Nitrogen Loading and Off-gassing - Clarification

[SkimFisher]

I think I have a pretty basic understanding of how loading works: greater depth>greater pressure>increased density/concentration of gasses under pressure>increased solubility of gasses under pressure>the body's inability to metabolize inert gasses (nitrogen, helium, whatever ISN'T oxygen)>excess inert gas dissolved into body tissue. Is that a fairly accurate, high-level grasp of what's going on?

In terms of off-gassing though, what's the actual mechanism? Every article I read just uses the term "off-gassing" without much explanation of what's actually going on. What I'm taking that to mean is that as you decompress/leave depth, the gasses drop out of solution due to the decrease in ambient pressure and are "off-gassed" via gas exchange in the lungs.

Before I ask the real question, I want to make sure that I've at least got a grasp on these two concepts. Am I more or less there?

 

[Diver0001]

I think I have a pretty basic understanding of how loading works: greater depth>greater pressure>increased density/concentration of gasses under pressure>increased solubility of gasses under pressure>the body's inability to metabolize inert gasses (nitrogen, helium, whatever ISN'T oxygen)>excess inert gas dissolved into body tissue. Is that a fairly accurate, high-level grasp of what's going on?

More or less.

In terms of off-gassing though, what's the actual mechanism?

Exactly the opposite of on-gassing. when you're on-gassing the "overpressure" is outside the tissues and pushing in. Then you're off-gassing the "overpressure" is inside the tissues and pushing out.

That's what people are talking about when they use fancy words like "gradient". It's just describing how much over pressure is built up inside the tissues.

So when yuo're off-gassing you build up an overpressure inside and as long as it's not too high then the dissolved gas passes back into your lungs (mixes with the gas in your lungs) and you breathe it out (ie, you're off-gassing). If the over pressure gets too high for this mechanism to keep pace while the gas is in it's dissolved state then bubbles will start to form inside the tissues.. etc etc. and then you knwo what happens after that.

R..

 

[SkimFisher]

aaaahhh, I like that explanation much better. That pretty much answers what my next question was going to be. Thanks.

 

[Blackwood]

The above is a reasonable concise description of gradient deco models. The following link is a description of bubble models (VPM in particular): http://www.hhssoftware.com/images/vpm_expl.pdf

 

[fnfalman]

More or less.



Exactly the opposite of on-gassing. when you're on-gassing the "overpressure" is outside the tissues and pushing in.

Ahem...greater ambient pressure, please. We must be scientific.

Then you're off-gassing the "overpressure" is inside the tissues and pushing out.

The "partial pressure" of the compressed gas is now greater than the ambient pressure.

And gradient is the rate of change of presures.

Let's say that you're opening a soda bottle. If you twist the cap off quickly, you get a massive "fizz" of the carbonation coming off the soda. If you twist the cap slowly, it doesn't have as much of a "fizz" but more of a "psst". Same thing happens with ascension. If you go up too quickly and the ambient pressure drops off rapidly, the partial pressure of the compressed gas will boil out and that's how we get the bend.

 

[rongoodman]

"Deco for Divers" by Mark Powell is a current, very readable source for information about decompression theory. Amazon has it.

 

[TSandM]

I would strongly second the recommendation for Mark Powell's book. If you have any itch of curiosity about this stuff, the book presents it in a readable, non-math-intensive way, with a lot of graphics.

 

[JanK]

Gradient is not the rate of change (with time) of pressures, but the difference of pressures at two points in space. We must be scientific. Rate is usually associated with time - dependent change, e.g., speed is the rate of change of position (so the phrase "rate of speed" is nonsensical in its common use).

But, the higher the rate of change of ambient pressure, i.e., the faster your rate of ascent (or the more you open the screwtop), the higher the gradients that will be established in your body (on in the soda liquid). And if these gradients exceed the limits at which the dissolved gas can be transported from tissues (or the soda liquid) to the lungs (or the liqid surface), the gas will form bubbles inside tissues.

To take the analogy further, if you open the bottle top for just a second, the bubbles in the liquid will be smaller and, once you close the top, they will be reabsorbed and the equilibrium established. In effect you're doing deep stops with the bottle. In the end the gas is released from the liquid, but without bubbles on the way.

 

[MXGratefulDiver]

We must be scientific.

Why?

Most of us aren't scientists ... we're not writing a thesis paper ... and scientific rigor is not necessary to explain or understand the fundamental concepts of ongassing/offgassing.

Makes for good cyberdiscussion, perhaps ... but completely unnecessary for diving applications.

I use an analogy of a series of sinks whose water levels vary with pressure to explain it to my students. It isn't very scientific, but it seems to be effective at getting them to understand why it's important to come up slow ... and for practical purposes, that is the objective ...

... Bob (Grateful Diver)

 

[JanK]

I agree - partially.

On this board and elsewhere often reads criticisms, for instance, when a reporter writes about oxygen instead of air in the tanks - and I am sure that there are many other examples where the loose usage of diving terms is frowned upon. Because they are not simply terms, they stand for concepts that one need to absorb to understand diving and to be a better diver.

In this thread the discussion was already becoming a bit technical and a term that has a well defined meaning was explained wrongly. Similarly to the above example, I believe that the proper usage of scientific terms, even when used in non-scientific discussions, can improve understanding of science (which, in itself, is not bad) and thus improve safety in diving.