What happens when

[miked]

Hi all,
This question occurred to me after I answered a bunch of questions from a few non-diving friends. Their questions and my replies got me thinking. I'm not sure this is in the proper forum, so, regulators, please move it as needed.

The question:
As a diver ascends, he is "offgassing/tissue unloading(?)" the nitrogen picked up at greater depths, but he is still "ongassing/tissue loading(?)" due to still being underwater .
the Question is : how do these 2 processes function together.

An example to clarify-and I realize that this may be a slightly different question:
The situation (and yes, each diver has proper equip., buddy, etc.)
Suppose Diver "A" goes to 99fsw (at 4 atm pressure) for 10 min,
then ascends slowly and safely to 33 fsw(2 atm).
There, he encounters diver "B",who is at his max depth for his dive. They stay there for 15 mins, then ascend to the surface-with proper stops,etc.

The question: What is happening differently in diver A, than diver B, while they are at that 33 fsw depth? Does each of the simultaneous offgassing/ongassing (in diver A) offset, intefere, slow down the other, or are the processes somehow (?)independent??

Thanks for the help,
Mike

 

[cpalerm]

As the diver ascends you are correct in that he is offgassing. He is not on-gassing, as it is the opposite process (both can not happen at the same time).

When your diver A went to 4atm, the partial pressue of nitrogen increases, so more nitrogen gets dissolved in the tissues. The gradient goes from high concentration to low concentration (in this case, high concentration of nitrogen in the air, low concentration in the tissues, therefore nitrogen goes into the tissues up to the point where the partial pressures of nitrogen in both balance out).
When he goes up to 2atm, now the gradient is going in the opposite direction; the concentration of nitrogen in the tissues is now higher than that in the air, so it goes from the tissue out to the air, again until both are balanced).

Diver B having just reached 2atm would still be at the point of having a higher concentration of nitrogen in the air than in the tissues, and thus would be on-gassing.

Hope this is clear enought... as it is not as short as I originaly intended...

 

[sheck33]

cpalerm once bubbled...
As the diver ascends you are correct in that he is offgassing. He is not on-gassing, as it is the opposite process (both can not happen at the same time).

When your diver A went to 4atm, the partial pressue of nitrogen increases, so more nitrogen gets dissolved in the tissues. The gradient goes from high concentration to low concentration (in this case, high concentration of nitrogen in the air, low concentration in the tissues, therefore nitrogen goes into the tissues up to the point where the partial pressures of nitrogen in both balance out).
When he goes up to 2atm, now the gradient is going in the opposite direction; the concentration of nitrogen in the tissues is now higher than that in the air, so it goes from the tissue out to the air, again until both are balanced).

Both processes DO happen at the same thing. Keep in mind here that you cant refer to the diver as if it is one tissue. The diver consists of a multitude of tissues each with there different halftimes. On the initial descent all tissues will be on-gassing until the very fast ones are saturated. When after a while, which i assume is shorter than needed to saturate ALL tissues, he starts ascending some tissues will be off-gassing while some of the very slow tissues will STILL have a tissue pressure lower than the surrounding pressure and will still be on-gassing. This is the reason why for example deepstops cant be arbitrarily long or they wont be useful anymore. If it only were so simpel that all tissues start off-gassing at a particular rate when we start our ascent.

 

[cpalerm]

I guess I over simplified by considering it a single tissue...

 

[The Iceni]

Hi Mike,

To add to Seck33's excellent answer most decompresion models assume ongassing and offgassing to be exponential. The higher the pressure gradient the faster the rate of gas transfer (like the cooling of a liquid). Half times are used to describe the rate of gas tranfer;

We are all saturated with nitrogen at 1 bar on the surface.

Below are the nitrogen half times for Professor Buhlmann's Algorithm: ZHL16

Number of Compartments: 16
N2 half-times(mins):
4.0, 8.0, 12.5, 18.5, 27.0, 38.3, 54.3, 77.0, 109.0, 146.0, 187.0, 239.0, 305.0, 390.0, 498.0, 635.0.

These are the times it takes to reach the halfway point. For all practical purposes it takes six half times to reach full saturation. So a true saturation dive must be longer than 635 x 6 mins = 63 hours or two and a half days!

Even the fastest compartment for diver "A" has not reached a state of saturation at the end of 10 mins bottom time but by ascending and staying at 33 feet for 15 more minutes (25 mins in total at that pressure or higher) he will saturate the fastest tissue compartment for that depth and all other compartments will either ongas or ongas (depending on their status following the sojourn at higher pressure) towards saturation at 2 bar.

All compartments will require 6 half times to completely offgass. (That is not to say that a deco stop will be essential. It depends on the M values etc.)

As for Diver "B" all compartments will be ongassing towards saturation at 2 bar during the whole of his dive but his total nitrogen load will be less than that of diver "A".

I hope this is not too complex, helps and does not confuse.

 

[jeffsterinsf]

(And no, nothing implied about your physique)

As long as you remember that a model is just a convenient way to create numbers that fit experimental data, not necessarily what is happening within the body, you might want to "play" with a dive planner and see what the model shows for the compartment loading.

You can download Suunto's Dive Manager from their website
http://www.suunto.com/
and simulate a few dives to "see" the model in action. No Suunto computer needed...

 

[miked]

Cpalerm, sheck33, Dr. thomas,and jeffsterinsf,

Thanks to all for the fast, thorough, and informative replies.
I thought that was the situation in general, but wasn't sure of the specifics.
I'll be doing some simulations when time permits.
Thanks again,
Mike

 

[Dr Deco]

Dear Readers:

The ATTACHMENT illustrates how some compartments can take on dissolved nitrogen while others are eliminating it. The graph is a hypothetical dive from surface to about 330 fsw on air. This is hypothetical but allows large differences to appear in the compartments. Plotted are short and long half times. In this example, the stop (here about 63 feet) eliminates gas from the short compartments, but gas is still be taken on in those that are long.

It is clear that stops would need to be very long to make much of a difference. If one were ascending from a deep enough depth, the difference would become apparent. This was a historical transition point in decompression procedures from the long and slow liner ascents of Paul Bert (about 1885) to the quicker ascents upward that were pioneered by JS Haldane (about 1908).

Dr Deco :doctor:

 

[The Iceni]

Dr Deco once bubbled...
The ATTACHMENT illustrates

A picture paints a thousand words!