CESA theory

[tursiops]

I only used SAC to have an estimate of exhale rate at depth and I knew that My thinking was probably wrong. But I do think that exhale rate is important if I want to resolve the equation. If you exhale like a seal (french expression), even from 30 ft, you won’t make it.

The point is not to exhale, the point is to keep your lungs open so gas can escape. Exhaling is one way to do that, but not the best choice. Hum, make a (high-pitched, not low-pitched sound which takes more gas) sound, whatever will open the throat. So SAC is not relevant.

 

[inquis]

If you exhale like a seal (french expression), even from 30 ft, you won’t make it.

I assume that's a forceful, complete exhale? That's the point. As you ascend, your lungs inflate -- just as if you breathed. Further, the urge to breath is less because the partial pressure of CO2 drops as you ascend.

Why not simulate this sitting in your chair? Normal tidal volume is 0.5 liters. The expansion from 30 ft is another liter (doubling of the residual volume). Exhale and start a stopwatch. Can you make it 30 seconds on two normal breaths? I'm pretty sure you can.

ETA: you may be able make 30 seconds without breathing at all after the exhale. This is harder on the surface because of the CO2 buildup. When ascending, that partial pressure is reduced by about 50% compared to sitting in your chair. Can you exhale and not inhale for 15 seconds? A real ascent from 30 ft would be easier.

Again, the issue will be avoiding barotrauma, not blacking out from O2 deprevation or fighting the urge to take a breath.

 

[johndiver999]

The answer is that if you have to do a cesa, then do your best. Ideally, you do not want to exhale all your air at the start of the ascent because it provides buoyancy. As mentioned earlier, the less exertion required the better, so you would not want to be neutral and then exhale and have to kick up even harder. In addition, the rate of gas expansion at considerable depth is slow.

There is no benefit to exhaling completely or very fast at the start of the ascent. As long as you keep the airway open, any excess expanding gas will be expelled passively- assuming you do not have a structural problem with your lungs and you are not very buoyant and coming up super fast.

Keeping your lungs moderately full on the ascent will also serve to dilute the quantity of co2 in the lungs- helping to keep the percentage as low as possible, which will greatly improve the feeling/comfort of the ascending diver.

Presumably, as the ascending diver approaches the shallower depths, expanding air in bc and wetsuit expansion should be increasing buoyant force and would be increasing the ascent rate at the exact same time that pressure is dropping most rapidly, so the final 10 meters or so might require a little more attention to exhalation. If the diver feels good and knows he will “make it”, he can really exhale more completely and thereby slow the ascent.

I bet that most people who have done such an ascent for real from 30 meters have probably ditched some lead on the bottom, so they are probably doing an emergency buoyant ascent rather than just swimming. That is what I would do anyway at that depth.

 

[Dody]

Just so that you understand my behaviour (or questions). I hate not being in control. I know control can be an illusion but like most human I happily accept this illusion. When I don’t feel something that I might have to do someday, I feel vulnerable like a baby.
I have a good and old friend. Very experienced diver certified in France, GUE Instructor (the only one who accepts diving with a cigar smoker ), over 1000 dives. We plan to go diving together for the first time in Thailand in a couple of weeks. I was drinking his words talking about the plan but when he told me about a story where he was alone at 30 m, had first stage failure, closed his valve and quietly did a CESA, I was like: I could not do it if I don’t understand the physics. Not only the theory. He told me: don’t worry, it’s easy, you will see. Well, I don’t want to see. I want to understand. Beyond simple explanation we get in diving training.

 

[Celt]

you don't have to think about exhaling, look up, open your mouth and head for the surface. physics will do the rest. when deep fully exhale before heading for the surface.

 

[tursiops]

Just so that you understand my behaviour (or questions). I hate not being in control. I know control can be an illusion but like most human I happily accept this illusion. When I don’t feel something that I might have to do someday, I feel vulnerable like a baby.
I have a good and old friend. Very experienced diver certified in France, GUE Instructor (the only one who accepts diving with a cigar smoker ), over 1000 dives. We plan to go diving together for the first time in Thailand in a couple of weeks. I was drinking his words talking about the plan but when he told me about a story where he was alone at 30 m, had first stage failure, closed his valve and quietly did a CESA, I was like: I could not do it if I don’t understand the physics. Not only the theory. He told me: don’t worry, it’s easy, you will see. Well, I don’t want to see. I want to understand. Beyond simple explanation we get in diving training.

Do you understand the physics of how your regulator is balanced?
Or the chemistry of the LI battery in your light?
Or the way your dive computer works out your NDL?

 

[Dody]

Do you understand the physics of how your regulator is balanced?
Or the chemistry of the LI battery in your light?
Or the way your dive computer works out your NDL?

Yes to the three. I have an unbalanced reg. Aqualung Calypso but up to 40 m and in warm waters, it is good enough and in the very unlikely event of an OOA due to stupidity, I should notice a few seconds before total failure compared to a balanced one. And I have not noticed any difficulty to breath at 40m.
Li or Li-ion battery, I have learnt before high school. I have two diving lights that I don’t use UW. I don’t do pictures or videos and I don’t really care if what I see too blue or too green as long as the visibility is fine. Don’t forget. I don’t do cave or overhead or night (forget the bad experience 6 months ago and forget about DM cert. I did it bit don’t want to exercise it and never talk about it. I just don’t feel night dives, period).
And I have almost learnt by heart my DC manual (I have three DCs that use two different algorithms). I have tried all of them. In regular and conservative modes to compare. But I haven’t used GF as I don’t do deco.

