What Deco Algorithm do you use? (Poll)

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06-06-2016 10:38 PM
PfcAJ

Quote:

Originally Posted by floridabob

Explain the science or at least the logic on this practice. And define deep and long a little more.

Two specific dives might help, one from each camp.

I'm curious about this theory.

Thanks!
Bob

Sure no prob.

*Warning! I'm not a deco expert! I could be super wrong (and probably am).*

So my rationale is based on a few papers and some conversations with some decompression researchers. It my understanding that a given GF (say...30/85) doesn't produce the same probability of DCS across all depths and times. For instance, the risk of DCS you get from a 60min dive at 100ft is less than what you'd get from a 120min dive at 100ft. In a nutshell, as deco time goes up, probability of DCS goes up.

http://archive.rubicon-foundation.or...123456789/8233
http://archive.rubicon-foundation.or...pdf?sequence=1

To me, that's bad. I'd like to keep the probability of DCS 'flat' across all depth and time combinations. I'd ideally like to be able to control what the probability of DCS is. Both of those things are unobtainable with current commercially available decompression software :( But that doesn't mean that I can't manipulate the GF settings to result in more conservative decompressions.

Also, it looks like 'deep stops' aren't great. In my mind, deep stops proper stops (more than a few minutes) before deco gases. Anything less than that is a 'slow ascent'. Maybe the software generates '1,1,1,2' before the 1st gas switch. Fine. But when its more like '3,4,6,10'....ehh, I'd maybe rather be on a deco gas.

http://archive.rubicon-foundation.or...123456789/5069

So knowing those two things, I'd rather have a higher GF Low so I get to my deco gas sooner, and a lower GF High on long dives so I'm on oxygen a bit longer.

Imo, on a relatively short BT at modest depths, almost anything goes. A typical south florida wreck dive is a good example. 150ft, 30mins, 50%, 18/45 backgas. Sign me up for some 20/85 GFs. Its fine, its worked fine in the past, and it works fine for tons of divers. But on a longer thing that's a bit more out of bounds (like 130mins at 190ft plus a 30min transit at 50ft, or 100ft for 280mins)? I'll make some adjustments to more like 30/70. Not wild, but it adds some shallow time and ditches some time before the 1st gas switch. I know those are kinda two extreme examples, but if its more than an hour of deco I think about backing off a bit and shifting to 30/80 or 30/70, and I won't even play with VPM. Its all an experiment, anyways.

Fwiw, I'm not bothered by having the last oxygen stop be at 10ft or 20ft, and typically do the bulk at 20ft.
http://archive.rubicon-foundation.or...123456789/8966

IRT to VPM, it diverges from Buhlmann rapidly and I don't even mess with it.

Clear as diving behind Derek?
06-06-2016 11:52 PM
Nitrogenius

Quote:

Originally Posted by PfcAJ

So knowing those two things, I'd rather have a higher GF Low so I get to my deco gas sooner, and a lower GF High on long dives so I'm on oxygen a bit longer.

very sound approach in my eyes..
06-07-2016 03:59 PM
Nitrogenius

Looking at the current poll results:

GF Users seemed to have moved away from trying to create a exaggerated Deep Stop profile
Very little users of Bubble based models here in the cave community (which makes sense looking at the load profiles in cave deco diving over deep bounces like oftenly done in OW deep diving)

Concerning: Quite a lot of divers seem to believe in the "concept" of even GF..
Or maybe those values just by chance did align with what they are using`?
06-07-2016 07:58 PM
sdenney

My Nitek HE uses Buhlmann ZH-L16 (no idea what the GFs are). Navy tables and EAN32 tables as backup for nitrox dives (along with bottom timer/ gauge). Trimix dive tables are generated with Buhlmann ZHL16/C with GF's of 30/85.
06-07-2016 08:11 PM
jcr

Here's an interesting exercise to visually demonstrate how little difference changes in gradient factors actually make in the shape of the deco profile. This is relatively easy to do in V-Planner or Multi-Deco and several other programs.

Plan a dive of say 300 fsw for 60 mins, which is deep enough and long enough to reflect some real differences. This just happens to be my go to profile for evaluating various factors and how they may affect deco. Using your typical bottom gas and deco gases for both, select two gradients factors that produce the same run times, one that represents a typical deep stop profile and one that represents emphasis on shallow stops as the latest research suggests. For example, 30/90 and 75/75 both have almost the same run times and are "radically different" approaches to decompression. Or are they? Plug these in and print out a depth/time graph of the resulting dive plans. Now overlay them and compare the actual differences. Yes, there are differences but I suspect most people will be surprised at how little.
06-07-2016 09:06 PM
floridabob

Frankly, I dont know what works best. I do know that a dive of 70 minutes at 100 ft is probably more concerning than 70 minutes at 300. The He vs Nitrox debate ensues.

Boils down to this: 2% rate of DCS across all recreational dives. Profiles, depths, times, etc are nearly irrelevant. Still 2% rate. Meaning not commercial dives--as in what we do.

Not my numbers but DANs.

I was bent. I've seen divers bent. I've seen Bonus Deco. I've felt great and felt like crap. I have ZERO clue what works and even if it does today it may not tomorrow. So, do as you please and pay your insurance premium. Enjoy the problem free days and wonder WTF when you head to the chamber.

For me, the deeper/longer then the slower the ascent rate and the more He I prefer. And to do all your deco at 20....well, that's just a bit silly when you can watch what happens to your loop using an ECCR. Oh, and swim on deco and watch what happens to the off gassing.

Worthless, but so be it.
bob
06-07-2016 09:44 PM
kwinter

Quote:

Originally Posted by PfcAJ

I do. The deeper/longer I go the more conservative the deco. More shallow time, less deep stops.

So on a very deep, long technical dive you adjust your GFs to get you shallow faster? I don't mind the theory if it works for you. But please, don't call that being "more conservative."

iPhone. iTypo. iApologize.
06-07-2016 10:01 PM
PfcAJ

Quote:

Originally Posted by kwinter

So on a very deep, long technical dive you adjust your GFs to get you shallow faster? I don't mind the theory if it works for you. But please, don't call that being "more conservative."

iPhone. iTypo. iApologize.

Yea. Based on the papers I linked and the conversations I've had, that's correct.

Less deep time, more shallow time, and more time overall.
06-07-2016 10:23 PM
Nitrogenius

Quote:

Originally Posted by PfcAJ

... and more time overall.

And that is the part that makes it more conservative..
I had understood it that way..
of course just moving up shallower faster will not be more conservative, but less..
i.e. moving from a 20/F to a 50/F will be more aggressive as the overall deco time will go down..
06-08-2016 07:51 AM
Nitrogenius

Quote:

Originally Posted by jcr

Plan a dive of say 300 fsw for 60 mins, which is deep enough and long enough to reflect some real differences.

I would argue that in fact it is already too long at this depth to provide significant differences..
After all GF are based on Buehlmann so it is the best base model.. tissue loading will be rather high hence lesser influence on the GF settings..
Would be really interesting to see some deco heat maps for that comparison though..

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