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Skip and Sludge-- how often do you find that you drain the 80 completely on exit, and move to backgas for the last 1/2/5 minutes? Does this happen, or do you find that you consistently exit using equal or less gas than your penetration?
Also, if diving with two 80s solo, would you keep breathing the second one until drained, or switch to the dropped one as soon as you pick it up?
Thanks!
I usually don't worry about it when using a Tekna or Mako - a 50-minute burn time is less than an AL80 will last me, so for example, at JB I'll exit with 700psi or more. However, last summer I borrowed an SS and had a couple of 90-minute dives at Devil's. When arriving at the Lips I was down to 200psi both times. I went to backgas until I was at 30' in the Eye, and went back to the 80.
When swimming I'll turn at half+200, so I've never exited with less than about 500psi.
Those of you that know me know how conservative I am, so if these numbers sound alarming (breathing a stage down to 100psi), keep in mind that I have several hundred cubic feet of untouched gas on my back.
I was doing the stage only diving a bunch while living in France. More for the gas filling, as I rather fill an empty 80 than top up doubles. I would factor some back gas to be used for the switches and BCD. As I would be on back gas when switching and dropping. These dives usually involved three bottom gas stages and scooters. I would go half on the first stage and drop, then bring the second two up from between my legs on the leash. With these I would run the whole bottle, then stop, switch and turn the dive. Even with these low flow caves I'd have gas left over on the return.
When I was working at Ginnie long ago I would do single stage swims, solo. They were my exercise dives. Would do a circuit 1000' into Devils in a wetsuit and 85's. I also did this because of filling gas. Because I did not like 28% in my doubles and it was easy to get 418psi of O2 into an 80 at Berman's shed.
With either of these examples I would plan and use some back gas, not to much. With 32% on tap every where in NFL, I don't do this any more. Rather just keep the stage full when diving solo. As you don't have to deal with the switching. I also like how a full stage rides when swimming.
Cheers!!
Kevin
Quote:
Originally Posted by JahJahwarrior
I believe independent tanks (in backmount or sidemount) to be as good or better than manifolded tanks for the vast majority of worst case limiting accidents.
Manifolded air management is a convenience but not a safety advantage and comes with some problems unique to manifolded systems. Some have suggested that the isolable center bar is a danger in gas mixing and tracking because of the possibility of getting misfilled, unfilled or un-monitored gas while others point to the danger of an unisolable leak taking out both tanks that might be isolable to only one with the isolable center bar. A more complex double-isolable manifold is an improvement to the unisolable leak scenario but worsens the gas error situation. I'm pretty sure gas errors kill more cave divers then gas leaks.
With independent tanks of whatever flavor those problems are minimized or completely eliminated. Any diver capable of monitoring stage bottle pressures is capable of tracking independent tanks.
I posted the following in another thread on the same subject:
------------------------------------------
About experience, and comfort level..........
There is an alarming trend for cavediving accident victims to be trained cavedivers. 40 years ago, it was only OW divers that didn't know any better. Today even OW divers know the dangers of overhead.
Most of the fatalities, among trained cave divers, have no gas left in their tanks. It is EXTREMELY obvious that some emergency caught them with insufficient reserve.
The accident analysis "rules" are for everyone, not just new divers.
I am like Phillip. I add 50-100 psi, or more, to turn around, for things like:
New Buddy
Low/silty passage
Major restrictions
Long dives
New exploration
Mapping
and anything else that *might* delay my exit
In the last couple of years, I have ended most of my dives with half to 2/3 of my gas remaining. If you don't believe me, ask some of my buddies.
I can't agree with Forrest more. I see creative gas rules being used with the only goal in mind to reach the dive objective,but minimal thought given to Murphy. The one I hate the most is people making a hypothesis on these creative gas rules based on a SAC rate,and average of depth,this sure leaves out a lot of variables. I never make flow a benefit to my exit,I never push more than one restriction in an area that I haven't been in,I never count on the viz being great even I know technique for our team was good,and I never bend to peer pressure to do a dive.Quote:
Originally Posted by FW
I've drained an AL80 just once (well to about 200psi) on the exit and switched to backgas (I was close to exit and could have made it on AL80, but didn't want it drained totally). I tend to use less gas on exit, even in no flow systems because I go in slow and look around, do the job I came to do, or whatever, but on exit it's time to go home without sight-seeing.Quote:
Originally Posted by JahJahwarrior
When using two stages I just do whatever seems reasonable at the time concerning pickup and gas switches. I do drain the one I'm breathing (well down to 100-200psi) before switching, but then sometimes have gone ahead and switched at higher pressure (400psi or so), just cause it seemed like a good time to do it.
