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Failure depth

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Well-Known Member
Aug 10, 2001
I remember Eric Fattah referring to the depth where lung pressure turns negative and below that it is difficult -or impossible- to bring air in the mouth for equalization. If I am correct he called it "failure depth".
I experience this at around 35m (with a mask) and was wondering how do people deal with it in order to go deeper. I can equalize for a few more meters if I pinch my nose but don't like this method.
Theoretically, the mouthfill technique where one fills the mouth right before failure depth would work in this case. I am just assuming it is not going to take me much deeper before having the same problem again -running out of air in the mouth. And then it's CERTAIN I won't be able to get ANY from the lungs.
There are plenty of divers that do 70m+ with a mask. So, here it is: how do they equalize at that depth?
Ideas/suggestions/tricks anyone? I'm counting on your experience guys.

Thanks in advance
Hydro, NY

A full mouthfill at 35m can take you to 120m+ with a mask, if done properly.

You can calculate your max depth for equalizing as follows:

(with a BUDDY)

1. On the surface, exhale about 95% of your air, and exhale the last 5% into your closed mouth, filling your mouth up. Now close your epiglottis. You have just performed the first step of the mouthfill.

2. Descend along the line until you cannot equalize anymore. Continue descending until you feel the pressure on your ears, and cannot descend anymore. Record the depth as depth X.

So, you went from a pressure of 1 atm, to a pressure of (0.1X + 1) atmospheres, with a single mouthfill.
Now, if you fill your mouth at depth Y, then your max depth will be:
10 * [ (0.1X+1)*(0.1Y+1) ] - 10

So, in my case, one mouthfill at the surface, with a minima mask, takes me to 22m, so my pressure goes from 1atm to 3.2atm, on one mouthfill.

I fill my mouth at 35m. If I try to fill my mouth at 40m, I cannot get a full fill.

So, at 35m, the pressure is 4.5atm, but I know that I can reach 3.2 times my original pressure once I fill my mouth, so my final pressure is 3.2 * 4.5 atm = 14.4 atm = 134m.

Or, using the full formula above
10 * [ (0.1*22+1)*(0.1*35+1) ] - 10= 134m.

Only a few divers have learned to do this technique effectively. For example, one of my friends, on a full mouthfill, can reach 12m. He can fill his mouth fully at 25m. So his max depth with this technique is:
10 * [ (0.1*12+1)*(0.1*25+1) ] - 10 = 67m.

Herbert told me that in Ibiza he filled his mouth at 45m, but then his mouthfill 'ran out' at 75m (and he just sank until 86m without equalizing). This means that he did not fill his mouth completely, because a full mouthfill would have taken him much deeper. I think a full mouthfill at 45m is nearly impossible.

This is the difficulty with the mouthfill. The body position required to fill the mouth at depth is very strange. To manage a full fill at 35m, you need to bend over (nearly doing a somersault), and then cock your head way back, and opening your jaw with lips sealed (keep your hood below your chin). You must then make a tremendous effort with your diaphragm to force all your air into your mouth. This is a very 'acrobatic' maneuver, and it consumes a lot of oxygen, but once you master it, you can forget about equalizing until you hit the bottom.

Eric Fattah
BC, Canada
Hi Eric, I was hoping you'd reply to this question.
Did I get this right? You only fill your mouth ONCE at the surface and then ONCE more at around failure depth or a little before?
If this is the case, I'm doing something completely wrong which is partially filling the mouth several times (2-3) before hitting 35m. At that point, when I try to do it again it just doesn't work anymore. I'm almost certain though I need more than one mouthfull to reach 35m.
So, the idea is to use up the first mouthfull and then do the second mouthfill at a depth that allows 100% of it, right? That means mouthfill is performed while pressure is still positive(?).
Could you elaborate a little on the acrobatics of the maneuver? Do you tuck the body completely as in an "egg" position but with the head tilted back? Does it interupt your freefall?
Sorry for the barrage of questions but I don't often have the chance to ask somebody who can actually do this.

Hydro, NY

The previous response was more than I could pray for.
Thanks again.

1. Swim down to 35m (or whatever depth), equalizing with the normal frenzel
2. Fill the mouth ONCE at 35m
3. Use that air to equalize to 120m+
(if your epiglottis opens for even a SECOND during the long trip from 35m to 120m, you will not be able to equalize anymore. There is no room for error)

Filling the mouth at the surface is only a method of testing your ability to do the technique, to calculate your maximum depth. Also, filling the mouth at the surface is useful for deep negative dives (with a BUDDY!--and watch for lung squeeze!)

Here is a picture of the acrobatic mouthfill as it should happen around 25-35m:

Eric Fattah
BC, Canada


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Using the tongue as a pump

Eric, in dry, when I completely exhale my lungs forcing to the maximum my diaphragm and intercostals and abdominal muscles to the point I can’t do it further I still can full my mouth pumping out air from my lungs to my checks using my tongue as a pump.

