First Impressions of the Evolution

Pelagian

While Decompression Strategies have always been evolving, I think one of most critical changes came in the 70's when we got Doppler technology, and started seeing that traditional neo-haldaneian profiles were leaving a lot of people symptom free, but still fizzing.

Since then people have been using a variety of methods to bend(pun intended) buhlman curves to reduce this fizz.

Gradient factors were a mathematical apprach which i liked.

In the 90's however the Tiny bubble group radically altered the way deco stategies are being formed, by looking at the source, at what bubbles do under pressure, rather than just post dive symptoms in divers.

The fall out from this has been the VPM-B and RGBM algotithms, that are getting us closer to what is actually going on in our bodies.

In general most approaches seem to be converging at the follwing very general criteria for safer diving:

1) Slower Ascents
2) Deeper stops
3) Usage of helium
4) Opening Oxygen window as wide as you can
5) Proper Hydration is crucial

The potential benefit of these ideas is we may one day have no need to doctor our tables or computers based on less tangible more subjective experiences we gain from our diving.

Untlil then there's a lot of assumpitons being made and doctoring of profiles will be necessary.

I guess we all need to read what's avaialble, learn from those who have gone before, and try and improve based on our own experience an experimentation, recognizing that we are always taking risks, and the deeper, longer, colder, farther in, the dive the further away from what we can bank on we go.

Happy deep diving

Seb

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UWSojourner

I’ll take a stab at a “Gradient Factors For Dummies (Condensed)”. Here goes.

Everything in the gradient factor decompression algorithms revolves around Buhlmann’s tissue model. Currently this means 16 hypothetical tissue compartments (TCs) that are constantly tracked during a dive in order to determine each TC's inert gas pressure.

As you ascend all those TCs start to off gas as the outside ambient pressure is decreased. The question is – How fast can you let those TCs off gas?

Buhlmann answered that question by coming up with an “M-value”. Basically, an M-value is a maximum overpressure value that tells you that, if you exceed the value, Buhlmann thinks you’re crazy and believes you are about to get bent. Because M-values are “overpressure” values, an M-value for a given depth will be above the ambient pressure for that depth (that is, you are not going to hit an M-value if your tissue compartment pressure is less than ambient pressure) .

One ascent strategy, therefore, would be to move up until the pressure in your TCs just reaches Buhlmann’s M-value and then let your tissues off gas a bit (reduce pressure), rise to the next level, etc. In this strategy, you would keep going up in such a way that you never let your TCs exceed Buhlmann’s M-value.

Unfortunately, deco sickness does not exactly track M-values. More sickness occurs at and above the M-values and less sickness occurs when divers never reach Buhlmann’s M-values. So, GFs were invented as a consistent way to never reach Buhlmann’s M-value.

Gradient factors are calculated as :

GF = ( TCInertGasPressure – AmbientPressure ) / ( MValue – AmbientPressure)

So what does this formula tell us? First, it tells us that at a GF=1.0, you are at Buhlmann’s M-value. Therefore, staying at or below GF=1.0 seems important. Second, it tells us that when our TC pressure just reaches ambient pressure, then the GF= 0.0.

Another ascent strategy might be to shoot up to a GF=0.8 and ascend in such a way as not to exceed that value. In this way you know that your tissue compartments are never over 80% of the distance between ambient pressure and Buhlmann’s M-value. Dive computers implementing GFs usually let you set two GF parameters. Moving straight to GF=0.8 and ascending in such a way that you always keep your TCs at GF=0.8 would be equivalent to setting your dive computer to 80/80.

ErikBaker’s strategy was NOT to ascend directly to a high GF. Instead, he said “Let’s all ascend to a lower GF initially, then slowly move to higher GFs.” So, let’s say you want to first ascend to a GF=0.25 and then slowly move to reach GF=0.75 as you surface. This setting on your computer is likely 25/75. As reference, I believe the Shearwater computer uses 30/85 as it's default setting.

