Mix choice

[rjack321]

what bottom time? what algorithm? how much time at 140? let's pick a realistic profile here...

Things I know/can reasonably guess at if this ends up being a "deep" system.

3600 psi fill in hp100s/lp85s, willing to use 1000psi of that = 60cf
140ft average (a bit deep to end up as the average depth but going with this as a deeper bound)
My SAC in 48F water = 0.5cf/min (realistically not any lower than this)
140ft = 5.25 ATA
DCR = 2.6cf/min
60cf/2.6cf/min = 23min BT @ the 140ft average.

I don't see myself doing a 20+ min dive at a 140ft average though. I might hit 150ft as a max, but I have yet to dive a North Island cave which is at a consistent depth. They are either going up or going down. The Quatsino formation is all tilted and and folded so its nothing like the basically horizontal bedding of MX or FL limestone.

So while the 140ft average is a deeper bound, a more realistic "deep" profile is 150ft max depth, an average depth of 110 or 120ft, BT of 30mins.

 

[LiteHedded]

exactly. of course you aren't going to incur a deco obligation like he's talking about. with 20 minutes of just wiggle room between the two.
we're not looking at what's the max depth and max bottom time you could squeak out with those tanks. you're obviously capable of keeping your dive reasonable. and you'll have to with the limits you're talking about logistically.

 

[rjack321]

I honestly have never done BG deco before...

Its one hella a long way from "help" though. 400m of cave with 5 rope drops to reascend, a 150m hike through the logging slash, and a 30min drive to the nearest hamlet with a nurse's station. Nearest chamber is a 8+ hour drive+ferry or 90min helo ride.

I'm starting to reconsider the BG deco and thinking about bringing a small deco bottle. Just watch, with my luck I get all this crap in there and the cave goes 35ft into a gravel strainer.

 

[LiteHedded]

I honestly have never done BG deco before...

Its one hella a long way from "help" though. 400m of cave with 5 rope drops to reascend, a 150m hike through the logging slash, and a 30min drive to the nearest hamlet with a nurse's station. Nearest chamber is a 8+ hour drive+ferry or 90min helo ride.

I'm starting to reconsider the BG deco and thinking about bringing a small deco bottle. Just watch, with my luck I get all this crap in there and the cave goes 35ft into a gravel strainer.

that is what I would do. but I have done recon in caves with no o2 in the water.
if it went deep quick i'd probably call it a minimum deco dive and be done with it. tie off and come back with the proper gas, or if i couldn't, not come back at all

 

[rjack321]

that is what I would do. but I have done recon in caves with no o2 in the water.
if it went deep quick i'd probably call it a minimum deco dive and be done with it. tie off and come back with the proper gas, or if i couldn't, not come back at all

That's one reason why I considered something like 25/25. I'm only hitting 140-150ft by myself if its super "easy". I would dip to 140ft on 25/25 in easy conditions. If by chance it ends up shallower then I'm a little further away from deco limits at 110ft compared to 18/45.

The coming back with different gas options are extremely limited. There's no dive shop and I don't really want to bring my compressor. I doubt my sherpas would want to setup once, abort due to depth and then return again a day later to push in all likelihood ~50ft deeper.

I have concluded that 32% is just too limited and any survey I get done would be unreliable crap.
25/25 is a possible choice, I'm familar with it. He content is a bit low for a super clear head but if the vis is terrible I'm not going to be pushing depth anyway.
21/35 gives me a slightly clearer head, 5mins less NDL time but a little more realistic for getting decent survey data if it ends up a 120ft dive.
18/45 is a bit too rich on He for me to be doing a BG deco dive. And I don't see myself getting into the depth range where I'd actually need 45% He (>140/150ish).

I am certainly willing to consider a non-standard mix, just throwing out where I'm landing right now which is either: 25/25 and air tables, OR 21/35 and air tables + 5mins shallow time.

I will admit I don't like the thought of rapelling an O2 bottle into a cave given the tank+valve explosion report recently released from the Tampa incident. That's steering me away from humping a deco bottle down there.

 

[LiteHedded]

25/35?

 

[rjack321]

25/35?

Maybe. O2 more in line with realistic profile. He more in line with getting good survey data, but not so high that BG deco gets scary.

 

[LiteHedded]

ya. i'd probably dive it like 3030 and call it good

 

[rjack321]

ya. i'd probably dive it like 3030 and call it good

Good use for that vplanner thingie I bought years ago and almost never use!

 

[Dive-aholic]

what bottom time? what algorithm? how much time at 140? let's pick a realistic profile here...

I was planning worse case scenario of square profile with 30 minutes at 140 using VPlanner +2. Total run time on 21/35 is 86 minutes. Total run time on 18/45 is 105 minutes. Neither is desirable but with limited options sometimes you simply have to accept things for what they are and go with the least of the two.