It was suggested that the rule of thirds be modified when diving solo. I'd like to hear suggestions for modifying this rule.
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It was suggested that the rule of thirds be modified when diving solo. I'd like to hear suggestions for modifying this rule.
Just curious, how did they suggest you modify the rule? I've heard everything from 1/6ths to 1/2+200psi. I'm in no way even close to being ready or wanting to solo dive, but I am curious to see what the standards seem to be.
Use thirds with a buddy bottle, fourths without bottle and sixths when scootering. JMO
This is just me, but solo sm diving, I dive to 1/4, this is my "base line" and I modify up or down from there depending on many things, siphon, how strong? Restrictions, how many and how restrictive? One tank off restriction(s) or two tanks off restriction(s) how many jumps? How many T's, what's the vis like, going in? what's the vis likely to be like on the way out? If I'm solo sm probably a reason for it, vis will more than likely be bad and slow my exit...way too many options to cover it all.
You really need to know a cave before you get *serious* about solo diving it.
In a word...experiance. You can't buy it no matter how much money you have.
Although I have heard of the "1/2 plus 200 psi" rule, I also remember that Sheck Exley cites it as a classic mistake in BLUEPRINT FOR SURVIVAL.
I don't want to go into details etc,but there are a couple solo dive accidents in the past that creative air rules are thought to be a factor,if not the factor.Quote:
Originally Posted by allen
Hey Kelly,I cant figure out your avatar.Is it a sports team or something?
http://siusalukis.cstv.com/Quote:
Originally Posted by stairman
Oh,a softball team from Illinois.Thanks chimie.I dont feel so stupid for asking now.Someone uses that same avatar on TDS.
Modifying 3rds is fine, as long as you stay on the conservative side. Like Mike said, save extra gas for anything unusual, like restrictions, siphons, unfamiliarity with a system, low vis, etc.
Like Kelly said, creative, going past thirds, air rules are suicidal.
If you do use a buddy bottle, don't use it as part of your thirds. On the other hand, some divers *only* dive their stage, and save *all* their "back gas" (or side gas) for emergencies.
In any case, you should finish the dive with at *least* a third of your starting gas.
No, it's Southern Illinois University, in general. It's just that the site you were sent to showed the current athletic schedule, and it just happens to be softball season.Quote:
Originally Posted by stairman
I happen to know three cave divers that are SIU graduates.
Gas planning for simple swim only solo dives is relatively straight forward. The modifying that you are pobably refering to is equating the size of your buddy bottle to the size of your back gas. For example:
Back gas lp95's filled to 3600psi.
Buddy bottle is an al80 filled to 3000psi.
Since the gas in the buddy bottle equates to only 1100psi of back gas, maximum penetration should be no less than 2500psi.
Hope this helps.
I mentioned something to that effect in an earlier post; perhaps that's what you're talking about.Quote:
Originally Posted by allen
In my view, modifying for solo entails adding a buddy bottle that contains twice as much gas as you'll need to get to your nearest gas source. This gas is not touched except in an emergency.
Quote:
Originally Posted by stairman
http://en.wikipedia.org/wiki/Southern_Illinois_Salukis
http://www.youtube.com/watch?v=hFThX...eature=related
Go Dawgs Go!!!
To me it means diving SM. No reason to adjust because you are diving solo. Cave conditions are reasons to adjust but not the fact that you are on your own.Quote:
Originally Posted by allen
If diving backmount solo (I don't do that) then it means a buddy bottle and limiting penetration to the buddy bottle content which in most cases would be less than a 1/3rd.
Why should gas management need to be adjusted when diving solo vs team based ? Realistically, there are two reasons that one would need to share gas.
1- Delayed exit. A buddy won't be much help since he will be about out of gas when you are since you are diving together.
2- Equipment failure. In the case of SM, it is statistically impossible to loose more than half your gas supply. That means that if proper gas management is followed (1/3 in high flow, less in no flow, less in low vis, etc) the loss of one tank can be dealt with. If diving backmount, the buddy bottle provides a fully redundant gas source in case of total loss of back gas.
