Deco for Divers: RGBM vs other algorithims

[CuzzA]

Hello,

These are good questions.

The issue of why use GFs at all is complex. Here is my take on it.

When bubble models emerged among technical divers in the late 1990s / early 2000s there was a perfect storm of factors that led to their widespread adoption. One was the fact that the widely used alternatives around at that time still resulted in "unexpected" DCS on occasion. Another was the theoretical attraction of the bubble model concept; that is to say that it seemed to make sense that the approach would work, (though this was NEVER tested). Yet another was Rich Pyle's contemporaneous observation that the insertion of a deep stop in his own practice seemed to benefit him (I will return to that issue below). Finally, a number of influential figures around at the time enjoyed getting on the bandwagon; I presume to seem knowledgeable and at the cutting edge. These people aggressively promoted the concept in the absence of any supportive data. Under these influences the use of bubble models, or GF modifications to make Buhlmann look more like a bubble model became very popular, indeed, almost ubiquitous. It went somewhat unnoticed that one of the most popular bubble models required adjustment to its early iteration because of an apparently high incidence of DCS associated with its use.

If we fast forward to today there are now two human studies which have demonstrated that a bubble model approach (somewhat against expectations) either resulted in more venous gas emboli after surfacing or a greater incidence of DCS. There have been extensive arguments about these studies (particularly the NEDU study) on the internet, but cutting through all the confusing detail, there is a strong signal in the data that emphasising protection of the fast tissues from supersaturation early in an ascent (which is what deep stops do) does not provide the hypothesised advantages. Indeed, it appears disadvantageous because it results in greater supersaturation of slower tissues (which continue to take up gas during deep stops) later in the ascent. Notwithstanding the argument that the NEDU dive profiles are dissimilar to those dived by technical divers, it has been convincingly demonstrated that this same disadvantageous pattern of tissue supersaturation identified in the NEDU study is seen in undisputedly real-world technical diving profiles controlled by a bubble model compared to a common GF approach (illustrated using Kevin Watts’ diagrams in my presentation that someone linked to on this thread).

This brings me to your question about why we should use GFs to impose any stops deeper than Buhlmann prescribed. The answer is that use of deep stop approaches became an “industry standard” as I have described above. The available data now tell us that use of a bubble model that emphasises deep stops is almost certainly not optimal. But these studies were not designed to actually define optimal decompression. We don’t know how raw Buhlmann would compare and we cannot exclude the possibility that some degree of stopping deeper than he prescribed might be useful. Thus, in the prevailing setting where the vast majority of divers have been using deep stops it seems sensible to be cautious in the approach to moving away from emphasising them. As I have said, my own approach is to raise the GF low to around 50. That is still a deep stop compared to Buhlmann, but it is not as deep as would be prescribed by a bubble model. I agree with those who have said that these adjustments are only going to have a significant impact on dives requiring substantial decompression.

I would like to briefly mention the issue of Richard Pyle’s “deep stops”. His practice of inserting one or more stops into an ascent deeper than his prevailing ceiling has often been confused or equated with a bubble model approach to decompression. This is incorrect. They are two different things. I talked at some length with Rich about this last year, and it is a source of some annoyance to him that “Pyle stops” are often equated with bubble model decompressions. He does not use a bubble model. I do not have a strong opinion either way on Rich’s practices because it has never been tested in any formal manner.

Finally, there are several other studies of relevance to this matter that will hopefully be published in the near future.

Simon M

Thank you Simon and everyone else.

Please update us to new information when it becomes available. I plan to start technical training at the end of the year and so I appreciate those who contribute on sites like this as it allows us to get a jump start on the theories, concepts and information which I believe is very helpful when someone goes into something new and knows a little about the subject rather than being completely oblivious and having to decipher loads of new information in the short period of time we have with an instructor.

 

[tbone1004]

Based on all this...

For my first time dive computer as a new diver, I'm looking between the:

Suunto Zoop Novo ($330), proprietary RGBM computer

VS

Aqualung i300 ($300), with their algorithim: "Dr. Lewis wrote the proprietary Pelagic Z+ (PZ+) algorithm employed in Aqua Lung’s computers. The PZ+ algorithm is based on the Bühlmann ZHL-16C algorithm and is a favored recreational algorithm for the safety conscious diver."

