Deco for Divers: RGBM vs other algorithims

[CuzzA]

I use the Buhlmann ZHL-16 as well. I don't have a hose in the race, but this presentation is worth a watch.

Thanks for sharing this. One of the best decompression presentation I've seen.

Of course, I'm slightly confused now about gradient factors.

 

[CuzzA]

I would like to hear more from the community on gradient factors. One thing I found confusing/contradictory about his presentation is that from using the Navy study the conclusion was drawn that bubble models increase the chance of slow tissue DCS, yet GF are an attempt to mimic bubble models. So if that were the case, then why use GF at all, or if you're going to use them for conservatism purposes, shouldn't the first factor/percentage be much higher and closer to supersaturation to eliminate slow tissue on gassing? Are GF's simply a result of divers not willing to let go of the bubble model idea?

Also, while we all appreciate the Navy's study, can someone explain Richard Pyle's observations. Deep Decompression Stops

Is there a place for deep stops in decompression diving? For example, would there be any benefit to add say one deep stop at half your depth for a minute or two and then continue on with a straight buhlman strategy (maybe adjusted for conservatism)?

 

[tbone1004]

Flexible Control of Decompression Stress - Shearwater Research
Review of Deep Stops - Shearwater Research
Gradient Factors | Dive Rite
Deep Decompression Stops

read up. Report back

Half pressure is much better than half depth in most peoples opinions.

 

[dmaziuk]

One thing I found confusing/contradictory about his presentation is that from using the Navy study the conclusion was drawn that bubble models increase the chance of slow tissue DCS, yet GF are an attempt to mimic bubble models.

It is my (quite possibly mistaken) impression that only GF Low is the part that "mimics" deep stops that are the part of bubble models everyone's unhappy with.

 

[Shotmaster]

The Navy tables in my opinion are too risky for Rec diving, as the Navy finds a 2% occurance of DCS acceptable, this risk is counter balanced as they are fully equipped with onboard chambers, a full support staff, a medical team, and a support diver when in the water, and is a luxury we don't have. Since working underwater it is not our day job and most of us probably aren't single digit body fat numbers, we must be cautious to ensure a very low probability of any DCS occurring. there are still a lot of unknowns even to the best in the medical/diving science field. All of the models are based upon the best evidence and the best guesses. I have been diving for a long time and did the majority of my dives using the navy tables. In hindsight I feel very lucky as I pushed the limits the first 20 years of diving. I was introduced to DECO in the 70's while diving with Navy divers hunting fish and helping salvage some brass port holes and other valuables from a turn of the century wreck. I have to admit I was a bit cavalier after that about bottom times and didn't think adding a deco stop was such a big deal. I luckily never had any problems, and I also did not know the tables were based upon a 2% allowable hit rate either. I found out by chance from a Navy diver at NEDU while I was back home on vacation in Panama City about the 2% DCS acceptable hit rate. I told him my background and talked about doing some salvage work, he keyed in on the Deco work and then enlightened me that he was one of the divers diving the tank to verify some new tables. He told me the study was started after the Navy had continuous DCS problems while diving the TWA800 crash and experience a very high and unacceptable rate of DCS. He said it had gotten everyone's attention and the realization that the tables were 40 years old. This incident led to the revised the Air Tables using Dr. Thalman's algorithm. Fortuneantly for me nitrox had made it mainstream at that time and the resultant increased bottom times eliminated the need to take those risks anymore. I became much more aware that we didn't know what we didn't know, and have become very conservative since. I am always interested in the latest research and findings. Safe diving!

 

[filmguy123]

Very interesting, learning a lot from this thread and video. Let me ask a more pointed question (not to re-direct away from more divers preferred model, but to co-direct towards also addressing this):

What is the consensus on the RGBM used by many dive computers? For dives that are unlikely to encounter any deco stops. Doesn't matter, use anything? Or, too conservative, buy a non RGBM system? Or, more conservative, but likely a non issue for most rec divers? Does this even matter much if at all if one is not currently into tec diving?