Hmmm.... Not so sure about statistically impossible!Quote:
Originally Posted by chimie007
That would be "statistically insignificant."
What about scootering with a buddy bottle?Quote:
Originally Posted by Wizard
It's pretty easy to add a single bottle to a scooter ride. Generally very worth the effort even if the dive plan doesn't require it.Quote:
Originally Posted by RN
Generally better to divide the air use up so that any single failure causes no more of a problem then any other one. So some dive plans that would mean using the buddy bottle. When dropping the bottle the air in it may be of less use to a person a couple thousand feet farther along so that using more of it then the gas with you might make sense.
When diving a manifold and solo I would expect the buddy bottle to be limiting. Although unlikely anything that can take both manifolded tanks out at one time leaves one with only the buddy bottle. Leaving that bottle untouched and limiting the dive to safe exit on that bottle is possible.
If solo scooter without a backup scooter planning the exit as a swim out becomes necessary. This could easily mean 6ths for many faster scooters. Combined with longer in water times, cold and deco obligations it may make swim out impossible even with 6ths.
More gear equals more failures. Extended range diving I think makes it necessary to at some point look beyond a single failure situation. Dual failures put almost any dive on more then simple thirds. When solo at least 4ths even with independent tanks and no scooter. Looking at combined failures that include both a scooter and a manifold for a swim out on only a buddy bottle could be impossible for a solo diver. If rebreathers are run off backgas then does a single failure disable both the rebreather and the OC supply? and does that provide enough for a diver to get out?
I just started scootering recently, and because of scheduling conflicts and some sinus issues with my regular buddy, most of my scootering has been solo. I'm diving sidemount in a high flow (again) system and have yet to come close to 1/3s during the penetration (barely reaching 1/4s). My first few scooter dives were within the penetration distance I had made swimming the system when the flow was down. Once I hit 3000' I started carrying a buddy bottle, not a stage. This leaves me with 3200psi plus whatever is left in the SM bottle left should one of the SM bottles have a total gas loss. I don't think the buddy bottle is completely necessary since I'm turning my dive with over 3/4s of my starting gas. But better to have it and not use it than need it and not have it. Besides, it really doesn't create any additional drag. I managed to increase my travel speed by 10fpm during my last dive.
Within the new CDG manual we give some time to thirds and the application of modified thirds.This is primarily for uneven cylinders sizes and used mainly in underwater digging.
Andrew.
LOL "statistically impossible" perhaps not the best choice of word. Sludge got it right: statistically insignificant. I mean it's a double failure. Even if you have two free flowing reg, you could feather the valves.Quote:
Originally Posted by contender
I wouldn't take off on a long run solo on a scooter. Having 75% of two jacked 104's to cope with exit or emergencies is still a lot of gas so I don't take a buddy bottle. With a scootering buddy it's back to thirds because swimming is less of an option.Quote:
Originally Posted by RN
That's just me. By all means it's not a rule to base anyones else's life giving air strategy on.
Another good thing to do is really know how much you need to exit swimming. Not just numbers but actual proven amount. One of my first scooter dive was to swim back from 3000' at Ginnie. Took me about 40 min and used a bit less than 1/3 of my gas supply. That was pushing the scooter ahead and carrying a stage. I would have dropped both in a real situation.Quote:
Originally Posted by Wizard
I use the rule on '1': How much gas do I need to get out? Other things you have to think about is; do I carry gas for another diver should I come across one in need? How do I carry my gas, in terms of redundant regs and bottles.
Chimie007 is spot on: make sure you DO know where you are in a dive and that given the worst circumstances you do get out. Actually swimming your gear out every now and then does give you an insight into this.
Of course...all of this is nothing more than the experience I mentioned in an earlier postQuote:
Originally Posted by chimie007
Oh what the hell, let's stir the hornet's nest a little.