I am also open to other options within a $350 price ceiling limit.

Not sure if it's worth shopping a dive computer based on the Buhlmann algorithim OVER the RGBM for non tech diving at a novice level, or better to have such a ubiquitious system such as the Suunto ZOOP line.

Thanks for input!

Save $50-$80 and go with the Hollis DG03, or spend an extra $70-$100 and get the Nitek Q. Personally I would argue for spending up to the Nitek Q. You get the digital compass, fully unlocked, and you get to choose which GF's you use. The GFLo obviously being irrelevant for NDL diving, but the GF High is certainly important as that is what directly controls your conservatism.

 

[CuzzA]

@tbone1004 I looked into the DG03 when I was puter shopping and while the features were nice it got really poor reviews for ease of use and intuitiveness. Thoughts?

 

[tbone1004]

buy the best or buy the cheapest. It's the cheapest computer you can buy new that will take you up to trimix for technical diving. The user interface sucks, but it is no worse than any of the other casio style computers out there. Shearwater and Suunto have ruined the casio screened computer market for ease of use. At this point I see 4 computers in the US market worth considering.

$250- Hollis DG03-if you're on a budget and can't spend up-this does have AI option btw
$400- Dive Rite Nitek Q-if you're on a budget, but want something a bit nicer than the DG03
$850- Shearwater Perdix-Nicest computer on the US market bar none, and the only one worth considering UNLESS
$975-Seabear H3- you need a watch form factor and want a really nice computer, in which case the H3 is worth considering. This is due to have compatibility with a wireless transmitter, but this was delayed due to the acquisition by Scubapro.

I have asked the question before, but if anyone thinks they have a computer that is worth considering other than those four, I'd love to hear it, but I am unaware of any.

 

[scubadada]

Based on all this...

For my first time dive computer as a new diver, I'm looking between the:

Suunto Zoop Novo ($330), proprietary RGBM computer

VS

Aqualung i300 ($300), with their algorithim: "Dr. Lewis wrote the proprietary Pelagic Z+ (PZ+) algorithm employed in Aqua Lung’s computers. The PZ+ algorithm is based on the Bühlmann ZHL-16C algorithm and is a favored recreational algorithm for the safety conscious diver."

I am also open to other options within a $350 price ceiling limit.

Not sure if it's worth shopping a dive computer based on the Buhlmann algorithim OVER the RGBM for non tech diving at a novice level, or better to have such a ubiquitious system such as the Suunto ZOOP line.

Thanks for input!

The Aqua Lung i300 is manufactured by Pelagic Pressure Systems. It is the same computer as the Oceanic Veo 2 but runs only the single decompression algorithm, PZ+ rather than dual algorithm, DSAT and PZ+. The Veo 2 can be purchased for the same or less cost than the i300.You will also have to purchase a download cable for either of these computers unless you don't intend doing that. If you would rather have a watch style computer, you could look at the Oceanic Geo 2, I have used one as a backup for 5 years, has worked flawlessly.

 

[Centrals]

The Aqua Lung i300 is manufactured by Pelagic Pressure Systems. It is the same computer as the Oceanic Veo 2 but runs only the single decompression algorithm, PZ+ rather than dual algorithm, DSAT and PZ+. The Veo 2 can be purchased for the same or less cost than the i300.You will also have to purchase a download cable for either of these computers unless you don't intend doing that. If you would rather have a watch style computer, you could look at the Oceanic Geo 2, I have used one as a backup for 5 years, has worked flawlessly.

Which algorithms, DSAT and Z+, would you use? I use Oceanic Veo 180 as a back up to Uwatec Aladin and found Veo 180(DSAT) way too liberal.

 

[Kevrumbo]

30/70 is the default, doesn't mean it is what we leave it set to. I wouldn't however do what Kevrumbo is saying and go all the way down to 50 on GF's because your CNS clock will get VERY high and the decos start to get excessive due to the air breaks. I will however bring it down. He is also diving in much colder water than I am with different conditions, so those GF values may be what is needed for his style of diving.

