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SCUBA-L  August 1999, Week 4

SCUBA-L August 1999, Week 4

Subject:

Re: Mixed drinks

From:

"Carl G. Heinzl" <[log in to unmask]>

Reply-To:

Scuba diving discussion list <[log in to unmask]>

Date:

Thu, 26 Aug 1999 13:28:45 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (214 lines)

Sotiris,
 
This email is not self consistent.
 
> B) SATURATION is the state where the MAXIMUM quantity of a gas is dissolved
> in a liquid in a given pressure and temperature. Saturation occur on the
> surface too. Any liquid (including all living creatures) on the surface of
> the earth is saturated with Nitrogen, Oxygen, Carbon Dioxide and a variety
> of other gasses, because we live in a constant pressure (1 ATM). This means
> that the maximum possible quantity of the Air gasses are dissolved in our
> bodies for this pressure and temperature (without having a dive).
 
So, you are stating that we are "saturated" at sea level.  I agree with
this statement.  When you are in "equilibrium" - i.e. no net rate of
ongassing
or offgassing -  with your environment you are "saturated".
 
> C) PARTIAL SATURATION is the state where SOME quantity of a gas is dissolved
> in a liquid. This mean that this liquid can dissolve some more of the same
> gas.
> A liquid (Or if you prefer, the diver) can switch from the Saturation State
> to the Partial Saturation State when it's environmental pressure increase.
>  I.e. A diver is saturated with Nitrogen on the surface -He has the maximum
> dissolved quantity of Nitrogen in his body-. When the diver start descending
> the environmental pressure increases, so the diver enters the Partial
> Saturation State -Some more Nitrogen start to dissolve in his body-.
 
I also agree with this statement.  You could also reach this state at
the
surface for example, by breathing pure Oxygen for some period of time.
This would reduce the level of nitrogen in your body.  Then, when you
stopped breathing the O2 you would ongas nitrogen until you achieved
equilbrium again.
 
> D) SUPER SATURATION is the state where MORE THAN THE MAXIMUM quantity of a
> gas is dissolved in a liquid. This occur when we decrease the pressure
> surrounding a Saturated liquid. For example think of a bottle of Coca-Cola.
> When the bottle is closed it's content is Saturated with Carbon Dioxide in
> let's say a pressure of 1,5 ATM. Coca-Cola and Carbon Dioxide molecules are
> floating together without any visible gas bubbles, because of the increased
> environmental pressure inside the bottle. When we open the bottle, the inner
> environmental pressure gets equal to the outer environmental pressure, which
> is 1 ATM. (Dropped down 0,5 ATM). This causes the Coca-Cola solution to
> enter the Super Saturation state which means that now the Coca-Cola solution
> contains MORE Carbon Dioxide than it can dissolve. This causes the excessive
> Carbon Dioxide to be expelled from the solution in the form of bubbles.
> Super Saturation conditions are HIGHLY UNSTABLE and tends to become
> Saturated by expelling the excessive gasses, with some times violent
> reactions.
 
> ( In a normal recreational dive, the divers tissue will NEVER -and I mean
> NEVER- reach a real Super Saturation State. Although in some cases
> Saturation state or Partial Saturation State IS SUFFICIENT TO CREATE A
> DECOMPRESSION SICKNESS).
 
This statment is false.  I dive to "x" feet for any amount of time.
My tissues have now ONGASSED whatever inert gas I have been breathing.
As I return to the surface and the ambient pressure is reduced my
tissues most certainly do reach a state of super saturation.  The key
is the amount of super saturation your body can withstand before bubbles
begin to form and cause decompression sickness.  Note that I am not
going to go into all the intricacies of microbubbles/etc which are
present on MANY recreational dives, I'm just pointing out the FACT
that divers DO INDEED REACH A SUPER SATURATION STATE.
 
Let me put it another way.  Super saturation is a state in which you
will OFFGAS.  Assuming a "square" dive profile, during the bottom
portion of the dive you are ONGASSING.  At SOME point
during the ascent the saturation level in your body will EXCEED the
saturation level for that depth and you will begin to OFFGAS.
 
If you were doing a "SATURATION" dive (i.e. you spent MANY MANY HOURS
at that depth), this super saturation state would be achieve virtually
immediately on the ascent.  Since recreational divers don't do
saturation
dive, this crossover point will occur closer to the surface, but it WILL
occur nevertheless.
 
If you run a bunch of simulations on your computer (for example I have
a simulator for the Solution) you can see the tissue loading graphs.
IN fact, one of the older dive computers around had a tissue loading
graph on it!  You can watch as the various "tissue models" reach
saturation/etc.  Then, on ascent you can see the loading bars go
over 100%.
 
Once you're back at the surface you CONTINUE to OFFGAS for some period
of time until you are again at equilibrium with the gas content
at the surface.
 
Again, the key is the AMOUNT of super saturation that you can withstand
without any ill effect.  I don't remember exactly what the current
limit is, however, I believe it used to be considered that you could
withstand a supersaturation of 200% (i.e. you could be saturated at
2 ATA) and withstand an immediate ascent to the surface.  Some people
were shown to have decompression sickness in this scenario and these
limits have since been changed.
 