I posted about this last year when I was doing some exploring in a downstream cave. I did that scariest of things - algebra - and came up with this:
http://www.andrewainslie.com/spreads...s%20Murphy.xls
It wasn't well received. Oh well... Gary and Kelly in particular hated the fact that although I wrote it for downstream calce, by putting a negative number in you can use it for upstream calcs... and it leads you to the conclusion that it's pretty safe when swimming Manatee or similar caves to go WAY over thirds and be safe. Doron Nof, one of the cleverer divers I've ever met, independently verified my math and concurs with my findings - so it's even peer reviewed. Yes I know, we PhD's are mere ivory tower idealists... blah blah... but there are the results if you want to play with them. Here's the original thread.
http://www.cavediver.net/forum/showthread.php?t=6312
For those of you who feel that math is actually useful, have a look through both the thread and the spreadsheet. My suggestion - don't hide behind overly simplistic "rules of thumb". We humans love heuristics - yet there's a massive literature on heuristic bias and how much trouble it gets us into. For example, merely adding an 80 when scootering or when diving solo BM may be overkill, or it may be underkill - you won't know until you do the math.
If you SERIOUSLY want to think about modiflying rules of thirds, start with my spreadsheet and modify it in line with the added contingencies you'd like to add. I have no problem making the claim that this spreadsheet is a more carefully though out approach to safe gas management than anything else out there. yet is still comes with the caveat that you're nuts to use it until you understand every single cell I put together in there.
Two additions that I'm actually going to play with right now - 1) I'd add a "minutes stationary" contingency, i.e. allow for something that stops you for a few minutes completely on the way back AFTER losing half your gas, and 2) if scootering, I'd use a different "speed in" from the "speed out", i.e. assume the scooter speed going in but the swim speed going out.
In all my calculations, I assume you're solo and diving sidemount. I see no other logical way to dive. This "air share" idea is one of the most dangerous ideas in cave diving. By all means train to do it, but IMHO you should never plan your gas management on any assumptions about someone handing you a hose. Just consider yourself really lucky if someone does.
I find that when solo i no longer need that gas reserve i carry for my buddy but rather it is now mine for my own personal consumption. So I modify thirds into .38 instead of .33. I call this the golden rule of gas consumption simply because it is based on the golden ratio (i like to think of that as the ratio of successive fibonnaci numbers F(n) + F(n+1) as n gets large, (1, 2, 3, 5, 8, 13, ....) although there are certainly more artfull ways to get at it (1.618)). So with 3600 I would consume 2225 and reserve 1375 for any untoward events. I certainly add some safety stock if conditions warrant and then I use a gas inventory control model with uncertain demand. I consider the uncertainty to be normally distributed but am now looking at alternative probably distributions. Probability based gas management is certainly the way to go. However, as always, ymmv. Bill
fyi
turns out that an eight followed by a closing parenthesis is 8) so the number above is 1.618...
Funny, Bill, I've never thought of you as a fibonacci sort of guy. I would have guessed you would lean toward root mean square...
This is a beautiful example of a silly heuristic. Sorry, but I just have to point it out.Quote:
Originally Posted by wingman
Fibonacci sequences are elegant. But, for crying out loud, what do they have to do with gas consumption?
Pi is sort of cool - why not reserve 1/pi of your gas?
Oh actually so is e. what about 1/e?
Etc etc etc.....
I never was any good with humor....pi and e don't work near as well phi in this context...the Fibonacci sequence is just one way to get to phi, there are many others and in one of those methods maybe you will see the light 8)Quote:
Originally Posted by aainslie
Back in the days i was writing about equity market efficiency i was hired as a consultant to explore the use of fibnacci numbers to drive trading rules...interesting but no joy...i do like a good root mean square but its the mean square i like the best...especially mean square error.Quote:
Originally Posted by Slüdge
My bad. I missed the humour. Damn - I need more coffee!!!Quote:
Originally Posted by wingman
Not to worry, i tend to work in the tongue-in-cheek domain and it often gets me in trouble. When i was presenting my research strategic plan for my university to the president, deans, other vp's etc. during the first term of the bush debacle i opened up my presentation with the statement that...There's nothing I like better then a good discussion of "Strategery." Unfortunately that was lost on the audience and i had to explain it as a bushism and not my screw up. Fortunately the rest of the presentation went well.Quote:
Originally Posted by aainslie
Good point Andrew,
Third's rule is too general. It's a baseline from which you should adjust based on cave conditions. Experience, as Mike said, tells you how too adjust it. Going full third in a low flow system is asking for trouble while doing the same in a high flow system is very conservative.