Personally, my computer is set to 50/75 and will typically not change unless a dive is going to be particularly strenuous and then I will drop it to 45/70

Regarding high CNS/O2 exposure over time:

Kevrumbo said: ↑
The disadvantage of tactically compensating for the deep stop slow tissue loading with elective extra O2 deco time of around 10 to 20 minutes maximum was as both you & I expected: an uncomfortably high CNS OxTox figure result of 300 to over 500 max as tracked by the Petrel dive computer. . .

Hello Kev,

Not to say there is no risk (because obviously there is), but we published this . . .which might make you feel slightly more comfortable:

Aerosp Med Hum Perform. 2015 Jan;86(1):41-5. doi: 10.3357/AMHP.4113.2015.
End tidal CO2 in recreational rebreather divers on surfacing after decompression dives.
Mitchell SJ1, Mesley P, Hannam JA.
Author information

• 1Department of Anaesthesiology, Faculty of Medical and Health Sciences, University of Auckland, New Zealand.

Abstract
INTRODUCTION:
Deep dives using rebreather devices result in oxygen exposures that carry a risk of cerebral oxygen toxicity. Elevation of arterial CO2 levels increases this risk. CO2 retention may occur during the deep working phases of dives, but it has not been investigated in 'real world' dives at the end of resting decompression when oxygen exposures are peaking, often to levels higher than recommended maxima.

METHODS:
We conducted an observational field study to measure end tidal CO2 (Petco2) in divers surfacing after decompression. Sixteen rebreather divers conducted two dives and two completed one dive (a total of 34 dives) to depths ranging from 44-55 msw. Bottom times ranged from 35 to 56 min and time spent on decompression ranged from 40 to 92 min. The first breaths on reaching the surface after removing the rebreather mouthpiece were taken through a portable capnograph. The Petco2 was recorded for the first breath that produced a clean capnography trace. Petco2 measurement was repeated for each subject 2-3 h after diving to give paired observations.

RESULTS:
There were no differences between mean surfacing Petco2 [36.8 mmHg (SD 3.0)] and the mean Petco2 made later after diving [36.9 mmHg (SD 4.0)]. One subject on one dive returned a surfacing Petco2 higher than a nominal upper limit of 45 mmHg.

DISCUSSION:
We found no general tendency to CO2 retention during decompression. It is plausible that breaching oxygen exposure limits during resting decompression is less hazardous than equivalent breaches when exercising at deep depths. Mitchell SJ, Mesley P, Hannam JA. End tidal CO2 in recreational rebreather divers on surfacing after decompression dives.

PMID:
25565532
[PubMed - indexed for MEDLINE]



If you want the full paper just pm me.

Simon

http://www.scubaboard.com/community/threads/deep-stops-increases-dcs.497187/page-34#post-7337595

 

[scubadada]

Which algorithms, DSAT and Z+, would you use? I use Oceanic Veo 180 as a back up to Uwatec Aladin and found Veo 180(DSAT) way too liberal.

I've been using DSAT since 2002, about 1,150 dives. Conservative and liberal is in the eye of the beholder, I'm perfectly happy with DSAT.

Out of interest, why do you think DSAT is too liberal? I would imagine it is because you compare it to your more conservative Uwatec primary computer. Perhaps the Uwatec is too conservative? I run an Oceanic VT3 primary and a Geo2 backup, they run within 30 seconds of one another, they are both just right. Perhaps you should run a backup computer that runs the same decompression algorithm as your primary computer?

 

[dmaziuk]

@tbone1004 I looked into the DG03 when I was puter shopping and while the features were nice it got really poor reviews for ease of use and intuitiveness. Thoughts?

Single-button computers get bad UI reviews, too, and I'm perfectly happy with mine. But that's because I don't change the O2% setting before every single dive, if I had to do that every time I'm sure one button'd get very old very fast. I strongly suspect that similarly, DG03's UI deficiencies come into pay when it comes to fancier functions like mid-dive gas switching. For basic use it's probably OK. Once you want fancy, I'm sure three buttons and the best UI in the business can still feel limiting.