 

[Angelo Farina]

Just so that you understand my behaviour (or questions). I hate not being in control. I know control can be an illusion but like most human I happily accept this illusion. When I don’t feel something that I might have to do someday, I feel vulnerable like a baby.
I have a good and old friend. Very experienced diver certified in France, GUE Instructor (the only one who accepts diving with a cigar smoker ), over 1000 dives. We plan to go diving together for the first time in Thailand in a couple of weeks. I was drinking his words talking about the plan but when he told me about a story where he was alone at 30 m, had first stage failure, closed his valve and quietly did a CESA, I was like: I could not do it if I don’t understand the physics. Not only the theory. He told me: don’t worry, it’s easy, you will see. Well, I don’t want to see. I want to understand. Beyond simple explanation we get in diving training.

I try to explain, based on my experience, so that hopefully you will understand.
First of all, every human, if he stops breathing, has still at least one minute, usually two, before passing out. Probably, if CO2 builds up, he/she will feel the need to breath much earlier, but this "need" isn't real, CO2 will not make you to pass out, it is the lack of oxygen causing you to pass out.
At depth, the pp of Oxygen is high. So, even if you take two full minutes for surfacing, the risk of passing out is just in the last 2-3 meters, when you are almost at the surface.
If you manage to surface in less than 2 minutes, there is no risk of passing out; so, if you are down to 30m, or less, you are 100% sure to have enough time for surfacing at a reasonable speed.
But of course there are two other risks: lung over-expansion and drowning.
Lung over-expansion occurs, again, in the last few meters close to the surface, where the gas expansion is large and quick. And this is avoided if you leave your glottis valve open, so that, when the lungs become full during the ascent, the excess air can escape. No need to exhale voluntarily, nor to start exhaling before the lungs are full. For being sure the glottis is open you can emit some continuous sound, usually through your nose, doing "mmmmh".
The other risk is drowning. This happens when the CO2 goes up (as said, this occurs much earlier than the oxygen becomes so low that you pass away). CO2 gives a strong stimulus to breath. If you breath underwater you can inhale water, if the reg is not working, and so you could drown.
People who started as free divers, as me, and only later became scuba divers, are used to resist the urge to breath when the CO2 gives you the stimulus. Some "pure" scuba divers are unprepared to control their body under the pressure to breath. You should train to resist to it, and this training is better done on your sofa, where there is no risk of inhaling water if you cannot resist.
Last point: if you do not have decent free diving capabilities (particularly about kicking efficiently and resisting the urge to breath), then it is quite logic and correct that you are worried to perform a CESA. Nothing to be ashamed of.
In this case there are just two options:
1) Get trained to free diving to the same depth you plan to reach with a scuba system (say 30 meters) - if you can swim down and up again to this depth without breathing, a CESA from that depth will appear to you always feasible.
2) Equip as tech divers do, so that you never need to ascend for solving your problem. This is what I do (albeit having never done anything really "tech"). Instead of a single reg, I always use two fully independent regs, mounted on the two posts of my tank (which is also equipped with a good old reserve mechanism) - it is a 15 liters steel at 232 bars (albeit in some places they fill it up to just 200-210 bars), so I have plenty of air without making use of the reserve, which stays there "just in case" and is not planned to be used, except for true emergencies.
If my main first stage fails, I just close its valve and breath from the other reg. When people here were yet using old yoke regs, I have seen enough failures for understanding that relying on a single reg is barely stupid. Now all my regs are DIN, but I still use two of them: better being safe than sorry...

 

[tursiops]

Yes to the three. I have an unbalanced reg. Aqualung Calypso but up to 40 m and in warm waters, it is good enough and in the very unlikely event of an OOA due to stupidity, I should notice a few seconds before total failure compared to a balanced one. And I have not noticed any difficulty to breath at 40m.
Li or Li-ion battery, I have learnt before high school. I have two diving lights that I don’t use UW. I don’t do pictures or videos and I don’t really care if what I see too blue or too green as long as the visibility is fine. Don’t forget. I don’t do cave or overhead or night (forget the bad experience 6 months ago and forget about DM cert. I did it bit don’t want to exercise it and never talk about it. I just don’t feel night dives, period).
And I have almost learnt by heart my DC manual (I have three DCs that use two different algorithms). I have tried all of them. In regular and conservative modes to compare. But I haven’t used GF as I don’t do deco.

I interpret what you've said as you do not have complete understanding of any of those three examples, but you have sufficient understanding to be comfortable using them. So what more do you need to understand about CESA to be comfortable using it? There is a wide range between understanding nothing and total, complete understanding of the physics and physiology. Where is your comfort zone in that range?

 

[johndiver999]

The physics is not that complicated, or at least the depth of understanding required is minimal. The biggest factors are unknowable and unpredictable in determining if you are going to “make it”.
One of the more important is your metabolic rate at the time of the failure. Are you relaxed and neutrally buoyant or are you stressed out kicking hard and out of breathe when the party stops? Also what level of fitness do you have? Are your legs strong and trained to function anaerobocally? Did the failure occur when your lungs are full or empty? All these factors are going to be hugely important and it is also why practicing a cesa while rested, calm and neutrally buoyant and with lungs full of air has got to be easier than the real thing. Not to mention the overwhelming fear of death.

The reality is that you are probably going to go up “fast” or the chances of making it are significantly reduced, in my personal opinion. Practice breath hold snorkeling ( with a spotter) if you want to improve your abilities to function without air and without undue stress. Trying to hash out details of physics ain’t gonna do much.