As for you spreadsheet, I'm a big fan of pre-planning and using as much info as possible but experience and testing are IMHO a better approach to learn the gas reserve needed. Anyhow, nothing says both can't be used.
Hey Raphael,
I agree... sort of. Where I think you need experience is in determining SAC, speeds with and without scooters, and actual flowspeeds in caves. Everything else is an input.
But why the hell am I dong this? I just remembered... I bought a rebreather. No need to reserve anything, just me and my 2x19 cu ft tanks....
(On the offchance someone misses it, that's MY version of humour...)
Or perhaps they got it and simply dissagreed with you :wink::-D:-DQuote:
Originally Posted by wingman
Quote:
Originally Posted by mmcauliffe
In the aftermath it was determined that was not the case 8)
I assume that suggestion was to make the T-rule for solo more liberal. And it was based on the assumption that you don't have to feed your buddy w/your air in case he lost all of it in the most far point. Thus we don't have a buddy to feed when we are solo. Here is the question - are you in sidemount or in doubles?Quote:
Originally Posted by allen
Doubles should be considered as something that will be empty in less than 5 minutes if manifold or neck o-ring breaks (try to drain full doubles and see how long it takes). Therefore with doubles we must use buddy-bottle for solo. Speaking here theoretically as i don't endorse solo. Think about doubles as it was 3 stages on your back. Since doubles can be discarded at any moment you cannot really count gas in them for emergency use. Hence you cannot use more than 1/3 which is approximately a volume of one buddy-bottle-stage (lets say AL80). That saves your lucky a$$.
Sidemount gives you more chance to survive without buddy bottle if you don't violate your thirds (each SM tanks contains 1-1/2 of stagebottle). In this scenario at the point of your fatal turn you loose the whole leftover gas from one of your bottles. At this moment you have only 2/3 available in your only working tank, and it is exactly 2/3-ds long way to the door (because you used another 1/3 from the failed tank on the way in plus 1/3 from your current tank).
We are talking here about rock bottom and didn't even mention: stress from gas supply failure, possible lost visibility, time lost while dealing w/accident, lost decompression gas reserve. Do you really want to go this way?
Quote:
Originally Posted by ARY
Which is exactly why I don't go to thirds when sidemount solo! Well said!
Okay, this has turned into a very interesting discussion. There has been some vague reference to my next question, but no one has quite come out and stated anything specific on it (and no one may do it).
What (or is there) is the general consensus on modified thirds when swimming into the flow in a high flow system such as Devil's, Manatee, or JB? Does anyone penetrate past thirds knowing that the flow will help expedite the exit?
I've never done this, but the thought has been on my mind for some time. If I'm penetrating a high flow system and turning 1200psi in and coming out with only 600psi then is there an issue with penetrating to 1400psi, for example, knowing that you'll only use 700psi to exit? You would still be coming out with more than one third of your total gas left.
Dropping the dead scooter makes sense. Dropping any viable gas supply doesn't. If you had a real situation, breathe the stage empty then jettison the empty tank.Quote:
Originally Posted by chimie007
The rule of thirds is meant to be the most liberal of gas management rules you should use when diving. Diving in poor visibility, deep, low-flow, syphons, or solo should make you pause and consider more conservative gas management rules; maybe that is diving to 1/4ths, or some other fractional, or maybe breathing a stage bottle and saving your back gas for emergencies. Unfortunately, there is not a single hard and fast rule which would be correct for all situations.Quote:
Originally Posted by allen
Also, please consider this. Prior to the rule of thirds the general rule of air management was "half-plus 200". Many people survived cave dives using the half-plus rule, but also consider that back in the early 70s a typical cave dive was quite a bit shorter than they are now. A dive to the hinkel restriction might be considered a boring non-event today, but when blueprint was published in the 1970s it would have been an amazingly extensive dive.