(And I play my games with the mouse and full keyboard instead of those crippling console controllers, too, and for the same reasons.)

 

[stuartv]

RGBM is supposedly proprietary, right? But to have any course to take legal action against someone unauthorized to use or duplicate it they would have to have a patent, right? And to have a patent you must publicly reveal and disclose what the idea or products is, right?

So, is there a patent? A quick google search turned up nothing.

All proprietary means is that it's their secret and they have no obligation to tell it to you. Just like the recipe for Coca Cola is proprietary. If you figure out all on your own a way to produce exactly the same results, you are free to do so - whether it's making your own cola that tastes the same as Coke, or whether it's a dive computer that always gives identical NDLs to someone else's RGBM computer.

I do not believe DSAT is a good algorithm for decompression. Not that it is not safe, but it piles on shallow (10 ft) decompression time very quickly, faster than one would think by looking at a planner, like MultDeco. I generally spend a minute or two at 20 feet, clear deco at 10 ft, and then do 3-5 extra minutes. As I have gotten older, I have become more conservative.

My current interest in this topic is because my next computer will run Buhlmann ZHL-16C. I've been diving DSAT since 2002, 1150 dives. My Oceanic VT3 is 6 years old, 745 dives, 795 hours, it won't last forever. I'm trying to understand the performance of the Buhlmann algorithm and the effect of GFs. Some of the commonly used GFs for tec diving make for an extremely conservative computer for rec diving. I am beginning to get a reasonable handle on the topic. I plan on getting a new computer in the foreseeable future and will dive them side by side as I gain experience.

Hey Craig,

As you know, I have been diving DSAT (Oceanic Atom 3.0) also, for the last year and a half. More recently, I have also been using a Seabear H3, which is Buhlmann ZHL-16C and I usually use GF of either 30/70 or 35/85 (2 of the default options on the H3). I completed my deco cert 6 weeks ago and have done 16 dives since then, with most of them being deco dives. Most of the deco dives were just doing a few minutes on back gas. A few were with 5 - 15 minutes of deco on 80%.

On all these dives, I have been programming the H3 for all the gases I was using and doing a gas switch on the computer whenever (if) I switch to my deco gas. For all those same dives, I also leave my Atom in Dive mode (versus Gauge mode) and set the FO2 to match my back gas, but I have not been doing a gas switch on the Atom to tell it when I switch to my deco gas.

So far, I have only bent my Atom once. I have put it into deco a number of times. But, even though the Atom thinks I'm doing all my deco on whatever Nitrox I have been using for back gas, its algorithm (I use DSAT) has been clearing my deco in the amount of time it takes me to do the indicated stops that my H3 prescribes. Even when I'm actually doing my deco on EAN80.

As I said, except for once. The day I did my first AN/DP checkout dive, I used 50% for deco gas and, for whatever reason on that day the Atom actually went into Violation mode when I got out and locked itself for 24 hours (though it still showed Depth and EDT the next day when I wore it during my second AN/DP checkout dive)

My conclusion: The DSAT algorithm in the Oceanic computers is pretty darn liberal, even for its deco obligation calculations. OR, maybe I should say that previous comparisons of the Atom NDLs (in the owner's manual) to Buhlmann w/GF 100/100 in Multi-Deco have led me to think that the DSAT implementation in the Atom is straight up Buhlmann with no GF (i.e. GF 100/100). Maybe that is indeed the case. Maybe, for the dives I've done, GF100/100 and deco on back gas would yield a run time that is equal or shorter to the run time for GF 30/70 with deco on 80%. Maybe when I have some time I'll play with Multi-Deco and see.

I think your approach of clearing off the Geo's deco and then staying an extra 3 - 5 minutes is a really good idea - if not staying even longer.

Which algorithms, DSAT and Z+, would you use? I use Oceanic Veo 180 as a back up to Uwatec Aladin and found Veo 180(DSAT) way too liberal.

How did you determine it was too liberal? Did you get bent?