Last time I did this manoeuvre on the sea I was at -30 m (this was the maximum depth this day), I waited there in the bottom at -30m for a few seconds and I filled my checks completely very quickly using only my tongue.

1 - Can this manoeuvre with the tongue supply the bending manoeuvre you explain to fill the mouth?

2 – There is any disadvantage if I fill my mouth for example at 23 - 25 meters when it is still easy to by done with the diaphragm. It is too early?

Saludos Agustín.
Tongue Suck

What you are describing is 'reverse packing'. This allows you to draw extra air from your lungs even after you can't exhale anymore. However, it is not ideal, because reverse packing will not allow you to completely inflate your mouth & throat, by its very nature. Try this: right now, tilt your head back, and fill your mouth by exhaling, and keep your throat OPEN. The fact that your throat remains open allows a huge extra amount of air. I have measured the air I can get into my mouth with a spirometer: 600ml !!! If you do the same with reverse packing, you get about 300-400ml. Once your mouth is inflated, close the epiglottis without losing any of your air back into your lungs. This takes a lot of practice.

To save oxygen on your dive, you should fill your mouth at the shallowest depth which will still allow you to reach your target. So, if you do the math and you calculate that a mouthfill at 20m will allow you to reach your target, then you should do it at 20m. Doing it any deeper will be more difficult, and waste oxygen because of the huge effort.

For example, I often fill my lungs at 25-28m, instead of 35m.

Eric Fattah
BC, Canada
thanks again Erick, thanks you very much.

Sorry Erick when you write:

"For example, I often fill my lungs at 25-28m, instead of 35m"

I think you mean: "For example, I often fill my mouth at 25-28m, instead of 35m."

Saludos Agustín.
(if your epiglottis opens for even a SECOND during the long trip from 35m to 120m, you will not be able to equalize anymore. There is no room for error)
Is there a chance that since you have negative pressure in the lungs and you open the epiglottis you might "suck" air from your ears to you lungs and might cause some ear damage?
Sounds pretty scary if yes, but still please answer. :D

Or, using the full formula above:
10 * [ (0.1*22+1)*(0.1*35+1) ] - 10= 134m.
So with fluid goggles your max would be.... 1743m? :D:duh
Last edited:
Descend along the line until you cannot equalize anymore. Continue descending until you feel the pressure on your ears, and cannot descend anymore. Record the depth as depth X.
At what depth should a blood shift occur in that sort of a dive? pretty shallow, no?
Answers to questions:
1. Is there any danger if your epiglottis opens accidentally, exposing the sinuses to the negative pressure in the lungs?

- No danger; the worst that can happen is that the air in your mouth rushes back into your lungs, but the negative pressure is not enough to cause a reverse equalization on the ears.

2. Blood shift during negative dives with the mouthfill

- Doing the mouthfill before a negative dive allows a much greater depth to be reached. In fact, you can simulate over 200m! This opens up a real danger. Because equalizing is not the limit, chest squeeze / lung fluid / blood shift now becomes the limit, and if you go to far, you can be SERIOUSLY INJURED!!

Please, when doing negative dives, always use a buddy, and never go to the point of discomfort.

You can calculate the depth that you are simulating as follows:
- Find your failure depth during a real dive, i.e. the depth at which you cannot exhale any more air: X
- After fully exhaling on the surface and filling the mouth, if you truly exhaled everything, then when you descend to depth Y, you can calculate the simulated depth as follows:

At depth X (pressure 0.1X+1), you reached empty lungs.

During the negative dive, you reached empty lungs at the surface (pressure = 1atm). You reached a depth of Y, or a pressure of (0.1Y+1), which is (0.1Y+1) times the original pressure of 1.

So, during the negative dive, you descend to (0.1Y+1) times the pressure at which your lungs became empty.

So, if your lungs really become empty at depth X, then assuming the ideal gas law at depth (which is not accurate), the simulated depth would be:
10*[(0.1Y+1)*(0.1X+1)] - 10

So, if you fail at 35m, and you did a negative to 10m with a FULL exhale, then you simulated:
10*[(0.1*35+1)*(0.1*10+1)]-10 = 80m

In my case, my failure depth for normal equalizing is 80m, and I have reached 22m on negative, so the simulated depth is:
10*[(0.1*80+1)*(0.1*22+1)]-10= 278m
Seeing this number, I realize that I probably didn't fully exhale on my 22m negative, because my lungs were only half filled with fluid at the bottom.

