Gradient factors - deep stops thread in DIR forum

[atdotde]

Hi @atdotde

You have may have taken a look at this probabilistic model Modern Decompression. So far, there is only a recreational dive planner. You can enter your desired risk, probability of DCS, and generate a bottom time. You can also enter your bottom time and generate your probability of DCS.

I have to admit, I had not yet looked at that model. But now that I have, I have a couple of things to say about this. This will need a bit more digesting, but I see a series of blog posts coming up looking at various aspects of that approach. Just as a spoiler: I think his probabilities are unrealistically big. The recreational diving industry would long be dead if the probability of DCS for a conventional NDL dive would be as high as 0.3%. That would mean you would get bent about every 300 dives. Imagine what that would mean: Many people do more dives than that in a year (they would be in a chamber more than once a year) while some popular spots probably see that order of dives per day (but don't have their own chamber to treat the daily case of DCS).

 

[jgttrey]

Hi @atdotde

You have may have taken a look at this probabilistic model Modern Decompression. So far, there is only a recreational dive planner. You can enter your desired risk, probability of DCS, and generate a bottom time. You can also enter your bottom time and generate your probability of DCS.

In the example, I chose 80 feet on 32% with a probability of DCS of 0.1 %, 1/1000. This generated a bottom time of a little over 34 minutes.

View attachment 548419

To put this into perspective, here are the 1st clean dive NDLs for DSAT, Buhlmann ZH-L16C at a GF high of 95, and PZ+ along with their respective probabilities of DCS

View attachment 548424

Edit: sorry, see the post by taimen that came up while I was typing. It references this topic in your blog. I read it before, but forgot Fraedrich follow-up – The Theoretical Diver

Lots of claims made on that website and much distracting hype. The theory of working backwards from probability is intriguing, but only as good as the underlying model generating those probabilities. Maybe I'm just missing it, but other than taking SAUL!'s word that is a better fit to the data than any model in the history of universe, I don't see what it is.

For right now, I'll put it in the same bookmark folder as "TruDive" and wait for some more fulsome explanation.

 

[KenGordon]

Hi @KenGordon

I dived an Oceanic Pro Plus 2 from 2002-2010 and have been diving a VT 3 from 2010 to present. My first backup was a Cochran EMC-14, then an Oceanic Geo 2. I switched to a Dive Rite Nitek Q in 2016, to learn about Buhlmann ZH-L16C with GF. I have only had my Teric since last May, and only have about 100 dives on it. I have not had a chance to use SurfGF much yet.

My dives are very different than yours, I have never pretended otherwise. The vast majority of my dives are no stop. From my brief experience, I would say the usual SurfGF for these dives is in the 70s. Yes, I always do a safety stop, often 3 minutes, sometimes longer. The SAUL probabilities of DCS include a routine 3 min safety stop in the calculation. For the majority of these dives, I never closely approach the NDL of DSAT or the NDL of a GF high of 95.

For my light deco dives I satisfy my deco obligation and then do additional time at the last stop, often 3-5 minutes. I have only done a few of these dives with my Teric. I would guess that the SurfGF is usually in the 80s. So, even for these dives, I generally do not approach a SurfGF of 95

My diving varies quite a bit. I will often do just 2 dives a day, with a relatively short surface interval 30-60 min, when diving at home in Florida, though 4 dives per day in not uncommon. On liveaboards and when traveling, 3-5 dives per day is common. I dive wet. Most of my diving is in temperatures between the low 60s and mid 80s (17-29). I have less than 50 dives in colder water from 47-mid 50s (8-13). I've not been to Norway or Bikini, but have been fortunate to dive Red Sea, Cocos, Galapagos, Revillagigedos. I have dived the same algorithm, DSAT, on all my dives, for the last 3 years, along with a computer running Buhlmann.

My style of diving works out very well for me, within the spectrum of dives I describe. I make an informed decision, everyone else should do the same. As I've aged, I have become more conservative regarding the longer safety stops and the padded shallow time on light deco dives. I'm now learning how I want to incorporate the SurfGF available to me on my Teric. Believe me, I'd like to continue diving for as long as I'm able.

