Mix choice

[rjack321]

When diving a complete unknown cave, far far far from civilization, what mix would you bring and why? Its a major effort to get to the sump with water, including 5 rappels. You have sherpas for one dive, possibly a second dive but that's not likely. Water's about 50F.

"Bring a CCR" is not really a viable answer, this assume this is OC and hp100s for tanks.

 

[Superlyte27]

30% Banked Nitrox purchased from my favorite dive shop Cave Country Dive Shop in High Springs, Florida

 

[LiteHedded]

18/45 always

 

[rjack321]

30% Banked Nitrox purchased from my favorite dive shop Cave Country Dive Shop in High Springs, Florida

High Springs is 3000 miles away...

EAN30?
No freakin' way. MOD too shallow, END too high. I forgot to mention caves in this area go up and down ALOT. If you couldn't figure that out from the 5 rappels (~200ft descent) to even get to the sump.

Nevermind that my surveys at >80ft END are total crap.

18/45 always

Possible, but I don't see myself getting below about 140-150ft solo in a completely unexplored cave on about 60ft of penetration gas. Bringing a stage doesn't help much, only getting the useable penetration gas up to ~80cf. Best I could bring along for deco would be 30ft of O2 and that's a huge extra effort, probably an extra sherpa just for that. I could slip in one of my 13cf tanks of O2 without requiring an extra sherpa. But that's almost pointless.

My gut reaction was to bring 25/25 or 21/35 since I have limited gas, limited depth, and minimal deco.

 

[LiteHedded]

if that's your max depth then you answered your own question. I'd probably take 18/45 anyway as it gives wiggle room for ****s n giggles and doesn't have much downside. the deco difference from 21/35 isn't even worth considering on a dive like this.
if that's all your gas you wont be doing much real exploration anyway and wont require much deco gas. if it were just a quick recon I might even do it without o2.

 

[rjack321]

if that's your max depth then you answered your own question. I'd probably take 18/45 anyway as it gives wiggle room for ****s n giggles and doesn't have much downside. the deco difference from 21/35 isn't even worth considering on a dive like this.
if that's all your gas you wont be doing much real exploration anyway and wont require much deco gas. if it were just a quick recon I might even do it without o2.

For all I know it goes straight down. Not likely though.

I don't think I'll have the sherpas to get O2 down to the sump. I will bring some O2 to the entrance although that's a long way up. I have some 95s doubled up and I could split them. But bigger tanks are not really plausible, nobody's ever done sumps up here with tanks >100cf.

 

[LiteHedded]

my concern would be getting all the way up there and not having the gas to follow the cave. but you've got a lot of logistical problems that are gonna keep you from getting a ton done at depth
18/45 will let you get in there and at least figure out what's going on to set up a proper dive the next time

 

[rjack321]

my concern would be getting all the way up there and not having the gas to follow the cave. but you've got a lot of logistical problems that are gonna keep you from getting a ton done at depth
18/45 will let you get in there and at least figure out what's going on to set up a proper dive the next time

The cave was first surveyed down to the sump in 1975. I'm the first person in 40yrs trying to go back and actually dive the sump. We don't have a whole lot of motivated sump divers up here. Tried dye testing it last weekend and that was a bust so entry from the downstream end remains a mystery.

I may end up with 18/45 in the end. I mostly wanted to hear other opinions and reasoning.

My other big decision is wet or dry. The thought of ripping a drysuit down there is pretty unpalatable. Wetsuit ditto. But I do think my wetsuit botties would be alot better than turbo soles or the floppy rubber things on my hog shell suit.

 

[LiteHedded]

can you put socks on it? then wetsuit booties over the socks. one of my suits is like this and it works pretty well

 

[Trace Malinowski]

Generally, air in an area in which trimix is too costly or not available. Or, the dive might not pan out and you are burning an expensive fill. Air is "good" to 130 according to cave diving standards for END and it definitely has greater range if needed. If you can get the sherpas to carry a small gas or electric compressor (depending upon what power source you'd prefer) you can remain on site for days exploring.

From what you describe, I'd say you are right with the 25/25 or 21/35. But, if you want to split the difference use 18/55 (boost helium a bit beyond the DIR standard gas content) because it's a 200 foot gas extended to 220 if you wanted to increase the PPO2 to 1.4 ATA. A Nitrox32 top-up around 1400 psi will give you the UTD standard gas of 25/25 which can be used at 140 - 150 feet at ppO2's of 1.3 and 1.4 ATA. A rock bottom plan should keep more gas in the tanks from which to top and keep the gases close to "standard" mixes for remaining dives. You can still get a third dive in the range of 28/11 with another Nitrox32 top-up to take the edge off a 120 - 130 foot dive.