Sounder wrote:I like Lamont's advice of always being the guy who surfaces with 1000psi in a large tank.
Its money in the bank. With a SAC of 2.0 on the surface (two stressed divers swimming for it) you can last 20 minutes. That'll get you back to shore with a dive buddy who is OOA and with a compromised BC (or 4+ foot waves) doing the swim back at EUP...
Also:
dsteding wrote:Personally, I wouldn't go past 110 feet on a single 130, that is doubles territory for me. It also just may be trimix territory-I can feel narcosis at 90, am definitely narc'd at 100, and just don't see the need to take a unclear mind much deeper than 110.
That is about my limit as well. I also never noticed that I was narc'd at 60 fsw before I did a dive down the boundary cable in cove 2 on 30/30...
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And speaking of the boundary cable, that is sort of "soft overhead" area due to the ferry going overhead, and that means you want to reserve more gas. You want to have enough gas for you and a buddy to swim upslope and then deco comfortably in the shallows, and it does take a few extra minutes to swim from the logs up to 40 fsw there (a better approach would be to do a direct midwater ascent to ~20-30 fsw and then swim back on a compass heading while doing an air-share, but I'd *plan* on having enough gas to swim upslope...)
If you work out the gas requirements for 2 divers at 2.0 SAC to spend 5 mins at 100 fsw then take 3 mins to ascend and do a 3 minute stop it works out to:
5 mins * 4 ata * 2.0 cuft/min = 40 cu ft
3 mins * 2.5 ata * 2.0 cuft/min = 15 cu ft
3 mins * 1.5 ata * 2.0 cut/min = 9 cu ft
40 + 15 + 9 = 64 cu ft
64 / 130 * 3500 = 1723 psi turn pressure
3500 - 1723 = 1777 psi usable
I usually use about 600 psi / 10 min at 100 fsw in a 130 (about a .55 SAC):
1777 / 600 * 10 min ~= 30 mins.
So, a 130 will give a diver, with a fairly standard (male) cold water SAC rate, 30 mins at 100 fsw with a 5 minute reserve at depth for two divers followed by an ascent from 100. That means that if your buddy has a free flow or another diver goes OOA on you, you have at least 5 minutes to sort things out while at depth, and then you still have enough gas to ascend safely and do your stops. That should cover just about anything that can happen.
On 130s I'd use 1000 psi as my rockbottom ascent time, and I'd turn the dive at 1500 psi at the I-beams or the logs -- that should give me enough gas so that I can still get 30 mins at 90-100 fsw (EAN32), even with a slightly short fill.
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Also a lot of divers tend to plan on doing the right thing in an emergency or cutting things short. So, some would say you are recreational diving, so you only plan on having to do a 30 fpm ascent and you do stops if you have gas left over. Some plan on two divers having 1.50 SAC or 1.75 SAC. Some would plan on doing the direct ascent and then the compass swim in. Some would assume that they'll be able to act correctly in an emergency and immediately leave the bottom without wasting any gas....
I would prefer to assume it all goes horribly wrong... I'll be mugged at the pilings below the boundary cable for my long hose and my mask will get ripped off by a diver not on my team, when I get my mask back on my buddy will be dealing with the OOA victim's panicking buddy which delays leaving depth, neither of the two victim divers will be competent enough to do an open-water ascent and we'll need to swim back upslope, and one or both teams willl probably have deco time to burn off shallow...