padi ow, why is CESA recommended to 9m?

[boulderjohn]

No. At BEST you'll get the occasional part breath at each depth and it'll get less and less the shallower you get. You'll be OOA again less than a second after getting "air".

I'm not sure what you mean there, but the change in pressure on ascent becomes more dramatic as you get shallower, and the ability of the regulator to provide air will increase.

 

[String]

I'm not sure what you mean there, but the change in pressure on ascent becomes more dramatic as you get shallower, and the ability of the regulator to provide air will increase.

Yes but not significantly. You'll get a bit of a breath but from 10-20m or so not a lot. Once you get to the last 5m or so you wont get enough for even that. Not that it'll matter as by that stage the diver will be heading up like an absolute rocket so will only be there 2 seconds or so.

Try it, you'll find the extra gas by expansion really isn't that much.

 

[flots am]

Try it, you'll find the extra gas by expansion really isn't that much.

Yep. The tank isn't changing size. The only thing that's happening is that the difference between the tank pressure and ambient is increasing slightly, so a little air might become breathable.

It's certainly nothing I'd count on.

flots.

 

[diver 85]

Go back(lol if you can find them) & look what Al Giddings got Shawn Weatherly to do(in the mini-series Ocean Quest, c1985)-----CESA from 100'------------she was learning how to dive to make the 'ocean----quest' of her lifetime...AND------she 'made it'.......

 

[scagrotto]

a little air might become breathable.

It depends on what you mean by "a little", and what tank you're using.

The actual interior volume of a Luxfer AL80 is 10.7 liters. That means that at any given depth an "empty" tank will actually still have (10.7 * ATA) liters of gas. That means:
42.8 L @ 99 FSW
32.1 L @ 66 FSW
21.4 L @ 33 FSW
10.7 L @ the surface

At 66' the 10.7 liters that becomes available from the ascent of 33' would have an actual volume of about 3.6 liters. The typical adult has a tidal volume of about 1/2 liter, but since they may be a tad stressed let's figure a single inhalation will use 1.8 liters. That means that the expanded air would offer 2 breaths, assuming a perfect world where the regulator delivers 100% of the pressure that exceeds ambient. That's not a lot during an ascent that would normally last a minute, but relative to being OOA it's significant.

At BEST you'll get the occasional part breath at each depth and it'll get less and less the shallower you get.

Actually, you get the same surface volume of air for each 33' you ascend. Effectively you get more air, because the reduced pressure equates to more actual volume as ambient pressure decreases. In ascending from 66 FSW to 33 FSW another 10.7 liters becomes available, and has an actual volume of 5.3 liters at 33'. Given the same 1.8 liters per breath, there are now 3 breaths available.

The 10.7 liters freed up by the final 33' of ascent would offer almost 7 breaths at the surface. Of course the atmosphere offers all the breaths you could ever want, so you might as well use that air under the surface as it becomes available.

I've sucked a tank dry in 3 or 4' of water, but never at a meaningful depth, and I haven't conducted an actual experiment on how many breaths you'd actually get in a real world scenario. Depending on the actual depth at which you take each breath and how much you inhale I expect it could range from 5 to 10 or more.

 

[beanojones]

It does seem like at least some of the people with opinions should have some experience about these things, since at least some number of us are actually instructors passing along knowledge about this topic (or maybe it's more like 'knowledge' about the topic.)

What does actually happen with an empty tank, how does the breathing effort change, both as it runs out, and on ascent with an empty tank? Can a diver do a complete exhale CESA from 60 feet? 80feet? 100 Feet? These things should be known for sure, through the diver's own actual experience and not as a thought experiment, at some level of the diver's experience and responsibility.