So what is happening when you use a GF setting of 30/85? First, your dive computer allows you to ascend until the pressure in your TCs first reaches a GF=0.3. This means your TC pressure is 30% of the way between ambient pressure and Buhlmann’s M-value. Then you sit there until your TCs drop enough pressure so that you can ascend to your next stop.

How much pressure must leave your TCs before you can ascend? Assume you hit your first stop (GF=0.3) at 110 ft. Well, then, we now have two known points. Point 1 is (110,0.3) – that is at 110 ft we are at a GF of 0.3. Point 2 is (0, 0.85), or at the surface we want to be at a GF=0.85. A natural way to ascend then (and this is what Baker did) is to create a line from the two points and ascend in such a way that you never exceed the GF generated by that line. The formula for the MaxGF at any depth, then, is

MaxGF = HiGF + (HiGF-LoGF)/ ( HiGFDepth – LowGFDepth ) * CurrentDepth

= HiGF - (HiGF-LoGF)/ LowGFDepth * CurrentDepth ( HiGFDepth is at the surface, or 0)

Therefore, if you hit your first GF=0.3 at 110ft, then your LowGFDepth=110. Before you can ascend to 100ft you must let enough TC pressure off so that, when you arrive at 100 ft, the GF of your TCs does not exceed 0.35 calculated as

MaxGF = 0.85 - (0.85-0.3)/ 110 * 100.

You can ascend to 90 ft when your TCs let off enough pressure at your 100ft stop so that when you reach 90 ft your TCs GF does not exceed 0.40 calculated as

MaxGF = 0.85 - (0.85-0.3)/ 110 * 90.

The GF method just walks the line all the way to the surface.

If you understood the above explanation, then you should see why some of the divers on this forum say that setting your GF parameters to 10/90, or 10/80, etc. helps generate deep stops. The low GF of 10 means a stop must be generated when your TCs are only 10% of the way between ambient pressure and Buhlmann’s M-value, rather than 30% if you were to set the low GF to 30. Simply, the GF line just starts deeper.

Hope this helps.


Talk to the pros before you rely on any of this to do any dive planning.

Pelagian

Wow. That was great.

Help me on one thing though.

The line between the Low GF and the High GF, is it linear or exponential?

I though that the GF's bent the buhlman curve to fit in between them.

CHeers

Seb

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UWSojourner

The line determining the MaxGF at any depth is linear. The GF line fits between the ambient pressure line and the Buhlmann M-value line. For example, using 25/75, the line starts (at whatever depth you hit that GF) at 25% between the ambient pressure and M-value lines, and ends (at the surface) 75% between those lines.

Pelagian

I think then that GF's apply a linear transformation to an exponential curve.

Thanks

Seb

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nitrojunkie

Now that's exactly what I was looking for, thanks for the context and practical explanation. Posts like that make suffering through all the "my unit is better than your unit" pain worth while.

Doug

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Decodiver

Good explanation. Have some green.

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Rebreather World Terms & Conditions

UWSojourner

I don't think there is any transformation function going on, at least not in the mathematical sense. The target maximum GFs really are linear when you consider them as a function of depth, as Erik Baker's target GFs do.

The attached PDFs show GFs based on one of my recent dives. You can see that when graphed against depth, the maximum GFs really show a linear pattern. That's because that is how they are derived - the DEFINITION of the max GF in Erik Bakers model is a linear function of depth.

But, when you graph the exact profile against TIME, then you see the typical exponential curve. Maybe that's what you meant? The reason for this behavior is that off gassing in the TCs is an exponential function of time.

FWIW

caveseeker7

Excellent graphs to illustrate your point, thanks for posting them.

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Cheers
Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

Pelagian

That's what I mean. The curve itself is still exonential vs. a time axis, but the GF's bend the curve using linear values. So it's linear adjustment to an exponential curve.

That makes sense to me now of a lot of the numbers.

Thanks

Seb

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