Decompression planning and cave diving

[karstdvr]

Unless someone is doing exploration,then most cave information is well documented,and it is prudent to ask questions of people before entering a system you've never been in. Your's is a very good question because all too often I see people take a full set of tanks,and perhaps a stage;then do their dive with inadequate deco gas. By inadequate deco gas I mean a small capacity tank where there is no reserve which allows for delayed exit,or a buddy's gas failure. When I first started cave diving the norm was a steel 72,mainly because there weren't any available haskels,but this large capacity was very handy. To this day I never use an aluminum 40,but prefer a steel 46 where I have the extra volume at 3000psig. This came in handy once when after a longer dive when my buddy lost the gas in his deco tank,so we were able to use my tank to complete both our decos. FWIW,still have my 72 with O2,and keep handy for real deep dives,but I have an 85 that has been converted to this role.

 

[Superlyte27]

I'd hate to imagine the deco I would have to do to drain an AL40. Most dives, I can do my deco on about 400psi or less in an AL40.

Hmmm, lets do the math.

20' / 33 + 1 = 1.6ata. x .3 SAC = .48cu' per minute at 20'. If I did all of my last stop at 20' my tank would last me over 80 minutes. Or, that gives me 40 minutes for me, and 40 minutes for my buddy. BUT, I don't do all of my last stop at 20'. I do my last stop at 10' so my consumption at 10' would be .39cu' minute at 10'.

See what I'm getting at? A 180min bottom time at Ginnie with an average depth of 100' requires 22 minutes at 20' and 42 minutes at 10'. So, I would need 11cu' at 20' and 16.38cu' at 10' for a total of about 28cu'. This is obviously an extreme example. But this above dive has a total run time of 311 minutes with 131 minutes of that being deco. If you shorten this dive to something a bit more realistic, like a Total Run Time of 3 hours, and an AL40 will easily be enough gas for you and your buddy, provided he's not an air hog. And at this stage of the game, he should have any SAC issues resolved.

 

[PfcAJ]

Fwiw, the avg depth of ginnie isn't 100' (the max might be, the the avg certainly isn't). You can get out of the water way faster than the time you listed for that dive, and a 40 is enough for any bottom time you're going to encounter there, even with a reasonable (1.5x) reserve.

I'm all about steel o2 tanks and deep cave diving, but I don't see a reason to drag a heavy steel along if its not needed.

Now, if you call the avg depth deeper than it is, and plan dives longer than you even have backgas (plus a stage) for, then I can't help you

 

[TSandM]

For me, almost all my cave diving is not goal-oriented -- by that, I mean that I don't have a place I am trying to reach, really. We decide before we go in, how much of the gas we are taking with us we are going to use (thirds, or something more conservative) and we dive until someone reaches that point, and we turn the dive. Since I do the vast majority of my cave diving in Mexico, the question of how much gas to bring is determined by whether you want to dive to the limits of a set of doubles, or one or two stages. As yet, I have been unwilling to haul two stages, so my gas determines my dive, rather than the reverse.

 

[Superlyte27]

Fwiw, the avg depth of ginnie isn't 100' (the max might be, the the avg certainly isn't). You can get out of the water way faster than the time you listed for that dive, and a 40 is enough for any bottom time you're going to encounter there, even with a reasonable (1.5x) reserve.

I'm all about steel o2 tanks and deep cave diving, but I don't see a reason to drag a heavy steel along if its not needed.

Now, if you call the avg depth deeper than it is, and plan dives longer than you even have backgas (plus a stage) for, then I can't help you

I've gotten average depths at Ginnie close to 100 on very long dives. But, you are right, most are 87-90'ish

 

[karstdvr]

I'm all about steel o2 tanks and deep cave diving, but I don't see a reason to drag a heavy steel along if its not needed.

In the grand scheme of things, a steel 46 is not tremendously heavier than an al40,but carries at least 12cuft more-will take that any day.

 

[PfcAJ]

Beats a 72, I guess.

 

[karstdvr]

Beats a 72, I guess.

Nice thing with the 72,was when we used to dive Little River in the early 90s,and the flow was spanking strong,the deco bottle would pin you to the bottom so you didn't shoot out of that place like a cork. I knew some WKPP divers that used steel 95s for deco,that was a killer tank.

 

[PfcAJ]

I use a 72 on deeper dives, others use 85s, 95s on occasion (all for o2). Take what you need.