First, a reasonably prepared diver (experience and mindset and forewarning) should be able to do a CESA from an arbitrary depth properly, that is exhaling continuously all the way to the surface, and maintaining proper ascent rate. After all, this is just a basic skill from the old days of basic OW certification, when CESAs were basically always potentially there. Just ask someone who got into a supposed buddy breathing (sharing a single reg) situation with an actual out of air diver in the old days, and ask them if they ever got it back. The thread from way back about this had several old timers point out that never getting the reg back from the OOA diver after a buddy breathing pass was basically something that one should expect to happen at any given time. (Actually, as a modern instructor, just train enough DMs and the reactions of Divemaster Candidates (not giving back the second stage during the gear exchange, and the spit and bolt, etc etc) should be enough to convince an active instructor that being able to handle things going to total poop should be part of the deal. Just imagine making your DMCs do the gear exchange swimming at 60 feet, and think about what they would have to be able to handle bailing on the exercize, to do that safely. A ditch and don from 60 feet. Also known as, to some degree of approximation, a CESA. I don't make my DMs do the gear exhange at 60 feet, but I have had OW students who traded gear at 50 feet on their first OW dive after certification, and because they were twins and that's what twins do, both ended up trying to play keep away with both sets of gear. I was ready to help, but I was also pretty comfortable with them playing around, because they understood CESA, because I understood CESA. I have left gear at 60 feet to go get other stuff. It's just not that hard. As instructors, we should know this).

I personally add that IMHO a working dive guide should actually go out and practice the continuous exhale CESA from the max depth they guide to , enough that they are comfortable and confident in their ability to do it, so that god forbid the situation ever happens that they cannot get to their own backup into place because their hands are literally full with an panicked OOA diver, they will not have to get into a slap fight with an OOA to steal the reg back, or at least get their hands free to get their own backup in their own mouth.

Note: this is a real world situation, not a theoretical for me, on both the practice side, and the actual real world side. My instructor for my DM course took my out to the Mahi and said go for it: get to the bottom, and practice the CESA until you can do a slow ascent from the bottom to the surface on a continuous exhale without breaking ascent rate limits. (I personally would tell a DM working for me, that if you cannot do a CESA from X feet, then that is beyond your safe limits for their own diving, and far more importantly for when he/she is guinding customers. There's plenty to see/show at 40 feet.)

And this has paid off, when I did not have to wrestle an OOA diver who was bear-hugging me, and keeping me from getting my backup in my mouth, because I knew I could do the CESA from wherever I was diving. I did not have to figure out how to hurt the diver to protect myself, because I alredy took care of that by actually making sure my skills were squared away well beforehand.

As to what actually happens to breathing effort with an "empty" tank, it's bizarre: Most regs get harder, but there are a few that get much easier when the tank pressure is slightly above ambient, usually the so called 'over-balanced regs. It's no problem getting additional real breaths, however from either of them. I have worked plenty of places where we had to breathe tanks to dead empty because each tank was a real cost, and real effort to get to the dive site, so every tank for the working divers was breathed dry enough that the reg could be taken off with the valve on.

 

[radtype]

Is there a good reference on being able to sip a little air from an empty tank as you surface. I keep seeing and hearing references that a tank will expand as you surface allowing you to get the extra air. It was referenced earlier that your regulator will adjust to the pressure change giving you the extra air which I understand.

 

[Redshift]

Is there a good reference on being able to sip a little air from an empty tank as you surface. I keep seeing and hearing references that a tank will expand as you surface allowing you to get the extra air. It was referenced earlier that your regulator will adjust to the pressure change giving you the extra air which I understand.

The tank will not expand.
A regulator gives you gas at the surrounding pressure. If you are at (sorry, this will be in metric) 30m, that's 4 bar ambient pressure. If a tank is "empty" it means it has 4 bar inside (just as an empty tank at the surface is not empty, it has gas at 1 bar, not vacuum). When ascending, if you are now at 10m, that's 2 bar ambient pressure, therefore the regulator can deliver you gas at that pressure, from the 4 bar that were in the tank.

 

[beanojones]

Is there a good reference on being able to sip a little air from an empty tank as you surface. I keep seeing and hearing references that a tank will expand as you surface allowing you to get the extra air. It was referenced earlier that your regulator will adjust to the pressure change giving you the extra air which I understand.

