend, there will be half a dozen shops represented; with anywhere form 2 to 30 students from each shop. Not unusual to see 150-200 students trying to get two or three dives in on any of three platforms in a sinkhole about 75 ft across & 85 ft deep.

Anyway, I happened to overhear one person, who I assumed was the instructor; give his group their pre-dive briefing. What caught my attention was the subject matter of his monologue. He was trying to relate the potential of lung overexpansion injuries to them.

In one of his comments, he indicated that because it was so much deeper, they might be able to feel that pressure. Having only been in the pool up till then, that they hadn’t been deep enough to matter. At least that was my interpretation of his comments. He went on to stress that they shouldn’t hold their breath, etc., etc.

I was taught that the first thirty feet had the greatest potential of this type of injury because the pressure/volume change was the greatest near the surface. Essentially it takes less change in depth in shallow water to achieve a unit volume change of gas than it does deeper. I was also taught that this injury can result in as little as 3-5’ change of depth, if you hold your breath.

OK. So here are my questions.

Is what I recall of my training correct?

Is the assertion by the instructor that they “wouldn’t have felt it in the pool” correct? My instinct tells me no. If you took a breath at 12”, held it, and surfaced, I’d say you stand a pretty good chance of hurting yourself.

And as to his comment that they might “feel the pressure”, wouldn’t that be an indication of a situation that shouldn’t be in evidence?

I think it’s important that instructors stress the importance of not breath-holding, but the gist of this guy’s comments gave me the impression that he didn’t fully understand the physics.

Clarifications and relevant comments greatly appreciated

 

[Spectre]

raybo once bubbled...
I was taught that the first thirty feet had the greatest potential of this type of injury because the pressure/volume change was the greatest near the surface. Essentially it takes less change in depth in shallow water to achieve a unit volume change of gas than it does deeper. I was also taught that this injury can result in as little as 3-5’ change of depth, if you hold your breath.

Yes... this is correct.

Think of it this way with boyle's law... 1 unit of volume is 1/2 at 2 ATA, 1/3 at 3 ATA, and 1/4 at 4 ATA.

So if you take a full breath at 4 ATA, you know it's 4 times the size when it hits the surface.

Take the intermediate range though:

4 * 1/4 (4 ATA) = 1xcapacity [100%]
4 * 1/3 (3 ATA) = 1 1/3xcapacity [133%]
4 * 1/2 (2 ATA) = 2xcapacity [200%]
4 * 1 (1 ATA) = 4xcapacity [400%] at surface.

Now lets assume that you'll collapse your lungs with an accent from 10 feet to 5 feet. 10 feet is 1.3 ATA. 5 feet is 1.15 ATA. That's a 13% pressure difference. A 13% pressure @ 4ATA is 3.58, or about 15 feet.

So yes... you have to be more careful in shallower water, not deeper.

Disclaimer. numbers are examples, I am not implying you won't suffer a lung injury ascending less than 5 feet @ 10 feet, nor that you can safely ascend less than 15 feet at 99 feet.

 

[raybo]

Way back in the dark ages of J valves & horse collars, my instructor had us do a little excercise to personally demonstrate just how significant water pressure was.

He indicated in class that as little as 18" of water could be sufficient external pressure to prevent you from being strong enough to take a breath. Few of us believed him, so he rigged up a really long "snorkel" and had us try it. You might be able to do it once, and maybe a little deeper than that, but not repeatedly or for any length of time.

Guess I've always figured that was a good example of how potentially dangerous pressure chnage was, so I've never taken the chance.

I certainly realize that each person is going to have a different level, and depth has a lot to do with it. I was just curious if ther is scientific evidence relating to this.

Goes more to what/how I was trained then compared to what I heard the other day.

 

[The Iceni]

raybo once bubbled...
Is what I recall of my training correct?

Is the assertion by the instructor that they “wouldn’t have felt it in the pool” correct? My instinct tells me no. If you took a breath at 12”, held it, and surfaced, I’d say you stand a pretty good chance of hurting yourself.

And as to his comment that they might “feel the pressure”, wouldn’t that be an indication of a situation that shouldn’t be in evidence?

Both you and Spectre are spot on but true to form I have to go and spoil it, as the anatomy makes this scenario even worse!

This subject was covered in detail on this thread.

Spectre once bubbled. . .So if you take a full breath at 4 ATA, you know it's 4 times the size when it hits the surface.

You do not even have to take a breath!

At the end of a normal expiration 6 litre lungs still contain about three litres of air (The Functional Residual Capacity). Thus 3 x 4 = 12 litres, which is twice the total lung volume and it takes far less than this to cause barotrauma.

If you take a full breath, to bring the total lung volume to 6 litres, at the bottom of a 10 foot (3 metre) pool, on ascent this will try to expand to 1.3 x 6 = 7.8 litres but cannot. The intrathoracic pressure will simply increase to 1.3 ATA which, again is more than sufficient to cause barotrauma.

And you do not feel any discomfort at all until your lungs go pop!

So my adbvice is simply this.

Never hold your breath during ANY ascent