Gradiant Factors / the VR3 and an explanation

schford

Split another thread to make this more logical....

nitrojunkie

I also find myself diving the VR3 for deco and really using the Vision as back-up. With the Vision gradient set to 30/85 the run times are essentially the same but the VR3 deep stops require that it becomes the "primary" computer of the two. The Vision seems to constantly express a slightly shallower ceiling than the VR3 except in the 40-60' stops range where the ceilings seem to be closest but the Vision still seems to be a few feet shallower.

Has anyone encountered a situation where the VR3 stops was shallower than the Vision? It could be different on shallower deco dives but I have yet to encounter it.
Doug

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schford

Hi Doug,

Try running the Evolution on 10/85 GF and I bet the ceilings come closer together (educated guess without running the models them selves and referring to the deeper stops) you are never going to get the deep stops the same as the VR3 uses a modified pyle stop, and in fact your still on gassing with some profiles and with GF you are neve rongassing at any stop. Although those deep stops do make sure you ascend nice and slow.

Stuart

nitrojunkie

Stuart,

I still don't have a solid enough understanding of gradient factors variables to start playing with them too much but I would guess your suggestion would keep the ceilings closer. I would only be concerned that the total run times would begin to get out of sync and potentially each computer might compete to take over the schedule. I'll give it a shot the next time I am on 2 similar profiles and see how this impacts timings and which computer controls the dive. I certainly wouldn't mind staying in the soup longer if conditions are friendly

The fact that the Vision uses a curve rather than stops kind of makes it a significantly different enough approach - I kind of prefer using the VR3. Although I hope and imagine the curve is a solidly tested method for deco, it seems like it would take me a while to get comfortable enough to ride it to the surface. I understand the concept but I kind of like the stair stepping of the VR3 compared with what seemed like shallower depths of the Vision approach. At least for warm water 240'-330' dives with 15 to 30 minute bottom times.

If the bottom times were longer I would probably switch to a 10/85 GF anyway or if I remember the Evo manual even suggested a low GF of 5 for deeper longer diving with over 20 or 30 minutes bottom time. Does any one have a recommendation on a good resource for understanding gradient factors in practice? I would like to understand the concept more fully I just never had the right resource and the time to delve into it at depth.


Doug

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jradomski

The VR3 does stops akin to pyle stops, a few stops rater than a nice progression.. it makes you get shallower too quickly in my opinion then makes you hang longer shallow..

GF is a much more natural limit progression.. The more stops required the smother the "curve" becomes.. If you get the right ascent rate the deeper stops will almost clear by arrival.. The deep VR3 stops are a pain to predict, sometiems they are too deep and require you to stop deeper than you should be (not really in the decompression zone)...

The problem you will find is that the VR3 will penalize you for staying at depth, so you will have to play with your settings and conservatism.. Your stops at depth should help reduce the shallows but this does not happen in practice on the VR3..

I can get a HH (whih is also a gf alg) and the explorer to line up much more eadily than the HH and VR3..

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All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

Pelagian

I found the following 2 papers a good start to understanding deep stop theory, icluding Pyle stops and gradient factros. I found that from here it was easier to go to the more technical texts, like Wienke's 'Basic Decompression' and 'RGBM in depth'.

http://www.bishopmuseum.org/bishop/t...deepstops.html

ftp://decompression.org/pub/Baker/Deep%20Stops.pdf


The VPM website

http://www.decompression.org/maiken/...m_Site_Map.htm

also has a bunch of information that I found very usefull.

Cheers

Seb

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nitrojunkie

The Baker paper on deep stops is the take I was looking for. It still seems like there should be a Gradient Factors for Dummies published... and inserted into the Vision section of the APD manual. I'll probably need to spend some time diving varying GF's to get a real feel for it.

Ever since I started diving deeper I have found deep stops certainly help make me feel better after a dive, with the Vision electronics it is the first time I have had to make the decision myself as to how to set them. The Vision manual gives some basic settings just not much on the rationale side as to the "why" you set what you set.

Thanks,

Doug

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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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