However, both of these calculations are too conservative. Near the surface, the ideal gas law holds, i.e. a double in pressure causes the volume to be reduced by half. However, down at 100m+, doubling the pressure does not cause the volume to decrease by half; the ideal gas law no longer holds, so you must use the Van Der Waals equation or something similar. If you do, the result will be an even greater depth, i.e. the above results (simulation of 80m, or simulation of 278m) are actually simulating deeper depths.

Eric Fattah
BC, Canada
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As I understood, when blood shift occurs, blood fills the capillaries that surround the alveolis to compensate some of the volume that the lungs have lost.

That gives us Risk no.1
"Erupting" some pulmonary blood vessels:
Can cause secondary drowning.
Scars lung tissue, might cause a small deffciency that might be permanent.
Coughing blood.

All the blood that was "shifted" to the lungs, need somewhere to go when you ascend/inhale, and the place where blood goes to after the lungs is the heart, via the "lung vein" (atrium? the only vein in the body that carries O2 rich blood to the heart).

That gives us Risk no.2:
"Lung vein" eruption since it might be unable to stand the sudden high amount of blood volume.
Heart "eruption"/failure, from a huge and sudden increase in blood flow.

Let's not forget the more obvious Risk no.3:
Fractured ribs, which I don't know how dangerous it is (I have no clue if a snapped rib can tear a lung/lung membrane), but is sure ain't pleasant.

What I was aiming at, is that a friend, as improtant as it is, is really not enough for empty lung dives.

What I do think, is that NO ONE should dive empty lungs to max depth in the first time he thinks he is able to, no matter how good he controls any mouth-fill equalisation, since the body needs time to adjust to that pressure and to make blood vessels/tissues and rib cage more elastic, and that should be achieved in a very graduated increase of depth, in some period of time which I have no clue about (weeks? months?).

I guess that the trained poeple have none to worry about all of this, since their body would no doubt can stand that pressure.

If anyone have an idea how to create such a training plan to make sure that a begginer freediver could adapt to those physical terms, I would be thankful.

If I made some mistakes I would be glad to be corrected, I see it as a matter of safety.
packing and failure depth

Hi friends
Is it correct to assume that packing would push the failure depth deeper?
I thought it would since it increases pressure in the lungs. Therefore it would take more external pressure (depth) to get to that point where pressure becomes negative.
Any ideas?

Hydro, NY

Packing increases failure depth, not because of pressure, but because you start with a higher volume of air in the lungs.

Eric Fattah
BC, Canada
my empty lung diving spiel...

yep I would agree that it's important to take it easy with empty lung dives. One variation that can be done to allow your body to adapt is to perform what I call 'neutral breath dives'. Maybe it's a silly name? Basically it means that you dive on a passive exhalation. In other words, you don't exhale with any force, just let the air out passively. What it means is that you dive with about a litre over residual volume.
This does allow you to dive deeper than with an empty lung of course, but you should still resist the temptation of diving too deep even if you can equalise. The good thing about them is that they're less strenuous that totally empty lung dives because the pressure gradient is shallower. My understanding of the blood shift (pulmonary erection) is that it takes a certain amount of time for the process to occur, possibly dependant on HR, amongst other things? If the changes in pressure occur too quickly, and the shift of blood can't 'keep up' then there is a risk of lung damage due to negative pressure. I can't be certain that this is true - it's only my common sense and logic. (Ben - this is your cue!) For normal dives, the gradient is very shallow, and so it's not likely to be a problem. But, consider a diver with an empty lung at the surface, who is dragged down to 10m (by a very heavy sled!) in about 2 seconds. I find it hard to imagine that the change in lung pressure could be accomodated by a shift of blood in that time... hence my theory. I could be wrong about this - it's just my gut feeling. So, there may well be a limit to the rate of change of pressure that the body can tolerate (beyond RV), but what that limit is, who knows...

So... it may be a good idea to either descend fairly slowly with empty lungs or dive on a passive exhalation instead. These are things I've always done, and such dives have never caused any problems for me.

I started off doing neutrals/empty lungs dives on the odd occasion at a time when i was beginning to dive reasonably deep (beyond the point of TLV=RV). In the beginning, they felt quite strenuous - new physical exertion on my body. Since then i've slowly built up the frequency and depth of these dives, and worked very hard on my flexibility in the chest/spine/abdomen/diaphragm area. These days, I consider doing such dives a normal part of my diving sessions. I do a few every time I dive without exception. My body has become totally adapted to them. Another good thing about them is that they keep your body fairly well accustomed to the pressures of deep diving, even when you can't dive deep, for whatever reason.

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Ok, Ok, Alun - time to dispell a few myths

Ribs cracking:

HIGHLY, HIGHLY, HIGHLY unlikely. Water pressure increase hydrostatically, and therefore external pressure will push the weakest resitance material inwards - which means your diaphragm. Only when your diaphragm inflexibility provides enough resistance that it overtakes something else, will the next tissue flex inwards.