Just some food for thought...
Of course... I was implying breathing the stage empty first.. Never said I would drop the stage and scooter at the same time. Thought that was obvious that no one would drop a non empty stage.Quote:
Originally Posted by kensuf
Well, I'll go here... I often use "half plus 200" on stages. I've done some spreadsheet work on this, and there's an added level of safety as you add stages, since this increases redundancy. i was surprised however at how SMALL the advantage is. So I only use 1/2+200 in high flow caves like Ginnie and Cow.
And I PROMISE that's conservative!
__________________________________________________ ___'
A slight change of pace.
Do this calculation - remember your PSI on entry and at turn, and look at your PSI on exit. Double the difference between turn and exit. Now take that as a % of the PSI consumed going in plus that number.
As an example, say you go in with 3600 psi and turn at 2400 psi. On exit you find that you're at 1800 psi.
You used 1200 psi going in, and 600 psi exiting. 2x600=1200. So to JUST survive the dive losing 1/2 your gas at turn you would have had to enter the water with 1200+1200=2400 psi of gas. You would have used 1200/2400 or 50% of your gas, and a bare minimum reserve would have been 50%.
That % is the min gas you need to exit safely, IN THAT CAVE AT THAT FLOW SWIMMING AT THAT SPEED etc etc. Add a safety margin depending on circumstances, and that'll give you a safe reserve.
Any safety you add will get doubled since it cuts into your ingress gas. So if we reserve 55%, that only gives us 45% for ingress, leaving a 10% gap in added safety.
So with tanks at 3600, we can now safely use 45% of 3600 or 1600 psi, making our turn p 2000 psi. And that's a safe turn.
Cow or Manatee will probably give you numbers similar to these.
But here's the interesting point. Assuming those calcs are right (and they're not far off from my memories of Cow dives) it's SAFER to dive to 2000 psi in Cow than it is to dive to 2400 psi in a no-flow cave.
Let's redo it in no-flow. You enter with 3600, turn at 2400 and exit at 1200. Your min gas needed is 1200 + 2x 1200 or - surprise! - 3600. Turn is at 33%. Now add 5% for safety and you should turn after using 28%, i.e. 2600 psi.
It is AS SAFE to turn at 2600 psi in Peacock (assuming you don't know where the alternative exits are) as it is to turn at 2000 psi in Cow (assuming my values for turn and exit p's are correct). it is way more dangerous to turn at 2400 in Peacock than it is at 2000 in Cow.
So if you want to be lazy and just turn at thirds, sometimes you'll be taking your life in your hands (no flow) and sometimes you'll be ridiculously conservative (high flow).
One caveat - you'd better be good at arithmetic before starting this stuff. "Math kills"!!
One other - sometimes caves change. There's a little no flow passage in Cow that completely changes these calcs. Be conservative in those situations.
So - I don't dive thirds any more. And I'm sure that offends some people. But at least I know my margin of safety. Do you?
Don't take it personally because there was nothing said with any intent to flame. I've just learned over the years to never assume what you may think is obvious is clearly seen by everyone else.Quote:
Originally Posted by chimie007
I can see the clarity in your Math and have informally used this in High Flow Sysyems but instead of using the spreadsheet I have always based the runtimes from actually numbers or PSi when diving those systems.Quote:
Originally Posted by aainslie
I have used on a regular basis the 1/2+200 rule at Ginnie on a stage bottle but have maintained the Rule of 1/3's on my backgas for the what if's.
Very interesting refence between the math on a High Flow versus a Low Flow System.
Very simulating thread....
IMO if you want to bust thirds that is your business (and your buddy's) but you certainly should not be advocating it on the internet, even if your math makes sense to you. Just keep it to yourself and accept that you are breaking one of the cardinal rules of accident analysis when you do so.
