conservative versus liberal algorithms

[Charlie99]

diatom once bubbled...
If I'm not mistaken, those 2 computers are considered liberal in their ndl's, while some of the computers using RGBM et al. are considered moderate to conservative. The most important question in my mind is, are the liberal algorithms actually conservative. i.e. Is there a sufficient cushion between their ndl's and the actual times until dcs becomes highly probable. I have read at least one opinion that the more liberal computers are frighteningly liberal when compared to other computers. In order to determine whether or not they are frighteningly liberal, one would have to compare the ndl's from the computers to actual data collected on instances of dcs/tissue loading rates. It seems to me that providing comparisons of the ndl's for the various algorithms to such data would be extremely useful. Then you could make a good determination on whether or not a computer is truly liberal, or simply liberal relative to the other computers. Does anyone have any idea as to where you could find that info? That is assuming it is publicly available.

DCS is a relatively rare event, so there really isn't enough data to make statements about relative DCS rates of liberal vs. conservative computers. There really aren't even clear statistics on the more basic comparison of the effect tables vs. computers on DCS rates. A large number of divers with DCS are within the limits of all computers, both liberal and conservative. What bends divers is their profile, not what their computer is reading.

Just like the tables, the computers merely track nitrogen loading. They don't control your ascent, and few, if any, make any recommendations as to intermediate stops before the safety stop (These are generally called "deep stops", but the term "intermediate stop" might get more rec divers to do them).

Fast ascents and/or short time-to-surface are often related to DCS incidents, even when the diver is within NDL. The RGBM-like computers like Suunto will adjust future NDLs for fast ascents and yo-yo actions on previous dives. Obviously, it is good dive practice to avoid fast ascents, sawtooth dives, etc.

The nice thing about the Oceanic/Pelagic computers is the the "tissue loading bargraph", TLBG, is a graphical indication of N2 loading --- unlike an NDL number that goes up dramatically as you head towards 30 or 40', the TLBG continues to show your N2 loading. This is a useful tool for controlling your stops, and you can adjust your level of conservatism by doing something like "stay at safety stop until TLBG returns to green". That will get you out with a reasonable margin (M value approx 85% for compartment closest to M0).
A more conservative computer that merely shows NDL won't give you the same cues as to how loaded you are, unless you remember NDLs for, say 37', and compare that to the NDL displayed at that depth, and it will give you NO hint as to N2 loading levels at all once you get to safety stop since NDL is infinite.
The purpose of NDLs is to tell you that you have reached the point where direct ascent to the surface is not advisable. A prudent diver may choose to add additional padding, either by staying further away from NDLs, or by spending extra time in the water doing stops.

Computers are like rather stupid bookkeepers. They track depth and time, and make estimates on tissue loading. Like any powerful tool, they can be both used and abused. A prudent diver will keep a rough estimate of his own as to how loaded he is, and not blindly dive right up to NDLs then shoot to the surface.

Charlie

sorry about the long message, I didn't have time to make it short

 

[Zept]

Two thoughts:

1. On identical profiles, my Suunto Vyper gives less bottom time than my PADI RDP. For example, for a square profile to 18m, the tables allow 56 minutes, but the computer only gives 52. I expect you would see the same thing if you modelled a stepwise multilevel profile and compared the dive computer to the wheel. In reality, it is unusual to dive a square or stepwise profile, but if you do, the Vyper gives less time.

2. There's more to diving conservatively than staying away from the NDL. Last year I got bent without exceeding table limits (using the wheel) or upsetting my computer. I did everything by the book... dives within NDL, slow ascents, safety stops on every dive, drank lots of water between dives... and still came away with a sore shoulder. Looking back, one thing I would change is the sequencing of the dives. It probably wasn't smart to do five dives on the third day of my holiday, finishing with a relatively deep night dive at 8:30pm, and then start the next day with another deep dive. Although all the dives were within limits, the sequence was pretty ambitious.

As they say in the instructions, no dive computer can prevent all instances of decompression sickness. You've still got to use your brain.

Zept

 

[Dr Deco]

Dear Readers:

Conservative vs. Liberal

As far as decompression goes all decompression devices are conservative. This means that all are quite far away (time-wise) from the bottom time that the most sensitive (or susceptible) diver will get DCS. This is the reason why so many different decompression devices can give you quite different dive times and yet leave the diver free of DCS.

