Avoiding DCS with GF and MB settings

[ChuckP]

A few recent things has me thinking about this

This thread https://www.scubaboard.com/communit...cozumel-until-chamber-bill-paid.563018/unread specifically where a few talk about DCS hits that occurred within recreational depths but they don't say what caused the problem.

This thread Where Is Your GF? mostly because I don't have any understanding of GF's

And finally on a recent dive: I had been in the water more straight days than the DM, both breathing same gas, 32% up until this dive which was on air. I have a G2 set at MB 0, she had a Shearwater set at ?? but she said it was conservative as she's had DCS before. Plan was no deeper than 110' (Cathedral in Cozumel) hit 10 min to deco and we'd get shallower. During one of the swim thrus at 100', my computer says 9 min to deco but we were ascending, we had meet another group coming in the opposite direction (plenty of room) and I commented to her after the fact that I'm glad we didn't stop as I was at 9 minutes, she said she was at 6 minutes at that time - this was pretty early in the dive.

This was the first dive I had spent "time" at depth and was glancing at my computer a lot - paying attention to how the NDL time changed as depth changed, sorta watching gas consumption, that kind of thing. Never at any time was I close to entering deco per my computer, but was closer when looking at her computer so....

These DCS accidents? Are they mostly due to rapid accents or not paying attention to the computer?

That leads us to the computer - trying to avoid the Shearwater/ScubaPro debate but - how do you know what setting is right for you? This certainly can't be trial and error based on if we lived or not. I want to believe it has to do with the type of diving we are doing and physical condition of the diver etc... But how and why?

The diving I currently do is recreational, greater than 95% of my dives I'd expect to be shallower than 80' with gradual accents staying well out of deco range but there again, where is deco for me and what settings are best for safe but not ridiculously conservative?

 

[hhmoore]

I have nothing to offer; but will be interested in the responses

 

[Lorenzoid]

There is a whole lot to your excellent post. Let me take a stab at just one item.

. . .
These DCS accidents? Are they mostly due to rapid ascents or not paying attention to the computer?. . . I want to believe it has to do with the type of diving we are doing and physical condition of the diver etc...

I don't think there have been studies that definitively say "doing X (or not doing X) is what causes the most DCS incidents." As you recognize, a lot of it may have to do with individual physiology, including not just being in good physical condition but also things about our bodies that we can't change. Someone who has suffered DCS before might do well to crank up the conservatism on their computer or proactively stay well below the displayed NDL. There has also been discussion of divers increasing conservatism as they age. Beyond that, I think the answer is that nobody knows for sure, and "medium" conservatism for younger divers in good physical health seems perfectly prudent. Not many people on SB advise "low" conservatism--maybe that's something Navy seal types might go with.

Regarding "ascents," whatever amount of conservatism one chooses to apply, diving conservatively most definitely includes adhering to the currently recommended ascent strategies in recreational diving. For many--and I think this was the crux of a DAN study--that may mean a steady ascent rate of 30 fpm, and at least a 3-minute safety stop at around 15 feet.

 

[RyanT]

The short answer is that there is a lot we still don't understand about decompression. As evidence, probably some of the most heated debates here on SB have been over decompression models! What we do know, however, is that for recreational diving, the modern decompression algorithms are quite safe. To put it in perspective, back in the day, people were diving the Navy tables that had longer NDL limits than any modern algorithm, and maximum ascent rates were 60 fpm (instead of 30, as is typical now). In addition, I don't think safety stops were so commonplace as they are now. Even so, people were not getting bent left and right. Now with more conservative algorithms, the chances of getting bent on a recreational dive are very low.

Yes, people do still get bent. Given how infrequently it happens (relative to the number of dives that are made) we tend to take notice and want to know the cause. In some cases the causes are identified (e.g. panicked diver bolting for the surface), but sometimes it just isn't clear. Everyone's physiology is a bit different, hydration rates, and physical fitness, and physical exertion probably play a role. These things can vary from day to day and divers have been bent doing the same profile they previously dove without incident.

What are some things you can do to minimize your chances of being bent? Probably what you are already doing. Don't dive right up to the NDL limits, make sure you are really doing an ascent rate no more than 30 fpm, and do a nice long safety stop at 15'. Additionally, if you're not diving nitrox to the NDL limits, that will provide a little safety margin as well.

