Re: Residual Volume of lungs and OOA

Ken wrote

Good depth to discuss. Most divers find out they are OOA when they have
breathed out, not when they have a lung full of gas. Without getting
into the question of tidal flow and minimum percentage content of the
lungs, assume for a second that your lungs are only 1/4 full when you
discover you are out of gas at 30 meters. As you note, you are at a
absolute pressure of 4 atmospheres. At the surface, the gas in your
lungs will have expanded all the way up to, you got it, one lung full of
gas.

Like so many things, it depends. Sure you tend to find out you're out of
air when you try to breathe in and can't - so your lungs are at the end of
a normal exhalation. At which point, for an adult with a total lung
capacity you'll find your actual lung volume is around 3L. This is made up
of what you can still forcibly breathe out - your exhalatory reserve
volume - plus a certain volume which you CANNOT breathe out, your residual
volume (both these together make up your functional residual capacity).

You'll note that the preceeding discussion quite specifically says "without
getting into the question of tidal flow and minimum percentage content of
the lungs." It also "assumes" the lungs are 1/4 full.

If you take 3L and multiply it four-fold (4ATA @ 30m to 1ATA at 0m) you
end up with a 12l vol - twice your total lung volume. If you don't exhale
AT SOME POINT on your way up, you WILL get a pneumothorax / gas embolus.

This is not suggesting it's a good idea to hold your breath while
ascending. It is only to point out that the degree of risk is not
normally as severe as taught. You should never ascend very far with the
airways closed, but breathing out is not the only way to keep them open.
They're open when you are inhaling too. As you ascend from our proposed
30 meters, the amount of gas available from the tank benefits from the
lowering of pressure. From the 30 meter dive to the surface, you can
actually get extra gas equal to 3 times the internal volume of your
tanks. One ata stays in the tank. The other two can be used. So figure
out what the internal volume of your tanks is, multiply it by 3 and
divide it by whatever you figure the tidal volume of your lungs is.

Not really.

Yes really. My statements are not assumptions and are not guesses. They
are based on actual experience. As it happens, my OOA ascent was from 128
feet, a bit short of 40 meters. I was wearing a single 72 cubic foot tank
at the time.

Your equipment is already malfunctioning (assuming it's not your lack of
attention that has put you in this predicament).

Out of air means out of air. It does not mean a total malfunction of all
available regulators. I'm reasonably certain that, if you were to count the
number of OOA situations that resulted from lack of attention, failure to
monitor gas, or some combination of circumstances that caused the diver to
overstay his supply, you'd find that they exceed, by far, the number or
times a complete regulator failure was to blame.

Note that a well functioning first stage regulates to around 10ATA above
ambient pressure, so at 30m you will need around
14ATA in your cylinder to get a breath.

Nope. A well functioning first stage stops flowing at about 10 ATA. It
starts flowing at a much lower pressure.

Sure, as you come to the surface your equipment will function with lower
cylinder pressures, as 10+ambient becomes a smaller
number.

Right, except for the 10 ATA part. That's exactly what I said. You get to
use more of your gas as you ascend. Trust me, if you ever have to use it,
as I once did, you'll be very glad we had this conversation.

However as you ascend it is the case that there is sufficient oxygen in
the mass of gas in your lungs so that you need never become hypoxic
during such an ascent, so you need not fear for that.

That is not something you can depend on. There are too many variables.
Shallow water blackout is caused by the combination of O2 used by the
body tissues and reduced pressure that combine to bring the PPO2 to less
than .16 ATA, the level normally considered necessary to maintain
consciousness.

No, this too depends on your rate of oxygen consumption.

Yes if you mean the rate at which your body consumes oxygen in the tissues,
which is what I meant when I said "O2 used by the body tissues." I'm not
sure what your "no" refers to.

Of course an option no-one has discussed is using your alternate air
source - and I don't mean your second 2nd stage connected
to the same empty cylinder!

Probably because having a redundant gas supply means it's not an OOA
situation. Note that's not a criticism, just an observation. It is clearly
best not to run out of gas included in your dive plan and second best to
have a reduncant supply just in case. If we're going to talk options, they
let's include all that are likely:
1. I sometimes wear twin tanks with an isolation manifold. A single
regulator failure does not mean I'm out of gas or that I can't access all
gas through a regulator working just fine. Even if I were to fail to shut
down a free flowing regulator before all available gas has escaped, I still
have a working regulator to let me take advantage of expansion to get still
more gas from the tank.
2. I don't normally carry a pony, but I do sometimes carry a deco gas
suitable for use from a bit over 20 meters to the surface (50% O2). In a
real emergency, I would use that supply briefly at still greater depths. Ox
tox is a function of both PPO2 and time. I would certainly use it above 20
meters.
3. My gas plan calls for ending the dive while I still have enough gas to
get myself and my buddy safely to the surface. I may have run out once.
I'm not likely to do so again.

If you do this (Breath from your BCD), you can breathe in and out
continuously as you ascend - but not enough gas there to do a safety stop,
just come up. The composition of the gas there is the same as whatever
your cylinder held. You breathe in and out from the hose and yes, you do
accumulate some CO2 BUT you'll have probably made the surface BEFORE this
becomes a real issue.

This method presumes you have significant gas in your BCD. The closer to
the end of the dive you are, the less gas you're likely to have there. If
you're out of gas, you're as buoyant as you're ever going to be and will
have less gas in your BCD than at any other point in your dive. It's better
than nothing, but not by much.

Lee


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