Effect of slow compartments size in relation to NDL and DECO

[boulderjohn]

Consider that the theory behind most dive computers is over 50 years old, and many of the theories have problems with band-aid fixes on them.

Just some information on this...

Different computers use different models, or Algorithms, pretty much all of which were developed in the last 30 years or so, not 50. These include the DSAT, RGBM, VPM, and Buhlmann models. All of these have been adjusted to different degrees over the years as a result of further research.

The original version of Ratio Deco was designed to approximate a Buhlmann computer program that was tweaked in accordance with the then popular theory of deep stops. The UTD version is different from that. It used a much deeper version of the deep stops theory than any of the other deep stops programs, like VPM and RGBM.

Research over the last decade has made the deep stops theory look less appealing, and divers are moving away from it rapidly. If you go to the technical diving forum in ScubaBoard and look for the deep stops threads, you can follow the debate. Right now the most popular theory among technical divers is the older Buhlmann algorithm tweaked with what are called gradient factors that are much farther away from the deep stops model that was popular just a couple of years ago.

The Ratio Deco system I was taught when i was a UTD student was very, very much a deep stop program. It was recently scientifically tested in comparison with a Buhlmann algorithm also tweaked to be a deep stop program, although not as deep as Ratio Deco. Ratio Deco came out very poorly in that comparison. I have been told that as a result, UTD has made a newer version that is not quite as deep, but still much deeper than the algorithms currently favored by most technical divers.

 

[TrimixToo]

This is one of the best explanations I have ever seen.

You ought to teach this stuff!

Oh. Wait. You do that already. Never mind. I'll go back to sleep now. ;-)

You may be thinking of body tissues as if they are bottles with a single opening through which nitrogen enters and leaves. Nitrogen actually enters and leaves constantly throughout the tissue, so the size of the tissue is not an issue. What matters for tissue halftimes is not the size of an opening or the size of the tissue but rather the degree of its liquidity and its perfusion--degree to which it is in contact with blood flow. Fat is a slow tissue primarily because it is not well perfused.

A second problem with that is the idea that the tissues fill up. When they are saturated, they are not full of gas the way a bottle is filled with water.

Molecules of gas are really, really small, and they are going pretty much wherever they please. They go into the lungs and pass through walls into the blood. The blood carries them around the body very quickly, but all the time they are flowing they are escaping into the surrounding tissues. At the same time, the gas molecules from the surrounding tissues are going into the blood and being carried back to the lungs. If we keep breathing at the same atmospheric pressure for a long time, everything evens out. We have as much nitrogen going into our tissues as out of the tissues. We are at equilibrium.

Now let's go to 99 feet. Now we have about 4 times as many nitrogen molecules in the lungs as in the tissues. The blood goes scurrying around like before, but now there are 4 times as many nitrogen molecules going into the tissues as coming out. The tissues begin to gain nitrogen. After they have gained nitrogen for a while, things begin to balance out. Eventually there are only twice as many molecules going in as coming out. The time it took for that to happen is that tissue's halftime. As the tissue gets more and more nitrogen, the process keeps slowing down until, once again, we have as many molecules coming out as going in. The tissue is saturated.

But it is not full in the sense that a bottle is full. If you now descend to 150 feet, you once again have more coming in than going out. If you instead ascend to 33 feet. That saturated tissue now has more molecules coming out as going in, and we are off gassing. As the blood flows through the tissues, it carries all that extra nitrogen out to the lungs.

 

[Duke Dive Medicine]

Take the slow compartment F representing Fat, with respect to ambient pressure the compartment has its given saturation half time and Decompression M-value.

Henry's law is assuming given temperature size and VOLUME, but divers have different volumes of F.

A compartment, F+, larger than F, can hold more Nitrogen volume than F.

Longer Deco stop.
Under identical dive profile and conditions, at same given depth during offgassing F+ will take longer to reach equilibrium because it's holding more gas dissolved in it, right?

Slower compartment.
F+ slower than F to reach saturation because it has more volume?

Lower M-value gradient value.
Saturated F+ will start bubbling at lower overpressure gradient than F?

F+ like a slower compartment than F.
F and F+ are different compartments with different NDL and Deco M-value?

Do we need to factor in F+ compartments to take in account obesity?

Hi Narke

First of all, nobody really knows how all this works. The best we (the relative we) can do is model it and hope that the physiologists and mathematicians listen to one another while doing so.

