Going into deco

[iceaxe]

Have there been any studies relating DCS/DCI with the algorythm used? I just bought an Oceanic VT3 and my buddy has a Nitek plus. I was comparing them in plan mode and the Oceanic has much longer NDLs even with the conservative factor on. I found that Suunto's RGBM actually takes into acount dives that were done days ago for figuring NDLs. What is the basis of this? Does the fact that Pelagiac computers don't do this make them unsafe? I was also curious to note that the Dive master at Bikini Atoll prohibits the usage of Oceanic computers--I assumed this was because at the time Oceanic did not have a computer capable of underwater gas switching--but now I'm not so sure. Sorry for the long post

 

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Since it wouldn't give him any credit for time served below 20 feet, he ended up with about a ten minute hang at 10 feet, which he did. Gas was no issue.

It does give credit for stops deeper than that. Provided you are above the ceiling it will off gas but at a slower rate than at the optimum depth. On shallow dives like that chances are it would off gas from 15m or so shallow and ceiling 3m < > 15m or so.

The up and down errors in the display show this, anywhere with an ^ displayed is off gassing but not optimum - 1 minute could take 90 seconds in reality etc. If you have both arrows, egg timer then its the optimum floor and the time is correct.

Suuntos tend to dial in lots of stops in particular if the dive is saw tooth, worse if saw tooth with one section faster than 10m/min.

They do not like deep stops below their effective ceiling though - it'll add on time in the shallows.

My question is: Who has deliberately gone into deco on a recreational dive,

Me, tens of times if not over 100.

and how did you figure out how much deco to do?

Follow the computer but making sure its sane with reference to last ditch backup tables. Also helps to check gas remaining.

Did you do what your computer asked for?

Always what it asks for, often more. Deep stops it doesn't ask for but i do anyway even if it does give me longer shallow.

Did it make any sense?

Yep perfect sense. Has always reacted exactly as i expected it to.


I dont use it for proper planned deep deco dives (v-planner for that) but light amounts of deco on a profile that im unlikely to know until down there, recreational profile if you like i have no issue in going 15-20 mins into deco provided ive got the gas.


Its also worth noting that despite their claim Suunto RGBM isn't actually RGBM. Its very basic Buhlmann with some very crude RGBM steps fudged onto the end.
If i follow my computers deco it gets me out of the water but often feeling like crap. Generally i make stops every 3m from 21m to the surface with it rather than rush up to the optimum decompression depth and sit there for the duration (ie bend and mend).

 

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FINDINGS: 1. Historically neither the U.S. Navy nor the commercial sector have prohibited reverse dive profiles. 2. Reverse dive profiles are being performed in recreational, scientific, commercial, and military diving. 3. The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any definite diving experience that indicates and increased risk of DCS. 4. No convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS. CONCLUSIONS: We find no reason for the diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 msw (130 fsw) and depth differentials less than 12 msw (40 fsw).

However there was another people some 18 months after this workshop that showed a massive increase in bubbles (via doppler) of something like 60% for reverse profile divers. Will try to find it here (its on paper) and pass the reference.

 

[lamont]

If you do a dive on a Suunto where you've forgotten to set it to EAN32 and the computer goes into deco you'll also get the little exclamation point warning signal even if you do the EAN21 deco that the computer is calling for.... And from then on it will mercilessly punish you, even after 24h it'll be giving you massively reduced NDL times...

Suuntos have a nice gauge mode though...

 

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If you're lucky you can get the exclamation point, if not you'll get full error and it'll refuse to dive for a day or two afterwards. Got caught out with this when i entered my mix during a food break, took too long diving it, it reset to air due to time period and i forgot to check. Bent it.

It also dislikes you violating what it thinks is 55 mins of deco if you forget to put it into gauge mode before an accelerated deco dive. Operator error on both accounts there though.