Very best and good diving,

Craig

My point is that a description of doing a lot of dives on a particular algorithm may lead to people believing that they can use that same algorithm and be safe. However, the particular circumstances - warm water, padded deco/safety stop, not approaching the NDL - may mean that you are not safe because of that algorithm but because you don’t push it. All the other factors talked about by Neal Pollock, as well as padding the profile.

I mention Bikini and Norway as they are both remote, one warm, one cold. A bend in a remote place is a special consideration.

 

[scubadada]

I have to admit, I had not yet looked at that model. But now that I have, I have a couple of things to say about this. This will need a bit more digesting, but I see a series of blog posts coming up looking at various aspects of that approach. Just as a spoiler: I think his probabilities are unrealistically big. The recreational diving industry would long be dead if the probability of DCS for a conventional NDL dive would be as high as 0.3%. That would mean you would get bent about every 300 dives. Imagine what that would mean: Many people do more dives than that in a year (they would be in a chamber more than once a year) while some popular spots probably see that order of dives per day (but don't have their own chamber to treat the daily case of DCS).

Lots of claims made on that website and much distracting hype. The theory of working backwards from probability is intriguing, but only as good as the underlying model generating those probabilities. Maybe I'm just missing it, but other than taking SAUL!'s word that is a better fit to the data than any model in the history of universe, I don't see what it is.

For right now, I'll put it in the same bookmark folder as "TruDive" and wait for some more fulsome explanation.

Initially, I was also quite skeptical. I would suggest you read the original literature along with the collaborating information. The background support for this is quite strong, unlike some dive algorithms. This blog post may also be of interest Fraedrich follow-up – The Theoretical Diver. The cumulative risk of DCS, when diving to the limit on every dive is illustrated here. Fraedrich follow-up – The Theoretical Diver None of us dive to the limit on every dive.

I remain open minded, I would like to make an informed decision regarding my choice of dive profile. See my recent response Gradient factors - deep stops thread in DIR forum

 

[scubadada]

My point is that a description of doing a lot of dives on a particular algorithm may lead to people believing that they can use that same algorithm and be safe. However, the particular circumstances - warm water, padded deco/safety stop, not approaching the NDL - may mean that you are not safe because of that algorithm but because you don’t push it. All the other factors talked about by Neal Pollock, as well as padding the profile.

I mention Bikini and Norway as they are both remote, one warm, one cold. A bend in a remote place is a special consideration.

Thanks @KenGordon

 

[scubadada]

Sorry, wrong link for the risk of DCS on repetitive dives. The Significance of DCS Probability on a Single Dive Profile | Modern Decompression

 

[atdotde]

None of us dive to the limit on every dive.

That is true without question. But still I would expect many dives either being close to the NDL or with a bit of deco and thus testing at least some boundary of the deco model used.

I was always under the impression that the generally accepted risk of a diving accident is more like one in a few thousand dives which is more than an order of magnitude lower.

 

[dmaziuk]

I was always under the impression that the generally accepted risk of a diving accident is more like one in a few thousand dives which is more than an order of magnitude lower.

SAUL explanation is it's if "you do the exact same dive 400 times in a row". I was always under the impression that's not actually possible IRL.

Poking a few random numbers in his planner, it seems the difference becomes significant when you change pDCS by an order of magnitude, below that it's only plus-minus a few minutes and a bar or two.

 

[rjack321]

I was always under the impression that the generally accepted risk of a diving accident is more like one in a few thousand dives which is more than an order of magnitude lower.

Mike Lever of the Nautilus explorer once told me his client DCS rate was ~1:1800 until he offered a budget nitrox package for the week and it then dropped to 1:3000. Probably because that was when he was operating in Canada and nobody dove to the 32% limits anyway while they had been diving close or at the air limits for the same dives.

 

[njdiverjoe]

That is true without question. But still I would expect many dives either being close to the NDL or with a bit of deco and thus testing at least some boundary of the deco model used.

I think there are more guys out there diving "at the limit" for "almost every dive" than you might expect.

For example I try to dive weekly(if not blown out) and besides a handful of dives haven't done anything but moderate deco (20-90 minutes) for all this year, with sometimes two of these dives per day. If you believe in taking the full helium penalty then I've been regularly surfacing significantly ahead of schedule (40 minutes in some cases). That's just me, and I know there are guys that dive more often and more aggressively.