I was taught and still believe 1/3 of your total gas volume is the maximum amount of gas you should use for penetration on a dive (in the most favorable conditions). I cant remember the last time I turned a dive right on my turn pressure, I usually turn 100 psi or more above it. If you want to drain your stages to 1/2 + 200 to make them easier to carry later in the dive or have them be "ditchable" in an emergency compensate for it off your backgas. 300 psi in an AL80 is approximately 100 psi in a set of double 104s. If I am breathing an AL80 stage down to 1700 instead of 2000 my new turn pressure on my backgas is 1/3rds plus 100.
Bob - I agree, I'm not going to say I have a lot of cave dives, but I've got a few and I have NEVER...NEVER dove past 1/3 of my back gas, or "side gas" as it where.
What you and your buddies do is your business but you dive with me and we turn on thirds or less period.
The first guy I heard of pushing thirds with with creative math was a guy at Little River who convieniently explained his method to a cave class before getting lost off the line and dieing nowhere near the exit. The last was John Robonson who pushed thirds without telling me as his buddy and nearly died as a result.
Both these guys pushed thirds, both thought they had justification to do so, both died in a cave.
You can follow Andrew's math spreadsheet on this if you want to but history shows this line of thinking is deeply flawed.
....because..... ?Quote:
Originally Posted by Gary
I hate debates based on "History tells us". Where's the flaw? you're merely being intellectually lazy. it makes way more sense to keep a consistent reserve than an inconsistent one. Always diving thirds under different flows will give you inconsistent safety margins - sometimes ridiculously high, sometimes ridiculously low.
Both this time and last time you guys (Gary in particular) debated this you got focused on the "when to go low" and ignored the "when to go high". Theres' some sense to this actually - some guys called Kahnemann and Tversky won a Nobel prize inter alia for demonstrating a form of irrationality in subjects' behavior called "loss aversion" and you're showing us a nice example of it.
There are two sides to this.
If nothing else, take away the fact that diving thirds in a low flow cave is stupidly irresponsible.
If you are willing to do the (mental) work, you can safely go below thirds on high flow caves. The simple, conservative thing to do is to stick to thirds.
One more time however:
IT IS UNSAFE TO DVIE THIRDS IN LOW FLOW CAVES.
Ah - and I'd never push this on a buddy.
Dude... everyone knows the Rule of Thirds is the most liberal rule in cave diving and that if you push it right to the edge in the wrong conditions and the silt hits the fan, you're probably not gonna make it.
Advocating using math to "beat the system" is reckless. It's pretty hard to mess up thirds and I typically don't take a computer to the dive site, let alone punch in numbers right before I get in the water.
And hell yes it's intellectually lazy, it's recreation, not rocket surgery.
Thanks for the math, Andrew. Although I'm pretty sure I'd still never push thirds, even in high flow, your low flow example has answered another question. This came to mind during a dive in which I was sidemounting AL80s. I didn't push thirds. In fact, I turned the dive at thirds plus 200. But, for some reason (probably the fact that 80s pumped to 3200psi only give me 160cf of gas and my usual 108s pumped to 3600 give me more than 100cf more than that!), the thought of how little gas I had with me suddenly occurred to me on this dive. I was already on the egress, but I didn't like having so little gas so far from the exit. That was the last time I soloed on 80s.
About pushing thirds in high flow....
High flow can actually slow your exit under adverse conditions.
I used to run a drill in Devil's Eye. The students closed their eyes, and followed the line out. I had my light on, so I could observe them. It they cheated, and opened their eyes, it was obvious, because they just moved their hands over tie-offs. A *large* precentage of them got pushed around at tie-offs, and some actually went back into the cave as a result!
Total light failure? Impossible, right? I have seen three cases of total light failure, one caused a fatality. A lot of divers negelect their backup lights, so when theri main goes out, they are in real trouble.
Another way high flow can slow you down is buddy breathing (which is the main reason for 3rds anyway). You have to stay close to your budddy, and the flow tries to separate you, so you have to slow down, loosing the advantage of high flow. Add a restriction or two, and you probably won't make it out if you didn't turn on thirds.