 

[Miyaru]

There is no hard boundary that says you're within the NDL or beyond. First of all, it's just a number on a computer screen that follows a predetermined model, and a dive computer doesn't know anything about what actually happens inside your body.
In order to still have significance, computers calculate on the safe side and show you a NDL that will most likely get you safely back to the surface.

Unfortunately the recreational diving courses hardly touch the subject of decompression. It basically comes down to shaking a carbonated bottle of beverage (time spent at depth), followed by carefully opening the bottle. The rate at which you unscrew the cap, is similar to the ascent speed. The goal is to not see any bubble in the bottle, while you still hear the gas hissing out of the bottle.
Recreational diving means you can keep unscrewing the cap slowly until you take it off (=surfacing) while you never saw a bubble.

Sometimes, you have to stop unscrewing the cap any further, and let the gas just hiss out while you wait. That's a deco stop.
You're in deco when you reach that point: if you would open the cap a little further, bubbles would form. And that goes beyond recreational diving, requires more knowledge in both planning and diving.

Back to your example. If you would dive with two different computers on the same arm, you will see differences in NDL. One computer will say you still have 9 minutes of bottom time left, while the other shows only 6 minutes.
That difference is caused by more than one factor. First of all, the decompression model that the computer uses. Next, the level of conservatism. Gradient Factors are suck a conservatism setting for gas models. And the time spent a specific depth plays a big role. If these 2 computers are used by different divers, doing the same dive, you'll see even bigger differences since your dive profiles will differ!
Back to the bottle: you can use a computer to open the bottle. The computer can measure the pressure of the gas, but cannot look in the fluids for any bubbles.
It will just tell you if you are opening the bottle too fast.

 

[100days-a-year]

It is my solely empirically based opine that recreatioal DCS is more a function of total time at depth and the diver's predisposition to DCS that any modern decompression " theory "
I dive 5 to 7 dives a day with a group of other commercial spearfishermen and those who stay longer on dives had far higher rates of incidents even if the total daily bottom time was equal , if that makes sense.

 

[ChuckP]

I feel pretty well versed and have a good understanding of diving where I'm at and just a tad beyond but I am wanting to learn more, advance my training and maybe even train in the future.

I like the shake the bottle description and have heard that before thru training.

What I haven't heard or read much about is the GF / MB settings and how it applies to folks - I've seen some dive profiles at a,b and c but....

51 years old, decent shape but I smoke, maybe I shouldn't be at MB zero. I'd just rather understand that now verses a physcian telling me that thru an intercom.

 

[scubadada]

Hi @ChuckP

You are running your G2 at the most liberal setting, 0, from a range of 0-9. Even at this, the ScubaPro deco algorithm is one of the more conservative. There is a link to the four simulated hyperbaric chamber dives from the ScubaLab 2017 testing in the attached article. Shearwater 45/95 and 40/85 were tested at the same time for comparison. 11 New Dive Computers Tested By ScubaLab In 2017

All computers assume you are ascending at the recommended rate, this is decompression you are doing on every dive.

 

[leadduck]

I think most recreational NDL divers' undeserved hits are caused by dehydration and post-dive exertion, and then of course lung barotrauma by too rapid ascents. I wouldn't care too much about computer settings with NDL dives but rather get these "soft factors" done right first.

 

[tursiops]

It is my solely empirically based opine that recreatioal DCS is more a function of total time at depth and the diver's predisposition to DCS that any modern decompression " theory "
I dive 5 to 7 dives a day with a group of other commercial spearfishermen and those who stay longer on dives had far higher rates of incidents even if the total daily bottom time was equal , if that makes sense.

What a scary post! It seems to presume that since deco theory is not perfect, that nothing is known. It misses the point that staying longer at depth is EXACTLY what deco theory is about, it is not a mystery, it is not a guess, it is not a coincidence. Because we know (from deco theory.....based on endless experiments and much testing) that two dives to 100 feet for 15 minutes each (with a surface interval in between) does NOT produce the same nitrogen loading as one dive to 100 feet for 30 minutes, we can try and avoid DCS. Saying deco theory is just a "theory" and then ignoring it is the height of silliness, bordering on stupidity.
Yes, I feel strongly about this.