Why scuba diver can't share gas with freediver?

[dumpsterDiver]

I decided to do some don and doff skills in the deep end of the pool when I got my double hose and a copy of "New Science of Skin and Scuba Diving". I scuba dove down to the deep end of the pool (12') and removed my gear setting it carefully on the bottom. Then I had to make a free ascent while blowing out air.
The tank valve had to be shut off otherwise double hoses will just freeflow away.
Then I freedove down and the first thing I did was turn the tank on and start breathing, then I put on the mask and all the other stuff on and swam away.
It's a great skill. I don't know why they don't still do it?
Probably because of the holding your breath part. Some people probably didn't get it.

I don't actively teach scuba anymore, but I have taught my kids to dive. This is my daughter's third time using a tank. We skipped the pool stuff and went right to open water (in a calm shallow area) and one of the skills I have her practice is: ditching the tank and swimming up while blowing bubbles. I didn't make her turn the tank off, since this was only her third time in the water and I don't make her take off any snorkeling gear for this time.

It is too dangerous to teach in modern scuba classes (from a liability standpoint), but in my personal opinion it is a great skill and confidence builder and is worth the risk for my children. If you skip to about 4;45 in the video we do the skill.

If a diver should ever have their scuba gear hopelessly entangled and they run out of air, being able to take the tank off and swim up could become a critically important skill.

Joan Training Dive Number 3 10-20-13 - YouTube

[video=youtube_share;NmeQmWZ_tzY]http://youtu.be/NmeQmWZ_tzY?list=UU1utDku8vJRJYgBZImLyLJQ[/video]

 

[Eric Sedletzky]

I believe there are two reasons for it.

1. It has not a skill that you actually ever use on a scuba dive.

2. It is dangerous.

The student is doing a CESA in that activity. A couple of decades ago, a study on dive accidents and fatalities during scuba instruction found that the overwhelming majority of them occurred during CESA training. Following that study, almost all agencies adopted very careful protocols for CESA instruction, and they dropped this exercise. For PADI, it is the only skill for which the precise procedures the instructor must follow, both for the pool training and the OW dive, are carefully spelled out in great detail. If a student were to have a lung expansion injury in a class in which the instructor was not following those protocols exactly, the lawsuit would not go well for the instructor, and I suspect the instructor would be expelled from the agency pretty quickly.

A few years ago the instructor of a scuba class in the University of Alabama (SSI) was having her students do doff and don exercise you describe, and one of them died of an embolism. I presume a very fat check was written immediately after discovery.

I know it's not something anybody would have to do in real life but that's not the point of the skill. The point is to build confidence and problem solve underwater. It's also builds comfort. Just like in old NAUI classes, there's a guy I know who still teaches them for SSU (since 1980) He does the 'jump in the pool holding all your skin diving gear and put it on underwater on one breath without coming up' (weightbelt/mask/flippers). You have to get a breath, jump in the deep end, roll on your weightbelt, put on your mask and clear it, and put on your fins all in one breath without coming up for air. It's a voluntary skill offered to the students if they want to try it and all of them always do, even the girls. Why? because their kids for one thing (or at least early 20's - kids to me), and because they get a friendly competition going and don't want to be outdone by the other. It's a skill that builds comfort and forces you to relax so you don't panic and use up your air in 10 seconds.
Just like the DM gear exchange. When would you ever have to do that? but they do it to build confidence and train divers to keep a cool head and think it through and come up with a plan under stress. If you are good at buddy breathing and can relax you should be able to use up a whole tank if you have to to get it done.

And yes, these days it is too dangerous because they don't have time to explain in detail how to do it without getting hurt. Plus, a lot of people getting into scuba these days would never be able to get past the skill.

 

[ican]

Another issue would be the scuba diver may have deco obligations.

This, I think is the biggest reason not to share air with a free diver. The moment a free diver takes a breath at depth, he is obligated to also do a decompression stop, regardless of how long he's been under. This may mean the SCUBA diver runs out of air because he has to continue to share air with the free diver for possibly many minutes... At the moment the free diver takes a breath from a scuba air supply, he should be staring at the scuba diver's dive computer. His life depends on it. I may be wrong, but even if the free diver can take a breath on a compressed air tank without any issues at whatever depth he is at, even if he exhales again as much as possible, the compressed air left can expand to the point of causing lung damage.

 

[mrdre]

This, I think is the biggest reason not to share air with a free diver. The moment a free diver takes a breath at depth, he is obligated to also do a decompression stop, regardless of how long he's been under. ....

mmm.... why?

 

[jboneng]

This, I think is the biggest reason not to share air with a free diver. The moment a free diver takes a breath at depth, he is obligated to also do a decompression stop, regardless of how long he's been under. ....I may be wrong, but even if the free diver can take a breath on a compressed air tank without any issues at whatever depth he is at, even if he exhales again as much as possible, the compressed air left can expand to the point of causing lung damage.