Good stuff Forrest, Thank you!!!
...Once more focusing on only one side of the equation...Quote:
Originally Posted by OneBrightGator
Thirds is NOT liberal in some circumstances.
Forrest, some good points but they only complicate the debate. First, it brings me back to why I really wish sidemount would take over from backmount. BM - and particularly gas sharing - is an inherently silly way to handle OOA situations. Secondly, the chance of a TOTAL light failure IN CONJUNCTION with OOA is astronomical. The probability of a set of independent events is the product of each probability. Let's say lights fail 1 in 100 dives, and you have 2 backups. Let's also say that you lose gas once every thousand dives. Then the joint probability is 1/(100*100*100*1000) or .0000000001. Not a frequent occurrence, particularly since for TOTAL light failure all three of your buddy's lights need to fail too, decreasing that number by another 6 zeros to .0000000000000001! Put another way round, you'd need to do 1,000,000,000,000,000 dives between such an event, on average. In fact, has anyone ever heard of two properly equipped divers losing all their lights, and one losing half their gas?
GREAT point on checking backup lights! Rich Courtney has a really good story about this issue, and about how carrying TOO MANY backups can lead to complacency. Checking backup lights often is critical. I actually keep mine switched ON (on my helmet), and change the batteries regularly - not only does it help with gauge reading but with verifying functionality.
On the issue of lights out and tie-offs, I did a fantastic class with Steve Bogaerts recently. I asked him to do a refresher on stuff he thought I needed work on. One was lights out drills, sidemount style. The idea is to use "bump and push" - at each tie-off, yu move one body length beyond the tie and wait. Your buddy then bumps into you, and pushes you to indicate you should keep moving forward. We first practiced in open water. After about one minute he got a pissed off expression and motioned for me to surface. "What do you think you're doing?" he asked. "You're moving so slowly that in a real situation you'll run out of air!". We practiced a bit more, went into the cave and essentially exited on the line, swimming as fast as we'd swim with lights on. It was an incredibly powerful lesson. BUT... it was in Mexico with no flow. I'm going to give it a try in Florida as well, and see how it goes. But here's the key point - try swimming flat out in a lights out situation on your own, and then try doing it while sharing air. That alone might convince you that SM is the only safe way to exit in a gas emergency.
Anyway... back to the key point. i agree, thirds is fine, as long as you adjust it by at least 200 psi in no flow situations. 'Nuff said. And if you want to do a bit of arithmetic and trust logic, I'd suggest you can safely go a bit lower. And I know that I'll always have people telling me I'm a lunactic when I do it - but until someone gives me a VALID reason, I'll be doing it.
One more point - KNOWING your rock bottom doesn't mean you have to go there. Although I've calculated that I can turn at 2000 with a huge safety margin in Cow, I'll be the first to admit that I've never actually gone that low. But it's nice to have an exact feel for your safety margin.
Thirds gives you a highly variable safety margin. Once more Ben, NOT ALWAYS CONSERVATIVE.
Or brain science, either.Quote:
Originally Posted by OneBrightGator
Andrew,
I understand what you are saying in high flow caves, but still disagree. For the low flow caves, you are absolutely right. If someone is diving to thirds they are absolutely fooling themselves about what reserves they have. I agree that very few have done the math for many of their dives. That's why 3rds is the starting point for ideal conditions and you should be more conservative for anything else, not less.
While in high flow caves, the math proves you have plenty to get out, but the extra gas is there to also give you time to deal with a variety of issues that can occur and compound. So the math says you will not likely have a gas and light failure at the same time. How about if you have a gas failure and in dealing with the issue you siltout the place and the flow pushes you off the line? Same end result, you cannot see even though your lights still work. Granted you may not have to do the entire cave exit in the silt cloud. The problem here was human error, not math.
Too much gas is not likely to be an issue, too little gas can be a big problem.
