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SCUBA-L  August 1999, Week 4

SCUBA-L August 1999, Week 4

Subject:

Re: Mixed drinks

From:

Sotiris Tzanlis <[log in to unmask]>

Reply-To:

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

Date:

Thu, 26 Aug 1999 18:03:21 +0300

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (160 lines)

> > I like to talk with FACTS, so here it is:
>
> The same facts that decompression _theory_ is based on correct?
 
This is CORRECT.
 
> Uh.... you can be saturated with O2 also and the excess that is not
> metabolized is then an inert gas under the increased partial
> pressures....
 
 
This is also correct. I just choose NOT to mention it in my example, in
order to keep the example simpler. Besides, the quantity of the inert Oxygen
in the tissues is REALLY SMALL. (Oxygen was only 10% in the first mixture
while the rest of the two remaining and REALLY inert gases was 90%).
 
 
> But what about the He that has not yet escaped the tissues
> and gone back
> into solution from the first dive assuming that some of the
> tissues became
> supersaturated during the first dive? If the diver is not
> completely clear
> of this remaing He then what remains is at an increased
> partial pressure
> during the second dive.
>
> I understand your examples with the gas laws sited but I
> still don't get the
> He that remains after the SI from the first dive.
 
 
I must admit that Henry's law is a bit confusing when it comes down to a
real diving environment. Let me give an simplified example, but before the
example, we must clarify some things.
 
A) THERE IS NOT SUCH THINK AS "PARTIAL PRESSURE OF A DISSOLVED GAS".
Once a gas is dissolved in a liquid, it DOES NOT force any pressure to the
liquid.
All we have is MOLECULES of both liquid and GAS floating together.
The correct term in physics for this case is "MOL", which gives the amount
of the dissolved gas molecules inside the liquid using a complicated
mathematical equation.
Therefore, Dalton's law on Partial Pressure DOES NOT APPLY in dissolved
gasses.
 This is a common mistake the divers do.
 
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).
 
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-.
 
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).
 
 
 ** 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.
 
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.
 
 
> Also, can you explain, in layman's terms M values and how
> they play a part
> in these examples?
 
Hmmm. I'm afraid I'm not familiar with the "Layman's term M Values".
Probably I know this by a different name. Remember my natural language is
Greeks.
Can you provide some input on this ??
Maybe a web page to look on.
 
> Mike
 
> So you are saying that after dive one above, when the diver
> exits the water
> he is He free from that dive?
 
I don't really understand your sentence here !!!
 
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.
 
 
>
> Mike
 
 
Sotiris Tzanlis

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