Seems like most popular use RGBM. I only see a few computers with Buhlmann 16.

 

[DevonDiver]

Which RGBM? Because the manufacturers that use it, can adjust it to their preferences. The problem is, being proprietary, the user doesn't know what those adjustments and tweaks are.

That's why I (and many) prefer Buhlmann or VPM.... as they're open source, understandable and predictable. That's a bigger consideration for technical divers pre-planning accurate and reliable deco, than recreational divers trying to understand the nuances and conservatism of a no-stop limit.

 

[scubadada]

...What is the consensus on the RGBM used by many dive computers? For dives that are unlikely to encounter any deco stops. Doesn't matter, use anything? Or, too conservative, buy a non RGBM system? Or, more conservative, but likely a non issue for most rec divers? Does this even matter much if at all if one is not currently into tec diving?...

You might want to take a look at the ScubaLab testing from 2014 to see how many of the most popular dive computers and their decompression algorithms perform on a series of dives. There is really quite a difference between the most liberal and the most conservative. These are all no stop (recreational) dives. There is a link in the description of the testing to the full test results The Best Scuba Diving Computers Reviewed by ScubaLab There is not much information like this available to help divers understand the implications of choosing a decompression algorithm for recreational diving

 

[scubadada]

I have been very interested in this topic for at least a decade. A while back, I compared DSAT NDLs to the results of simulating those dives in MultiDeco using a variety of Buhlmann ZHL-16C GFs. I found the results very interesting and learned quite a bit from the exercise. If I owned a Petrel or Perdix, I could simply run the planner and find out what the Buhlmann NDL is for each of these settings, maybe someone will do that and share the results?

As a single example, here is the MultiDeco output for a 60 foot air dive for 57 minutes (the DSAT NDL):

GFs MultiDeco output
100/100 No deco
95/95 1:55@10 ft
90/90 3:55@10 ft
85/85 6:55@10 ft
45/95 0:20@20 ft, 2:00@10 ft
40/85 0:20@20 ft, 7:00@10 ft
35/75 3:20@20 ft, 12:00@10 ft
30/70 0:45@30 ft, 2:00@20 ft, 17:00@10 ft

 

[KenGordon]

I have been very interested in this topic for at least a decade. A while back, I compared DSAT NDLs to the results of simulating those dives in MultiDeco using a variety of Buhlmann ZHL-16C GFs. I found the results very interesting and learned quite a bit from the exercise. If I owned a Petrel or Perdix, I could simply run the planner and find out what the Buhlmann NDL is for each of these settings, maybe someone will do that and share the results?

As a single example, here is the MultiDeco output for a 60 foot air dive for 57 minutes (the DSAT NDL):

GFs MultiDeco output
100/100 No deco
95/95 1:55@10 ft
90/90 3:55@10 ft
85/85 6:55@10 ft
45/95 0:20@20 ft, 2:00@10 ft
40/85 0:20@20 ft, 7:00@10 ft
35/75 3:20@20 ft, 12:00@10 ft
30/70 0:45@30 ft, 2:00@20 ft, 17:00@10 ft

Are your DSAT numbers from a table? If so you should be comparing with ZHL16B. The C variant is for PDC as the PDC will calculate based on actual exposure. Tables introduce conservatism due to rounding depth, time, descent time and ascent time (for tables which use time to first stop).

You don't say what ascent rate you used. It is a key parameter.

You don't mention the one minute safety stop which I believe as assumed when planning actual buhlmann tables. Remember that the stop times are only useful in the context of the rest of the model or tables parameters.

FYI, on DM5 a using the regular Suunto RGBM gives 6 minutes of mandatory stops and 3 minutes of safety stop. For those computer which allow P-2 (eg HelO2, EON Steel, DX etc) this is a no stop dive on that setting, so just the 3 minute safety stop.

It looks to me that the limiting factor would be gas for this dive. Like the old boys used to say, "Why do I need a watch, gauge or tables, you can't get bent on a single?"

PS, I have previously asked for people with greater patience than me to post such NDLs from the petrel but there have been no takers.