Blood shift:

Alun is correct - it is dependent on blood flow rate:

1) Remember the heart is inside the rib cage.

2) Blood shift is therefore influenced by the rate of blood leaving the rib cage and the rate of blood flow into the rib cage.

3) Blood flow out of the rib cage needs to be maintained at a rate that keeps enough blood O2 supplying the brain. This is influenced by the proportion of the blood flow that goes to the brain and the pressure of that blood flow going through the brain.

4) If we (incorrectly) presume that all blood leaving the rib cage is going to the brain only - the difference between that and the blood returning to the rib cage is the blood shift (blood from the limbs, organs etc).

Next point;

5) you can't have more blood shift than residual volume - incompressivle air spaces in the lungs. If you did the blood would be bursting through membranes and filling up the bronchioles, bronchi & trachea. Not a nice thought.

So, what does this mean:

What we are trying to achieve is just enough blood coming out of the heart to keep happily concious, and as much coming in as possible. This will allow you to descend as rapidly as possible. In reality (as Alun suggested) the slower you go the more likely this is to happen. However, you can go too slowly - where you are wasting O2 and getting no exta blood shift benefit. This is where training comes in.

You need to train your lung tissue to fill with as much blood as possible (erection!) and as fast as possible. To do the latter your heart (control) needs to be trained to give as little blood out as is needed to maintain conciousness. Your body will give up blood to the rib cage passively.

All this counts most at depth (post 40m).

Empty lung diving is a great way to do this. The more you practice, the faster a descent at depth you will be able to face.

There are quite a few confounding factors in this but I won't go into that now.

There was a bit there that needs clarification:

residual volume - incomplressible air spaces in lungs


residual volume minus incompresible air spaces in lungs
I had wondered whether Pipin's "total ventilation" technique (I think that's what he calls it?) helps to prodcue a sufficient blood shift to keep up with the fast pressure changes during no-limits. Equalisation of pressure in the lungs would also make inner ear equalisaiton easier - depending on the equalisaiton method used. Pipin's ventilations are designed to increase the HR prior to and during immersion. I think the main reason behind this approach is to counter the extreme bradycardia experienced at depth, and so minimise the risk of deep water black out, by maintaining adequate blood flow to the brain.
Maybe having a raised HR is also beneficial for the blood shift for those sorts of fast descents where you can reach 100m within about 50secs!
To me it sounds strange that the idea of blood shift (pulmonary erection) can be trainable, because I think of it as a physical effect rather than physiological. This is despite experiencing the benefits of training myself! :confused:

Maybe you're right Alun, but I don't think that was his intent....he teaches that breathing to spearos as well. I think it was just to oxygenate and put off the breathing reflex through hyperventilation....fairly dangerous for spearos who are usually alone, or alone for all intents and purposes.
I do know this: when I am rec diving for over an hour, this fast HR at the surface happens automatically. My resting HR is around 48, and 55 during the day. After a number of dives in one session, my surface HR while ("whilst" for the Brits ;) ) inhaling is around 100 BPM or more, but instantly drops back to sub-60 BPM as soon as I hold the breath, then drops at depth even more.
Another thing that might be significant: when in heavy training, doing 5 statics a day plus diving, I can induce shunting at the surface at any time with just a big inhale and hold, even with no breath up. I can feel the blood "emptying" from my limbs, without any submersion at all. I have never seen this on paper from a lab, but I know it's what is happening. Anyone else concur?
Erik Y
Ah, right, I didn't know that he uses that method for all kinds of diving. I can see how it could be useful for extreme no-limits.

I don't think I've ever noticed that kind of effect just from surface statics. I suppose that would be 'peripheral vasoconstriction' which isn't the same as pulmonary erection. I would consider that a physiological effect, unlike pulmonary erection.

One thing I have felt quite distinctly is when doing fairly deep constant weight is a weird numbness in my arms. Maybe numbness isn't the right word, but whatever it is, it's most probably due to a shift of blood from the arms. Maybe the same thing happens in the legs, but I notice the arms more because they're not moving.

empty lung/ears

ok this is a weird question...

I tried a totally empty lung dive recently (in the tank at dolphin so it was pretty safe..)

compared to the half lung dives I often do this was REALLY uncomfortable, not so much on my ears as on my chest but my whole body felt kind of horrid...

I only went to about 7m but it was not nice

Then on my next normal dive my ear drum started leaking... went to the doc and he couldnt see an obvious rupture but there were definitely bubbles coming out of it and it is only just feeling normal a month later.

A team mate in Hawaii suggested that empty lung was a good way to exercise the ear drum... have I over exercised mine? is it possible to stretch it to the point that it would leak air without an obvious hole? should I never do empty stuff again?

am I asking too many questions?

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