John already explained in detail why a body tissue is not the same as a container full of gas, and it's worth emphasizing again that theoretical tissue compartments are not equivalent to actual body tissue. I personally think that the word "compartment" is deceptive because tissues are not necessarily compartmentalized. The M values (also theoretical) of your F and F+ tissues would likely be similar unless the owner of F+ has some kind of vasculopathy that affects the circulation to his tissues.

Best regards,
DDM

 

[rossh]

Take the slow compartment F representing Fat, with respect to ambient pressure the compartment has its given saturation half time and Decompression M-value.

Henry's law is assuming given temperature size and VOLUME, but divers have different volumes of F.

A compartment, F+, larger than F, can hold more Nitrogen volume than F.

Longer Deco stop.
Under identical dive profile and conditions, at same given depth during offgassing F+ will take longer to reach equilibrium because it's holding more gas dissolved in it, right?

Slower compartment.
F+ slower than F to reach saturation because it has more volume?

Lower M-value gradient value.
Saturated F+ will start bubbling at lower overpressure gradient than F?

F+ like a slower compartment than F.
F and F+ are different compartments with different NDL and Deco M-value?

Do we need to factor in F+ compartments to take in account obesity?

The formula used in gas kinetic tracking of models, is designed to factor out all volume equations, and instead use "time to full" as the measure of state. This simplifies matters as we no longer need to know about each individual tissue volume (which would be impossible to measure anyway). These formula have been in use for most diving history, and are still considered reliable today. The beauty of this design, is that the array of gas tracking cells will overlap and correspond to all the tissues within the body, but it need not know the specifics of each tissue. The body is (mostly) considered to on/off gas uniformly without restrictions, the time based measure is sufficient to track gas load.

Then the model has to decide, what is the allowable over pressure condition value / time combination, which forms the ceiling limits. This is where the difference in models exist, and the arguments start about what is effective, or required, essential or not. Everyone has an opinion where to place the stops, but deco time is required in some form or another.

.

 

[narke]

Even if their respective computers took into account their individual body compositions, and calculated different

That is an interesting scenario I wish to give it some thought. Our computers are doing just that, rounding up on us.

 

[narke]

That is an interesting scenario I wish to give it some thought. Our computers are doing just that, rounding up on us.

... hmmm...rounding down?

 

[narke]

The formula used in gas kinetic tracking of models, is designed to factor out all volume equations, and instead use "time to full" as the measure of state.

Very Well explained. I didn't do better.

All: Seen we have more overall F+ in total volume of and individuals diving, do we need to do anything about the model or just tweek on our GF Hi-s and Low-s (when we can)? Or nothing, the approximations are still good enough?

 

[leadduck]

Obese divers have a higher absolute amount of nitrogen stored in fat tissue. The partial pressures may be the same in the model compared to a non-obese diver. But for bubbles blocking lung capillaries and passing through a PFO, the absolute amount of nitrogen matters. Obese divers are known to have a higher risk of DCS.

See Obesity and DCS

Related to the models, I guess obese divers better use smaller GF.

 

[DavidFL]

"all models are wrong, some are useful". Quotation from George Box.

 

[stuartv]

Take the slow compartment F representing Fat, with respect to ambient pressure the compartment has its given saturation half time and Decompression M-value.

Henry's law is assuming given temperature size and VOLUME, but divers have different volumes of F.

A compartment, F+, larger than F, can hold more Nitrogen volume than F.

Longer Deco stop.
Under identical dive profile and conditions, at same given depth during offgassing F+ will take longer to reach equilibrium because it's holding more gas dissolved in it, right?

Slower compartment.
F+ slower than F to reach saturation because it has more volume?

Lower M-value gradient value.
Saturated F+ will start bubbling at lower overpressure gradient than F?

F+ like a slower compartment than F.
F and F+ are different compartments with different NDL and Deco M-value?

Do we need to factor in F+ compartments to take in account obesity?

It seems to me that the explanations given thus far are making it too complicated.

Since you are implicitly talking about a dissolved gas model, the difference between F and F+ is not that F+ will start to bubble sooner. The difference is that F+ will simply have more (or bigger) bubbles when it does start to bubble (at the same time as F).

Since you have specified that F and F+ are both the same "theoretical compartment", just of different physical sizes, then BY DEFINITION, their on- and off-gassing rates are the same. Conversely, if F+ off-gasses more slowly than F, then, by definition, it is not the same "theoretical compartment" as F - because they don't have the same half-time.

I think it is a mistake to try and think of the theoretical compartments as physical tissues. That is what leads to the wrong conclusions that you reached. A 720 minute compartment could be a small deposit of fat, or it could be a big lump of muscle tissue (or anything else). The compartment is defined by the half-time and could be any kind of tissue (that manifests the defined half-time).