Why scuba diver can't share gas with freediver?

[CT Sean]

This probably explains it better than I..

"We have already seen how, for the breath-hold diver, Boyle's law predicts that compressible air spaces will shrink on descent and re-expand on ascent, and that the situation is different for scuba divers because compressed air is continuously inhaled. Even if the scuba and breath-hold diver go to the same depth and spend the same amount of time under water (e.g., one minute), the effects of water pressure are radically different on the two divers. Because tank air is inhaled at the ambient pressure, the scuba diver's lungs and other compressible air spaces do not shrink."

Effects of Unequal Air Pressures While Scuba Diving

From my limited knowledge it would seem that if a freediver continually exhaled during ascent (similar to CESA) he might survive, assuming that he was able to actually inhale a breath of air from a regulator after his lungs were compressed to very low volumes due to the pressure. But I really wouldn't expect that is something a freediver is really trained to do.. Freedivers don't breath!

If a freedivers lungs do not shrink, than I've having trouble understanding the difference between lungs that are containing a smaller volume due compression (a free diver at 30m) vs. lungs that are containing a smaller volume due to exhalation. A freediver would certainly have to know that that breath of compressed gas needs to be exhaled all the way to the surface, but my question/comment was specifically in regard to the statement:

it doesn't seem like it would be a good idea to suddenly inflate them by taking a breath off of scuba.

Which has nothing to do with ascending. not trying to argue - just trying to understand what, if any difference there is between a "compressed" lung and an "empty due to exhalation" lung.

 

[Bob DBF]

I have not done this at 100' so I can't answer that part of the question. The point to be taken away is that a freediver who breathes compressed air at depth becomes a SCUBA diver. What he does after the breath is limited by the choices a SCUBA diver has, not those of a freediver. If you decide to do it, don't get confused between the two.



Bob
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I think that advocating unsafe and dangerous practices is both stupid and foolish. That is why I don't tell people to do what I do. Dsix36

 

[Roatan Joe]

If a freedivers lungs do not shrink, than I've having trouble understanding the difference between lungs that are containing a smaller volume due compression (a free diver at 30m) vs. lungs that are containing a smaller volume due to exhalation. A freediver would certainly have to know that that breath of compressed gas needs to be exhaled all the way to the surface, but my question/comment was specifically in regard to the statement:



Which has nothing to do with ascending. not trying to argue - just trying to understand what, if any difference there is between a "compressed" lung and an "empty due to exhalation" lung.

Yeah, I agree, that's the question and it's quite possible that not a lot of scientific research has been conducted to identify all of the possible issues. I do believe that as a breathhold diver goes to 30, 40, 50, 60, 70 meters deep, the compression of the lungs due to pressure at depth is MUCH more significant than anyone could ever achieve by simply exhaling. Speaking for just myself, I have no idea what I don't know about all of it and that is why I didn't question Walid when he told me it would be dangerous. If a breathhold diver was capable of taking a breath, maybe accepting air at depth would increase the risk of DCS?? Maybe introducing compressed air into lungs that are already compressed and full of plasma has serious negative physiological implications? Maybe nobody knows for sure and because of that it's just better for a breathhold diver to just breathhold dive and recognize that breathing off of scuba once at depth could have very dire results.

 

[Mustard Dave]

There seems to be a lot of assumptions about the knowledge of scuba by freedivers. The bloke who taught me to scuba dive was the runner up in the AIDA (Association Internationale pour le Développement de l'Apnée) world's best coach category, he's an AIDA instructor trainer, international judge, organiser of the UK national pool championships and the first UK depth competition. He has also represented the British freediving squad six times as a competitor and as head coach. He is also a PADI IDC Staff Instructor and BSAC Open Water instructor.

There is a very active freediving community within my club and the vast majority of them are also trained scuba divers. I don't think any of the freedivers I know would have any problem knowing what to do if they took gas from a scuba diver.

I can't find the answer. The scenario is: the SCUBA diver is at 100 fsw, the free diver descends to 100 fsw, SCUBA diver shares the gas with the free diver, they both ascend sharing the gas.

What can go wrong with this? It seems that there is an opinion that this can kill the freediver, but I can't find any clear answer why and can't think of any reason why this is dangerous for the freediver.

If they make a normal ascent and nothing goes wrong, there is no reason it would harm the freediver.

What I would question is in what situation would this happen? If it was an attempt to prolong the freediver's time underwater, it is a very bad idea. When two scuba divers act as a buddy pair (assuming both are on single tanks), they act as each other's alternate air source. What if the scuba diver had an OOG situation? Who will he share air with? He could dive with another scuba diver, but if they have to share air for some reason, who does the freediver go to? If the freediver wants to extend his time underwater, the answer is his own scuba. Personally, I would never get involved in such an activity.