You got the first part corrected by the above hopefully perfectly clear post by radtype.

For the second part, the reg will most definitely not make it easier to get the last breath out of a tank. It is (usually) very much like sucking marbles through a straw to get the last bit of air for most regs, because there is very very little excess pressure over ambient in the hose running from the first stage to the second stage.

By comparison, hose runnning to the second stage usually has something like 10 times the ambient pressure running to it. (That's a vast simplification, the sort of vast simplification, that as a divemaster, you should really think about not having to need, but ...)

This is not how gear is designed to be used, and you can flood a first stage, and a tank easily doing this stuff, requiring overhauls etc. etc.

There are some distinctions at play here:

1. Divers: I think recreational divers should be comfortable handling CESAs from whatever depth they dive to. Easy and safe enough to practice with a full tank and all gear in place. Divers should always maintain there gear in place, and should never intentionally ask for trouble by running tanks to empty, etc. If a diver questions whether they can CESA from some depth, then they probably should not be diving that depth. It's not that they will ever need to do it, necessarily, but someone who dives regularly to 100 ft/30 m can get comfortable and complacent. Constantly thinking "could I get up from here on a continuous exhale?" will probably make a diver less likely to hit depths complacently.

2. DMs are however a different matter. This is not meant as a criticism of you, but it certainly is a criticism of whoever certified you as a DM. DMs should simply more than a little bit better than a recreational diver. And they absolutely should have the theoretical stuff down to the point that they are not misunderstanding things as badly as you are. DMs are supposed to have instructor level theoretical understanding. But since many instrcutors don't have it, where is one supposed to get that knowledge from?

3. As instructors, we should all be able to state facts about diving from experience, and not from "Some dude at Scuba Board who might post about diving more than they dive, and repeats received information as if it is personal experience." Personal experience for the physical aspects of diving are key. It makes little sense for an instructor to have an opinion about what OOA is like if they have never been there or done that, and it is more or less an ethical misdeed to repeat unconfirmed second hand received information as fact, at the instructor level. There is a reason why I have strong opinions that vary from the received wisdom: actual personal experience. I don't trust that someone knows what they are talking about, when I can go verify it myself. And then, I know it not from them, but from me.

There is nothing to prevent people who claim to be 'active' instructors from doing repeating unverified second hand hearsay as fact, though. Because there is very little mechanism (other than actually doing things yourself) to correct those spouting nonsense, and outlying opinions in diving can be either crazy or prescient. It on us as professionals to actually do the things so we can actually see which an outlying opinion is: crazy, or prescient.

There is an ethical issue here about repeating second hand knowledge as fact. That ethical issue can largely be, and usually is, covered up by the fact that recereational diving is mostly safe even when people fsck up fairly badly, or instrcutors present second hand nonsense, that they have no ground for holding beliefs about, as fact. Luckily for us the basic safety factor is what allows people to get away with having such silly ideas about recreational diving.

 

[flots am]

It depends on what you mean by "a little", and what tank you're using.

The actual interior volume of a Luxfer AL80 is 10.7 liters. That means that at any given depth an "empty" tank will actually still have (10.7 * ATA) liters of gas. That means:
42.8 L @ 99 FSW
32.1 L @ 66 FSW
21.4 L @ 33 FSW
10.7 L @ the surface

You don't get to breath all that.

Depending on the reg design, you only get roughly what's left over when you subtract the ambient pressure, and the intermediate pressure from the tank pressure, and even that's all theoretical.

It's entirely possible that any particular reg won't operate when tank pressure gets that low. That's why people say "you might" get another breath. It's a maybe.

There is no guarantee and certainly nothing you can calculate and say "You'll get X breaths". In fact, I've actually tried it and my reg delivered air until the tank was pretty much empty, then I got nothing on the way up.

Happily, you don't actually need anything on the way up from recreational depths with no deco obligation, and you'll need to have screwed the pooch so many times to get to this point that Darwin should rise from the dead just to congratulate you when you surface.

flots.