shallow water deco?

[SailNaked]

recently on the fourth dive of the day we had sand at about 35ft and towards the end (maybe 50 min,) my wife noticed that she was out of bottom time, she moved up to her safety stop and in the few seconds it took her deco was showing more than 10 min. after a few minutes the deco had built to over 16 minutes, she moved up to 10 ft as that was what the aeries computer was telling her to do, however they were drifting into a cruise ship safety zone and she was not allowed to complete the obligation. After I got over my emotions about the situation and was thinking again I made the decision that 10 ft was not a significant pressure difference from the surface and that she would likely be ok. and she was.

later as I think about it more, I believe that what happened was that she exceeded a slow compartment, thus the fast buildup of deco and the no credit below 10ft. anyone know the history of diving under 40ft? apparently they only asked haldane to come up with a theory when trying to exceed 45ft, did they have unlimited diving before that?

Im sure some believe I was lucky, but I am not so sure that sort of deco is not a function of the algorithm and not actually a serious concern. either way I thought it might be a good discussion. I will download the computer info later tonight, and see what exactly happend, but I am interested in knowing what yall think about it.

 

[DevonDiver]

As you suspect...it's a tissue compartment issue. Repetitive (multi-day too?) dives stacked up her slow tissues which then became the controlling compartments for the deco. That can lead to some 'unexpected behavior' from your dive computer, if you dive with the assumption that deco will always clear quickly on ascent.

When I was working in Thailand, diving 4+ times per day, 6-7 days a week, my SDM was showing me hitting the water in the AM 90%+ saturated in the slowest compartment. Scary implications should a fast ascent occur...and something dive pros certainly need to be aware of (especially when deciding to arrest that panic ascent by an OW student, or when conducting multiple CESA skills). I wish I hadn't lost all that info on when my hard-drive clutzed up..

Downloading the dive/s will provide some good insight. Share them if you can.. it's good to spread the education...too many divers 'surf the curve' using their dive computers. Some parameters can bite them in the ass.

I made the decision that 10 ft was not a significant pressure difference from the surface and that she would likely be ok.

Seriously flawed thinking/lack of knowledge from someone at your certification and level of responsibility as a dive educator..

 

[Bob DBF]

It may be a dive computer issue. My buddy got popped with a large deco after one of our dives, we kept him under 'till the air ran out. Our backup plan involved the fact that my computer was fine with the dive. Later we ran the dive with another computer from the logged profile and computer should have been OK. It seems that under certain circumstances his computer adds substancial deco penalties to a dive.

Even though we thought the computer was zooming us, we conducted as much of the deco it specified as we could. In your case, I would have insisted on a fast move, less than 5 min total, and dropped back in to finish the deco, since the infrastructure was available to do it.


As for the 10' deco stop, that "was not a significant pressure difference from the surface" is the pressure difference between happy and bent. Talk to some tech divers about why holding a deco stop is important.



Bob
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I may be old, but I’m not dead yet.

 

[SailNaked]

As for the 10' deco stop, that "was not a significant pressure difference from the surface" is the pressure difference between happy and bent. Talk to some tech divers about why holding a deco stop is important.



Bob
-------------------------------------------------------
I may be old, but I’m not dead yet.

Yes bob I absolutely agree, about that,
to be a little more specific, I was only thinking that with a shallow deco obligation that it may not be an emergency. if the obligation stems from only one atmosphere difference and it requires a lot of minutes at 10 ft then the difference between 10ft and the surface may not be out of bounds. and I had little to do about it at the time. She was pissed at me for being upset, and we had to get out of the water or be shot by the federallies. lets just say I was hoping that 10ft for a slow compartment was not going to cause the bends.

this dive was a mistake!. Even though I thought we had learned that you can go into deco on a 33ft dive and that it builds up relatively quickly when it happens, she should have been monitoring her watch better, and she should have gone to 10ft immediately. My watch was reporting no such problems and while our profiles were similar they were obviously not the same. I am very glad it worked out ok but this is a lot closer to the line than I would like to be.

 

[supergaijin]

What computer was she using? Aeris.....

If I read correctly, the order of events is:

1. Run out of bottom time.
2. Perhaps a slight delay before ascending (getting buddy's attention etc.)
3. Go to 15' or normal safety stop depth (computer at this point asking for 10min deco stop)
4. Computer continuing to add more deco obligations at this depth? (assumming approx 15' depth)

I'm very surprised to hear that during the first deco stop extra deco obligation was being added by the computer. Perhaps the diver descended just a bit, out of the computer's desired depth range?

 

[SailNaked]

Super, no she was getting less than 1:1 credit at 15ft, the computer was only going to clear the obligation at 10ft, and we usually do our safety stops at 20ft and it was adding time at that depth. it is an aeries manta.

we have seen this before, and it happens when your obligation is from shallow depths, to many days/time at 30-40 ft

 

[rjack321]

You really think that there's a difference between a 15ft and a 10ft deco stop?? There isn't, its completely a computer issue. That computer was not going to give her deco "credit" for even an 11ft stop.

Cheap solution, understand what your computer is telling you.
Expensive solution, get a better computer with a modern algroithm capable for giving you proper credit for intermediate deco stops.

 

[p1p]

I once had a similar situation like that with a recreational computer, that suddenly started adding up deco obligation like a slot machine. pounded me with 20m of deco. After that I crap canned it, and bought a non recreational unit with better modeling, and havent regretted that decision since. Some recreational computers will heavily pile up deco for committing sins you arent even aware of.

 

[CT-Rich]

hmmm, interesting... I was watching a BBCprogram on Youtube today and was thinking about posting a link. It was about the effects of air diving on proffessional divers in the North Sea. It was not painting a good picture. I will say that I am not sure exactly which tables these divers were using (the vintage of the show seemed to be 1989 according to some google research), but they were talking about deco hits that were not noticable to the divers themselves, but were detectable histologically.

https://www.youtube.com/watch?v=5UugcnA3zKM

 

[SailNaked]

You really think that there's a difference between a 15ft and a 10ft deco stop?? There isn't, its completely a computer issue. That computer was not going to give her deco "credit" for even an 11ft stop.

.

No I do not think there is a "significant" difference between 15 and 10ft, the computer did. (.15 atm -- .45 - .30)
I also assumed that if the obligation was generated by a compartment that was between 40ft and the surface, that the reason the computer required 10ft was because otherwise there would not be a significant delta to off gas. with that assumption I hoped that 10-0 would also not be a "significant" difference to cause the bends. .3atm

I do wonder about an algorithm that builds deco at 2:1 for time in the 35-10ft range but that is really my question is this sort of deco worth worrying about or can it be ignored. we had 3 other computers that had no issues but the difference could have been a few minutes on the previous dives +10 ft or so. I can look at the averages of my computer against hers, maybe that will provide insight maybe not. i have heard people complain that their watch gives them a penalty for fast ascents.

how much of the algorithms are just math guesses and how much is based on real research, I think we know the answer to this and I think we got fouled in the math and not the reality but it is also very scary to have your wife leave the water with lots of deco time still showing, I have read all of the Doria books and the like, so I have vivid pictures of that kind of death.

I think this may go off the rails but Anyone know what the delta atm/time is that will cause the bends? this may not be the place for this so please ignore if appropriate.