If a freediver came up to me and signalled he wanted to share air, he'd get it from me. If he doesn't understand the risks from breathing compressed gas underwater, that is his problem, not mine. I'd feel better about a freediver dying from a lung over-expansion injury from my gas than a freediver drowning because I refused to donate gas on the off-chance he doesn't understand the risks. I doubt this is a situation that occurs often though.

If it can be done from 100 fsw with air, can the same be done from 200 fsw with SCUBA diver using trimix?

No reason why not. Another issue would be the scuba diver may have deco obligations. Assuming he is using normoxic trimix with an O2 content of 18% and a separate deco bottle, the freediver would have the option of staying on the scuba diver's back-gas (the trimix) as he deco'd, or continuing to the surface and exhaling on the way up. The freediver should not have any deco obligations if they don't hang about at depth, so staying on 18% in shallow water won't cause him to incur a deco penalty.

Then there is this little problem called buoyancy control. Most free divers only become neutrality buoyant at some depth well below the surface. So even if they know how to keep their airway open durning an ascent on compressed gas, you will both end up on a free ride to the surface once they become positive.

Not a good plan for a happy ending, there are too many things that can go wrong, resulting in two victims.


Sent from my iPhone using Tapatalk

Generally, freedivers weight themselves to be neutrally buoyant at 10 metres. Above this depth, they will be positively buoyant, but they will still be able to control their position the water.

I have not done this at 100' so I can't answer that part of the question. The point to be taken away is that a freediver who breathes compressed air at depth becomes a SCUBA diver. What he does after the breath is limited by the choices a SCUBA diver has, not those of a freediver. If you decide to do it, don't get confused between the two.



Bob
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I think that advocating unsafe and dangerous practices is both stupid and foolish. That is why I don't tell people to do what I do. Dsix36

This statement sums it up perfectly.

 

[dumpsterDiver]

There is a significant difference between doing this at 100 versus 200 feet. The reason has been referenced several times. I used to do ditch and don in 60 feet (for fun).

So each time, I did that, I was in the exact same position as a freediver ascending after taking a few hits (although my nitrogen load was higher). I sure wish we had GoPro's back when I did my one (and only) tree stand, rope-swing, scuba entry..

If anyone wants to learn what it feels like to be a freediver who takes a hit of air at depth- do the experiment yourself.

I often like to do quick descents when scuba diving, so sometimes I just swim down hard and fast without taking a breath from my regulator. Allowing your chest to compress on descent certainly facilitates your descent speed and once you get pretty deep and you want some air; it always feels weird to inhale when descending fast. If you inhale slowly (and descend fast) you just sorta keep inhaling and inhaling without filling the lungs.

 

[Lost down under]

Did it with a mate at 10m, we had both talked about if before hand and both ascended safely together.

Not advised but we are only now getting old enough to recognise our own mortality!

 

[mrdre]

Assuming that, as everyone else has said, that the freediver knows about lung expansion injuries and breathes with you on the way up from 100' just as an out of gas diver would, it would be fine. However from 200' it would not work. I'm not sure the specific depth it happens at (probably depends on lung volume), but as a freediver descends their lungs keep compressing with depth until they are about the size of a fist and contain no gas, but instead are filled with fluid (blood plasma? Surfactant? I can't remember). At that point the freediver couldn't inhale even if they had gas available because their lungs are effectively collapsed, and they would not be able to inhale. Deep safety divers are generally just there to get the body back to the surface, so it can hopefully be revived.

I think that's the explanation for why the support divers are instructed not share air. So after certain depth it's basically impossible for the freediver to breathe.

As dumpsterDiver mentioned it actually could be quite fun to swim to scuba friends, take few "hits", and swim around them for a while. I've seen people do that, but not at great depths: around 50-60 feet. So I guess it's possible and relatively safe only up to some depth, which makes sense.

Thanks for the replies.

 

[Eric Sedletzky]

That is an interesting point, I'm not sure what the correct answer is, but I don't believe it is actually an issue.

My initial scuba training in the 1970's involved doff and don practice, with the "final exam" being a freedive into the deep end of a pool (about 20' deep, under the high dive boards). All of our gear was arranged on the bottom, including mask and fins, the the air was turned off. You take a deep breath, swim down to your tank, turn on the air take a breath, then don the rest of your gear and surface normally.

My point is that there is a fair amount of compression descending that first 20'. There were zero issues with breathing compressed air after freediving to this depth, and I am guessing this would remain true at greater depths.

But again, this is just my opinion, and it would be interesting to find out for sure; I've never really given it much thought and I could certainly be wrong.

Best wishes.

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.

 

[Bob DBF]

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".
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.

As fast as some of the classes are taught, it is lucky that they teach as much as they do.

I always thought that all the quality time in the pool was the most fun, although it is looked on as foolish and a waste of time by a lot of instructors today.



Bob
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That's my point, people, by and large, are not taught that diving can be deadly, they are taught how safe it is, and they are not equipped with the skills, taught and trained to the level required to be useful in an emergency.

 

[boulderjohn]

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 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.