**** Side track for a minute ****
I'll let this bring up another pet peeve of mine - that of PFO and
"undeserved hits".  I have maintained over the years that dive tables
are overly conservative because they were developed using an
average cross section of the population (yes, this is true, they
were NOT developed using 18 year old fit navy divers so don't even
bother trying that *wrong* argument).  This cross section includes
20-33% (actually the MAYO clinic puts it at 25-33%) of people with
some form of PFO.  This only makes logic sense.
 
DAN recently responded to a request I sent them with the statement
that roughly half of the people who took unexpected hits (where an
unexpected hit is a dive within the tables) and were tested were
positive for PFO.
 
If people with a PFO had been removed from the original testing that
created the dive tables I maintain that we would have a very
different looking table.  I maintina that this is why many people
are often able to "break the rules" and get away with it with no
ill effects.  I have been tested using a high resolution cardiac
echo and been given a clean bill of health.  I suggest that all
divers get tested and I maintain that this test should be required,
just as it's often required to get a doctors approval.
**** end of side track ****
 
>  ** And now the example of Henry's law. **
> Imagine a glass of water, connected with two sensors that measures the
> amount of Nitrogen and Helium in the water. The environmental pressure of
> the water is 1 ATM because it is on the earth's surface. According to the
> above statements, the two sensors indicates SOME dissolved Nitrogen (we
> don't care for the exact amount), and NONE dissolved Helium. Now, we place
> the glass of water in a pressure chamber and pressurize it with PURE Helium
> at 3 ATM. In this condition gas exchange start to occur in the water FROM
> ITS SURFACE, because its surface is the only place where the water comes in
> DIRECTLY CONTACT with the gas. Over the time, the Helium meter will indicate
> an INCREASE of the Helium dissolved in the water, while the same time the
> Nitrogen meter will indicate a DECREASE of the dissolved Nitrogen although
> the environmental pressure is 3 ATM.
> This happens because the PARTIAL PRESSURE of the Nitrogen (outside of the
> water) has dropped from 0,8 ATM (the surface pressure of Nitrogen) to 0 ATM.
> In simple words, our water has MORE Nitrogen molecules than it's surrounding
> environment. This causing the Nitrogen molecules to move from the solution
> to the gas in order to equalize the molecules amount in both the water and
> it's surrounding space.
> In the same time, we have a large quantity of Helium molecules around the
> glass of water, but no Helium molecules inside the water. This causes the
> Helium molecules to dissolve in the water until the Helium molecules
> quantity is the same around the water and into it.
>
>  After a few hours, we stop pressurize the glass of water with Helium and we
> start pressurize it with PURE Nitrogen at 3 ATM.
>  Now the opposite process start. The environment around the water is full of
> Nitrogen molecules but NONE Helium molecules. So Helium molecules start
> escaping the solution to equalize the amount of molecules around the water
> and inside it, while in the same time Nitrogen molecules enters the water in
> order again to equalize the Nitrogen molecules amount around the water and
> inside it.
>
> The releasing and dissolving of those two gasses (or any gas) is irrelevant
> to each other.
 
That RATE at which they release/dissolve is independant, based only on
the
partial pressure gradients, however, to say it is irrelevant is not a
true
statement.  In this simple example you have held the AMBIENT (outside)
pressure constant and only varied the partial pressures of the gasses.
I agree that in this case the on/off gassing of the two gasses are
independant.
 
> The above situation happens in the dive. the diver is the glass, the water
> is its body and the surface is his lungs. The lungs is the only place that a
> gas, with a combined partial pressure, come in directly contact with the
> diver and therefore it is the only place that a significant gas exchange
> happens.
 
Decompression sickness occurs when the SUPER SATURATION that a diver
DOES INDEED have on a dive causes bubbles in their blood.  This is
a very individual situation since injury sites and/or other site that
can allow microbubble formation can facilitate DCS in some divers.
 
Let me briefly go back to the "two dive" scenario in which there
are TWO inert gasses - Gas "X" and Gas "Y".  If I breathe a combination
of X/O2 on dive 1 and Y/O2 on dive 2 then what SOME people are
saying here is that gas "X" does not have to be considered at all
during dive 2.  This is NOT TRUE.
 
If Gas X is still present in the tissues during dive 2 and is super
saturated at some point during dive 2 when the diver is ALSO
offgassing Gas Y then both X and Y will be offgassing at the same
time.
 
Once a small bubble forms BOTH GAS X AND Y will flow into it by
diffusion
until the partial pressure inside that bubble is consistent with the
ambient relative partial pressures.  This mean that X could indeed
contribute to DCS on dive 2.
 
Indeed, if a diver does get bent and starts to breathe pure O2 there
is sometimes a transient worsening of the symptoms before they
being to resolve.  This is due to the increased partial pressure
of the O2 which is being ONGASSED for a brief period while the
N2 is being offgassed.  The observation of the worsening of
symptoms has been taken to mean that the O2 is diffusing INTO
the bubbles at a slightly faster rate than the N2 is being removed.
Once enough of the N2 is removed the bubble will collapse.
 
> I'm saying that the diver when he finish his first dive has an amount of
> Helium in his tissues. This amount of Helium decreases over the surface
> time. During the second dive the amount of Helium in his tissues continues
> to decrease over the time and it is completely irrelevant with the amount of
> Nitrogen dissolved into the tissues during this dive.
 
Not necessarily true as shown above.
 
Carl

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