Planning dives so as not to go into deco

[grassgreen]

I have done my full cave course with IANTD, and this wasnt taught to us, i dont know if it was missed or isnt in the course?

Anyway, my question is: how do you plan a dive as not to go into deco if you dont already know the exact depths of the cave your going to? when to turn and return?
For example , you have been diving for 30 mins and you are now getting shallower, your computer says you have 30 mins until you go in to deco but if you now turn and go back the way you came you will be going deeper, therefore the minutes until deco will decrease, therefore you are not back in time.......!!!

Thanks Marc

 

[Jim Lapenta]

In the dive planning phases of my ow and aow courses contingency plans are covered. It sounds like you are talking about expedition type dives.

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[TSandM]

What we did in both my C2 and Full Cave classes was sit down and look at the map -- the map will give you the depths in various parts of the passages where you are going. You figure what the average depth of the deep section is, and then you look at your no-deco limits for that depth. Then you figure your gas duration for that depth. Whichever one you hit first will turn your dive.

For example, Mayan Blue has an average depth for a lot of the passages between 60 and 70 feet. On 32%, that gives me roughly 50 minutes of no-deco time. 50 minutes, with a SAC rate of 0.7 (average adult male) at 70 feet, will use just about 100 cubic feet of gas, which is 2/3 of a set of Al80s (one third going in, one third coming out). So deco and gas will turn the dive at roughly the same place, about 25 minutes in. (In reality, because the first portion of the cave is shallower, the average depth will end up more like 50, and gas will turn the dive -- and my SAC rate isn't .7, either.)

 

[grassgreen]

yes, cave dives where you cant go directly to the surface or choose your depth.

 

[Peter Guy]

grassgreen -- it is an interesting question and one that is pretty hard to answer. As far as I know, NO deco algorithm really works for caves -- especially ones that have a significant deep and then shallow portion because "real dives" don't go deep, then shallow then back deep again.

Whereas TSandM's instruction had her look at the map and depth average (which works if you have a map but not in your hypothetical), my instructor told me "If all else fails" just to plan the deco based on the deepest depth -- which is undoubtedly the "most conservative" method.

Cave Deco is, again as far as I can tell, a true example of each one of us running an experiment on our own bodies every time we do one of these dives.

 

[grassgreen]

Ah right, get it now, so you have to have a map of where your going and plan that way, thats what i thought, but as it was never mentioned i wasnt sure. cheers tsand m

 

[TSandM]

Yes, and if you're doing exploration diving and have no map and no clue, you need to have a) some kind of method or structure for assessing your deco status as you go along, and b) play it conservative with gas supply and deco gases.

But for a newly certified cave diver, you should be in places where there is plenty of information available, so make use of it!

 

[BabyDuck]

i'm surprised it wasn't covered. it would have been covered somewhat in the nacd class, but my preference was to take adv nit/deco proc during app/full. many folks feel the bit of coverage in app/full is enough. anyway, it's off-putting it wasn't a part of your class.

 

[boulderjohn]

i'm surprised it wasn't covered. it would have been covered somewhat in the nacd class, but my preference was to take adv nit/deco proc during app/full. many folks feel the bit of coverage in app/full is enough. anyway, it's off-putting it wasn't a part of your class.

I think I am close to this in my response. I don't think you should be doing a dive that MAY take you into decompression if you don't know what to do if you go into it. If you think it is going to be close, then you should be prepared for it, in which case it is no big deal. If you are not prepared for it, though, it can be a very big deal.

 

[ucfdiver]

When I was basic cave, I just used the rule 1min deco for every 1min I went over NDL. I always tried to stay within it, but sometimes I was 2-5 minutes over by the time I exited the cave.