Effect of slow compartments size in relation to NDL and DECO

[narke]

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?

 

[tbone1004]

here are your assumptions

the tissues have to be fully saturated for the volume of a tissue to matter because as slow as it offgases, it is also that slow to ongas. Based on this, outside of saturation diving, it wouldn't matter. Gas moves across a gradient created by the pressure of gases across cell walls. The volume of cells does not change the rate at which gas moves in and out of those cells.
If your assumption is correct, than individual body composition as well as total body mass would have to be factored in. Your assumption is that a single compartment is "larger" than the others.
So who does more decompression?
I'm 275lbs, mostly muscle but could lose a few pounds and 6'3"
My female dive buddy who is probably ~140lbs but has a higher BMI than I do due to various womanly features and she's 5'9"
My male dive buddy who is obese but only weighs 200lbs because he's 5'4"

I weigh more, so will have more total fat on me than my female dive buddy, but she has more fat as a function of her total body mass
I also weigh more than my other buddy, but we probably have roughly the same total mass of fat, but his BMI is significantly higher.

Now, there are other physiologic factors with obese people and circulation that may impact decompression efficiency, but to my knowledge it isn't the different ratios between the tissue compartments as a function of mass

@Duke Dive Medicine can hopefully comment further

 

[narke]

First of all thanks but....hmmm...your buddy Paul has a twin brother, Peter, they have identical equipment and make identical dives and are identical in every way except Paul is 200lbs while Peter 150lbs due only to fat.

They do a saturation dive.

Paul's and Peter's dive computers are giving them same Deco stop depth and time.

Shouln't Paul computer tell him to ascend to a first stop at deeper depth than Pete's?

 

[victorzamora]

First of all thanks but....hmmm...your buddy Paul has a twin brother, Peter, they have identical equipment and make identical dives and are identical in every way except Paul is 200lbs while Peter 150lbs due only to fat.

They do a saturation dive.

Paul's and Peter's dive computers are giving them same Deco stop depth and time.

Shouln't Paul computer tell him to ascend to a first stop at deeper depth than Pete's?

I don't think so. I think you're wrong for two reasons:

1. Compartments don't actually/directly correlate to body tissues....so it's not like slow-compartment "F" or compartment 14 is "bodyfat."
2. The biggest issue at play is, I believe, that all of these tissues function on half-lives. Total dissolved content doesn't matter when half-lives are the mathematical representation. You lose half your Nitrogen in that tissue every half-life, regardless of the total content.

 

[narke]

again thanks...hmmm....in the half life model you will lose half of the original amount. But half of Paul's original amount is more than that of Peter's, no?

Paul can saturate more, no?

 

[victorzamora]

again thanks...hmmm....in the half life model you will lose half of the original amount. But half of Paul's original amount is more than that of Peter's, no?

Paul can saturate more, no?

So: Stating directly that body tissues directly correspond to compartments in the algorithms is wrong. You absolutely cannot make that logical jump. Also, half-life IS how all deco algorithms function. All of the models I'm aware of function on a half-life principle: RGBM, Buhlmann, and VPM are the main ones.

If we ignore the other stuff: Paul's body would be able to absorb more inert gas than Peter's body. That is true.

However, I think that saturation is low enough that it's not a dramatic difference. I would bet that other factors that are harder to control, account for, and understand have a much bigger part to play (hydration, sleep, exhaustion, work, seemingly-minor internal differences, etc).

On the other hand, I don't think you'd get much disagreement in saying that unhealthy people are generally more prone to DCS than healthy people. I think the bigger factor isn't that it takes longer for the nitrogen to escape. I think the bigger factor is that the heavier diver is offgassing at a faster rate (not in terms of percentages but in terms of molecules). I believe that that faster off-gassing produces more stress on the diver and is more likely to cause DCS.

 

[victorzamora]

To answer your first post directly: Let's take your assumption that F+ belongs to Paul and F belongs to Peter.

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?

Nope. It's half-lives so they both reach equilibrium simultaneously.

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

Nope. Both have the same half-life and therefore both reach saturation at the same point. However, F+ offgasses/ongasses MORE nitrogen/helium than F.

All of the above is, of course, understanding that compartments are NOT directly correlated to different tissues.

 

[sigxbill]

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?

no - half life of that theoretical tissue determines off gassing - not volume

Do we need to factor in F+ compartments to take in account obesity? Shouln't Paul computer tell him to ascend to a first stop at deeper depth than Pete's?

This is your most important question - by far. There are other factors to consider besides the half times of f. Although I don't consider myself an decompression theory expert by any means, you are correct in questioning why Peter and Paul's computers might calculate the same ascent - assuming they are both wearing identical computers with identical settings. This is why maybe computers are not the best way to 'not plan' your ascent! It is likely that neither computer will compute a deep enough stop for either of them, and that Paul's shallow stop will be too short.

Even if their respective computers took into account their individual body compositions, and calculated different ascents accordingly - what would they do then? Blindly follow their individual computers and split up during deco? LOL!

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. This is why you might consider studying ascent strategies - like the one offered by UTD. The decompression theories module in their Rescue Diver class is one of the most informative and easiest to understand that I have come across. And Ratio Deco is covered in many of their courses, and will help you develop your own ascent strategy that you are comfortable with - and will keep you and your buddy together for the entire ascent!

Find an Instructor · UTD Scuba Diving

cheers

 

[boulderjohn]

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.

 

[boulderjohn]

And Ratio Deco is covered in many of their courses, and will help you develop your own ascent strategy that you are comfortable with - and will keep you and your buddy together for the entire ascent!

You do realize, don't you, that ratio deco was just studied in comparison with other algorithms, and it did not do so well?