They have good download features in SDM v1.0 software as well for logging even in gauge mode. (v2 is crap)

 

[Bismark]

I know that a lot of people do know what a computer is simulating but it seems that the more I learn about deco, the more conservative I get. Most people are just "most people" and not everyone so no need for anyone to reply specifically. Good point about the fact that most people would then be placing their life in the hands of something else they don't understand, that being tables. I certainly am not the expert here and lots of people on this board know a hell of a lot more about deco than I do. I just don't like trusting my life to something I don't actually understand. The information on a computer is just as good as the software designer who made the program to run the thing, and it seems like with so many apples and oranges approaches to deco, we (as those ultimately responsible for our own wellbeing) owe it to ourselves to educate ourselves on the topic as best we can. (I know that last bit was the worlds worst run-on sentence but I am overwhelmed at work today.....)

 

[halocline]

One nice thing about the Pelagic/Oceanic/Aeris line of computers is the graphical display of N2 loading on the bargraph. I pay less attention to NDL than I do to the bargraph and what my depths have been during the dive.
Charlie Allen

I do this too, as the NDL on my aeris towards the end of a dive is ridiculously high. You can be an hour into a dive, down to 500 PSI, at 15-20ft with the N2 in the yellow, and show 2hrs+ of NDL. That's a misleading indicator, where the N2 bar is at least a graphic calculation of your N2 loading.

There are no studies (that I know of) that demonstrate any corollation between computer algorithms (liberal vs conservative) and incidence of DCS, even though many people buy suuntos on the (mistaken) belief that they'll be safer. Just another example of liberals getting a bad rap!!

Regarding the often argued opinion that people using computers are sometimes trusting their lives to a microchip, I guess that's at least in theory true. More accurate a statement would be "There are many divers who don't dive responsibly and expect their gear to keep them out of trouble" or something like that. There are so many safety factors that computers are not aware of, like dehydration, exposure protection, general cardio vascular health, and a simple respect for the dangers inherent in diving that results in more informed and cautious dive practices.

Okay, I'll try not to stub my toe stepping off the soap box

 

[DeepBound]

Again, my impression on reading most of these posts is that the average diver just buys a computer that suits their level of conservatism and even though they might not understand it, they are happy with it and just follow the computer prompts. This scares the bejezzus out of me as I have some pretty fancy equipment in my business that regularly breaks down for want of a 3$ part. I for one do not want to trust my life to a piece of equipment with parts made by the lowest bidder and assembled by somebody who will never use it. Bob, like you, the more I learn about Deco, the more Ratio Deco done by me before a dive and then adjusted if necessary by me with my bottom timer/depth gauge and back up bottom time/depth gauge makes me feel safer.

I actually had a computer malfunction on me, in that it kept showing the time and depth correctly, but the NDL got 'stuck' at 20 minutes. I eventually realized that teh NDL wasn't changing, and at the end of the dive, told my buddy, and we did about 15 minutes of deco based on what his computer was telling him. My computer was still saying that I wasn't in deco. It turned out that a component had broken off the circuit board inside the computer, which affected NDL calculations, but not the other functions. Who woulda thought that could happen?

 

[ChikkenNoodul]

I use tables generated from V-Planner for dive planning - use the computer as a backup, haven't sent my Oceanic Veo 250 into deco yet even with a couple of light deco (According to doppler tables and V-Planner) dives.

One interesting recent experience with the Oceanic however, we did two dives in the morning on 28% and somehow the computer reset itself to 50% O2 from 28% before the afternoon dives. On the third dive it was flashing me a ppO2 of 2.5(!) and did not return to normal function until ascent past a ppO2 of 1.6.

Now surprisingly, this did not 'break' the computer and it functioned just fine for the fourth dive dispite the fact that according to it my O2 clock was maxed for the day.

The self-reset of O2 % must have been a known bug then

 

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somehow the computer reset itself to 50% O2 from 28% before the afternoon dives. On the third dive it was flashing me a ppO2 of 2.5(!) and did not return to normal function until ascent past a ppO2 of 1.6.

Now surprisingly, this did not 'break' the computer and it functioned just fine for the fourth dive dispite the fact that according to it my O2 clock was maxed for the day.

The self-reset of O2 % must have been a known bug then

That is a design feature (safety) on the older Data Plus. It can be a bit of a PITA but you get used to it quickly. I suspect your manual will tell you it is the same with your VEO 250.