For me that makes no sense... I don't know of any physiological effects that magically makes the tissue loading of inert gases spike to critical levels for a free diver taking a sip of compressed air, and not holds his breath longer than the NDL for that gas mix on that depth (which would be a feat diving with air or nitrox at recreational depth), but I might be wrong. If you by "Decompression stop" means "Safety stop", The safety stop is optional but recommended, so the free diver can safely skip the safety stop and go straight to the surface.

A normal adult male have about 5 liters of vital capacity in his lungs, lets say after exhaling as much he can and have 1 liter left (normal residual volume for adults is 1-1.2 liters), the free diver need to experience a pressure difference of 5 bar to make the residual air in his lungs expand to 5 liters, 10msw =1.01325 ~ 1 bar, so the free diver have to ascend through the equlant of 50msw (or exactly 49.346 msw), which is within the realm of possibility, but keep in mind this is for filling his lungs to normal vital capacity, I am not sure how many liters / how much pressure you need to actual rupture the lungs, also this is based on the assumption that he exhales as much as he can on maximum depth.

 

[Storker]

The moment a free diver takes a breath at depth, he is obligated to also do a decompression stop, regardless of how long he's been under.

mmm.... why?

Count me in as another who'd love to learn why. I'm all ears.

I may be wrong

That's definitely a possibility.

 

[shark63]

Ok let me put my 2 cents into this discussion. A freedivers inhales air into lungs at the surface , as they are fully expanded. As he descends the lung volume decreases as pressure increases, as he ascends pressure decreases and his lung volume will increase. As het took his last breath at a place with minimal pressure he will not be able to over expand his lungs however if he breathes gas from let's say a safety diver and ascends the following scenario's could take place: As he ascends and holds his breath his lungs could easily overexpand and rupture, this alone could be a fatal problem there is however is 2nd problem, as he is inhaling a mixture of oxygen and nitrogen at depth and as freediver tend to ascend a lot faster then 60ft per minute chances of explosive DCS are also a real possibility. The only option a freediver has if he has to breath gas is to ascend slowly as if he or she has become a scuba diver.

 

[Storker]

Ok let me put my 2 cents into this discussion. A freedivers inhales air into lungs at the surface , as they are fully expanded. As he descends the lung volume decreases as pressure increases, as he ascends pressure decreases and his lung volume will increase.

Ergo, his lungs already contain compressed gas at depth. Albeit with a volume that doesn't lead to lung overexpansion and embolisms on ascent. But still compressed, and still providing increased dissolution of nitrogen into the tissues.

As he ascends and holds his breath his lungs could easily overexpand and rupture

No contention here.

there is however is 2nd problem, as he is inhaling a mixture of oxygen and nitrogen at depth and as freediver tend to ascend a lot faster then 60ft per minute chances of explosive DCS are also a real possibility.

So, one sip of compressed air at depth can lead to "explosive DCS"? Sorry, you've got to provide some plausible evidence for that.

Yes, DCS can happen to freedivers, exemplified by the Taravana syndrome. And freediving after SCUBA diving is a bad idea, because the repeated, fast ascents may provoke bubble formation (AKA DCS) in tissues where there already is a significant amount of dissolved nitrogen. But unless you can provide some good evidence for a freediver undergoing "explosive DCS" after a single sip of compressed air at depth, I'm going to assume that you're speculating wildly. And I'm not talking about lung overexpansion and embolisms, we agree on the danger of that. I'm talking about the bends.

 

[boulderjohn]

Ok let me put my 2 cents into this discussion. A freedivers inhales air into lungs at the surface , as they are fully expanded. As he descends the lung volume decreases as pressure increases, as he ascends pressure decreases and his lung volume will increase. As het took his last breath at a place with minimal pressure he will not be able to over expand his lungs however if he breathes gas from let's say a safety diver and ascends the following scenario's could take place: As he ascends and holds his breath his lungs could easily overexpand and rupture, this alone could be a fatal problem there is however is 2nd problem, as he is inhaling a mixture of oxygen and nitrogen at depth and as freediver tend to ascend a lot faster then 60ft per minute chances of explosive DCS are also a real possibility. The only option a freediver has if he has to breath gas is to ascend slowly as if he or she has become a scuba diver.

How does one inhale? What makes it happen? Never having been presented with this problem before, I think that mechanism is an issue here.

You can lower the diaphragm, expand the chest, and raise the collarbone. These actions increase the size of the chest cavity, which causes air to come in through the mouth and nose. A free diver frequently does all three, at which point no more air can be inhaled because the chest cavity is already at maximum size.

When the diver descends, the gas within the lungs becomes more dense, causing the lungs to shrink. Does this cause the chest cavity to shrink as well? Descending also cases the air in the sinuses and middle ears to become more dense as well, but does that cause the skull to shrink?

I certainly have not tried it, but I would guess that a free diver who has descended with full lungs would be unable to inhale without first exhaling at least some of the air inhaled at the surface.

 

[Lost down under]

This, I think is the biggest reason not to share air with a free diver. The moment a free diver takes a breath at depth, he is obligated to also do a decompression stop, regardless of how long he's been under.

Get yourself a copy of 'deco for divers' should seriously be compulsory OW reading. As the others have said and having read Powel's book I can't see the mechanism.