So that idea of breathing half of AL80, well it's usually a bit less than half on the way in, turning at 1600 or 1700 psi (3000 to start).
Just so you know, I've done a total of 20 solo cave dives (Florida and Tennessee). I do prefer a buddy, but have noticed that more of my dives are solo these days (as the economy worsens, traditional dive buddies work more travel less).
I am all in favor of conservative gas use and if anyone thinks I'm not being conservative enough post it! I can rethink my position on this, but it does seem fairly conservative in solo diving to keep all backgas for emergencies.
I also dive sidemount sometimes (and solo) and sometimes do not take a stage, but rely on the two tanks. In the ear on sidemount I had planned to leave my reel for a second dive, then changed my mind when I saw all the traffic (lines) in there. I returned to the stop sign to retrieve my reel and had it jam with debris on the exit in the cavern zone. Cleaning it out then took me into the emergency reserves, but I could have dropped the reel and exited anytime. It did drive home just how quickly that last third can disappear. Like a fuel gauge in a car - it seems to stay on full a long time, but the needle really drops during that last quarter of a tank.
I'm not sure why I feel solo sidemount is ok without a stage, but solo backmount requires a stage....I must have some unconscious lack of trust in the manifold? or in my ability to actually shut it done in sufficient time?
-skip
Are you seriously trying to learn how to solo dive off the Internet? I am amazed your instructor hasn't thumped you yet.Quote:
Originally Posted by JahJahwarrior
I always dive sidemount when solo. I do turn slightly before thirds. If scootering, I turn much sooner or take a bailout bottle or both.
Quote:
Originally Posted by Gary
I absolutely agree that the x-over isolation valve is responsible for errors that definitely have killed or severely harmed several divers, but I have to disagree on the backmounted independent doubles. I believe this to be the most unsafe configuration of all possibilities...
Assume a failure always at 1/3rds turnaround - the superior configuration is certainly the one that gives you the most usable gas present to get out...
Given BY FAR the most common gas loss failure mode, an isolable leak - say something on one of the regs fails. In all cases, you isolate (lets say quickly and lose little gas). In SM - you have 2/3rds of ONE tank to get out, same Backmount Independent (BMI). For any x-over BM (XBM) you have access to 2/3rds of TWO tanks (through the x-over). One can (and has) argued that if SM, you can feather or swap regs underwater - true, but you will lose some gas, adds to task loading and ADDS to exit time. For BMI this is difficult to out of the question because it is behind you (lets agree it is not very easy...say, a hard possibility). Given the gas mixing issues mentioned, then clearly for ISOLABLE gas lose = XBM no valve is BEST, XBM w/valve 2nd best, SM 3rd, and last is BMI. NOTE - not saying you still won't get out - just you have less gas available to do it.
Ok - now move onto the by far, less common gas loss failure mode, and unisolable leak. SM and BMI again have 2/3rds of ONE tank. XBM w/valve has something under 2/3rds of ONE tank. XBM no valve eventually has none. So here, SM=BMI is BEST (may need a buddy), XBM w/valve next (likely needs a buddy) and last is XBM no valve (definitely needs a buddy). Now you also have the leak rate factor. It has already been pointed out that it takes 1-5 minutes to drain a set of doubles with MASSIVE flow. Most likely, the flow will not be massive, blowout plugs are restricted, tank o-rings are fully captured and there is the thread + most of the o-ring to slow the flow, and x-over's now usually have double o-rings on both sides that are axial and require 2 failures (some older ones like the OMS have single face o-rings...not as good). Restricted flow will look and sound bad, but you will have time to breath it down as you exit before going on buddy gas (assuming you have one). Scooter damaged valve where you knock the piss out of it - likely a restricted leak, but who knows.
Now take the probabilities into account - take your pick on the ratio of how probable isolable leaks are compared to unisolable and consider massive versus significantly restricted leakage on the unisolable. It starts to add up - for the VAST majority of the gas loss issues encountered, the governing case is isolable or not significant enough to hurt alot...ergo my statement regarding BMI and buddies definitely give you a gas volume present safety margin compared to solo given the same tank configuration on the diver.
If you are going independent, put them where you can get to them otherwise put them connected on the back.
Oh, learn to drive and do something "special" with the blowout plugs.
Bob