In addition to that, most divers are considerably more resistant to DCS than the most sensitive. Divers are not accustomed to thinking that there is a spread in DCS susceptibility, but there is. There are many commercial divers (and black coral divers) who could tell you stories of decompression that you would not believe. Many have become paralyzed in their quest to determine if they were DCS resistant.

The distribution of resistance/susceptibility in humans is easiest to see in altitude decompression but most divers do not have an acquaintanceship with this field. It is very apparent in laboratory studies.

Decompression Meters :scuba:

The vast majority of divers are of the distinct impression that they are protecting themselves from DCS by the choice of a decompression device. This is only partially true. The activities of the diver that surround the dive that are directly under the diver’s control play a big role. These activities are refraining from climbing, running, lifting, and such. These activities promote the formation of gas nuclei by stress-assisted nucleation.

Deco Models

There are not any decompression devices that will adjust the gas loads as a function of you activity level. The Aladin (I believe) claims to do something of this sort based on air consumption, but I have no idea of the algorithm and I doubt that there are any test results.

One big culprit in mechanism of DCS is stress-assisted nucleation referred to above. However, not a single deco device on the market today incorporates anything to account for this factor. In truth, they do not need to because they are already very conservative. Conservative is good – no one wishes to get DCS and go through the time and expense of a chamber treatment.

Dr Deco :doctor:

Please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Rev. Blade]

I just happen to have a square profile dive on my computer.... I went to see some sharks getting fed. A pretty cool dive considering I was kneeling in the sand. Since I had intentionally stuck an extra 10 pounds of weight in my pockets, there was very little bounce on the bottom.

Dive profile:
2 minutes to 71'
28 minutes at 71'- 72' (30 minutes dive time, 80' rounded down on my SSI tables)
3 minutes accent to 18' not counted as dive time as it is part of my direct accent to the surface.
3.5 minutes "stop" from 18' to 10'
slow to surface

My tables show 30 minutes at 80' as max NDL. My Suunto Vyper showed it as about 5 or 10 seconds past NDL. (It said I would need a deco stop, but cancelled it, because my accent rate allowed for some deco on the way to 18'.) If I had stood on the bottom, and been in 69'-70' of water, the tables would have given me another 10 minutes of bottom time.... All this assuming I had more air. Even though I was kneeling during nearly the whole dive, air management was not my main focus, the sharks had me somewhat excited. SAC for the dive was decent .54 SCFM.

Another thing that makes some computers "Conservative", is that they insist on the safety stop. If I don't stop for 3 minutes between 20' and 10' before the surface, my Suunto beeps. I get a minute to go back down, and do my stop. (If I drift down to 21', start the 3 minutes over. Another good reason to practice bouyancy control.) If I ignore the computer, it becomes a "gauge" (non-computer) until it says I'm safe to FLY!!!! If you dive a Suunto computer, and don't do what it says, it will stop telling you what to do. If you're a "computer only" diver, that means your done for the day... perhaps more. (It's also somewhat conservative on the "time to fly" countdown.)


FYI - take 2 feet off the dive (to 69') and Suunto says I would have 8 minutes NDL when I was 30 minutes into the dive. (Dive planner software) My tables say 40 minutes at 70' is the NDL. About the same between my computer and tables for a square profile dive.

 

[diatom]

From the posts here, it seems to me that the take home point is that by far the most important consideration is not what algorithm is used to minimize the likelihood of DCS; rather, it is how you plan and execute the dive. While the importance of dive mechanics is obvious, it is nice to understand the degree to which their importance overwhelms the differences among computer algorithms.

Thanks for all the info.

 

[Dr Deco]

Dear Readers:

DCS and Post-Dive Activity

Divers have it right when they note that executing the dive (especially post dive activity) is important. The differences in algorithms are really minimal for recreation (no deco) diving. Deep stops are important for decompression diving because they minimize the sequestering of gaseous nitrogen in tissue bubbles.

Most readers of SCUBA SOURCE are aware that decompression is a factor in all dives and does not end when you reach the surface. The tissues of your body are still supersaturated, although the degree of oversaturation falls with surface time. Physical activity will generate nuclei that grow to decompression bubbles very quickly. If you would perform a safety stop and prevent the growth of nuclei into gas-sequestering bubbles, why would you not also try to prevent microbubble formation during the surface interval? While it requires tens of minutes to hours to remove dissolved nitrogen, heavy exercise/activity will provoke bubbles formation in one or two seconds.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[zboss]

Would climbing the ladder with all equipment on then helping your buddies get their gear on the boat be considered bad? What about long boat rides on a small boat where you are getting banged around a lot?