I'm an engineer and not a professor; in my field when you cannot account for something or it is impractical to account for all of the variables one option when testing to failure is impractical, you give yourself a larger margin of safety. Many engineering failures are from pushing the math for one case too far without properly accounting for all the other variables or cases.
Quote:
Originally Posted by Ben M
Then dive sixths. Hell, dive twelfths. They you'll REALLY be safe.
If you're an engineer then you should REALLY realize what I'm suggesting. A constant safety margin. Thirds, once more, gives people an inconsistent margin because people are too lazy to compute it properly.
Extra stops, extra problems, all that stuff can happen in high flow, low flow and reverse flow caves. None of that cahnges the argument for calculating a consistent safety margin.
Sheesh. Ben, you're right - this isn't rocket surgery (thanks for that nugget, I'd forgotten about it!) It's simple arithmetic. I bet i could explain it to a 15 year old... If they hadn't been indoctrinated into thirds as being sacrosanct.
See... if I was a consultant I could have made $1,000 off that...
Every dive is different, just because you are reserving the same percentage/amount/whatever of gas does not mean you have the same safety margin. Conditions, flow, buddies, emotions, physical fitness change dive to dive.
Thirds is a great place to start, modifying conservatively is a good way to go. Anything else is unadvisable, and to condone it on the internet is reckless.
I still don't get why it's reckless. Care to explain?
I'm suggesting keeping the SAFETY MARGIN constant, not the RULE.
Thirds is like setting a speed rule. We all know that increased speed increases the probability of an accident. but we also know it's a function of the road conditions. So we a) alter speed based on e.g. weather (slowing down in fog, etc) and b) alter speed limits based on road conditions (e.g. 65 on a highway, 35 in a town).
My suggestion is to implement something like the speed variation based on road conditions, which (like cave flow) are reasonably invariant. Thirds is like setting a speed limit of 50... whether you're on the highway or in the middle of High Springs. And flow is a very objectively measurable aspect of a cave.
You should add more margin if the cave is narrow... silty... unknown... deep... etc. etc..
Attached is a sheet with the calcs I put in the earlier discussion. Note I put a piece in there to add a psi "safety net" for conditions other than flow. And notice how easy the arithmetic is. Measure psi on entry, on turn and on exit. Input next dive's starting psi. Based on just those 4 numbers, plus the addition of a safety margin, you can keep a safety margin that stays constant as cave flow changes. I am ONLY controlling for flow. You need to control for other sources of gnarliness.... which btw you'd better be doing with thirds also.
It's another reason I hate thirds. it's such a simplistic rule that people think it's good enough. It isn't. It's frequently too liberal.
Andrew, I haven't looked at your spreadsheet so I don't have an opinion on it either way. I have noticed that this discussion comes up from time to time. You are in a no win situation. The % of trained THINKING divers is small in the real world, here in cyberspace it is even smaller. Look at some of the answers in respoinse to the original posed question. They show no understanding of the contingency being planned for when soloing. For ex: "I'll dive to sixth's so I don't need a buddy bottle!!!!". Give me a break. Some of the more experienced have correctly stated that more complex gas planning is involved in more complex dives. Essentially, have enough alternatiive gas to swim out and make a safe ascent from deepest penetration allowing for as many critical failures or situations as you can invision.(For example loss of all backgas, DPV, Complex navigation in Zero viz). All of this gas does not necessarially have to be on your person at all times. Some can be staged and placed along a planned escape route.Quote:
Originally Posted by aainslie
Since the training agencys don't take a position on Solo diving, little contingency gas planning is being taught in the normal course of cave training. (DPV class is an exception to this). This is why you see chest thumping and Rule of Thirds shouted at anyone who suggests anything else. (Many would not even notice that a more conservative approach might be the result, That would require THOUGHT). IT'S TIME FOR THE MAJOR TRAINING AGENCYS TO CHANGE THIS. With the tremendous growth in CCR diving a need for this planning has now become comonplace.. An OC diver with a CCR diver is essentially diving solo as far as gas planning. Can it really be within a training Agency's mission to ignore this? I hope not.
Take care,
John
Some points to ponder along with the finer points already made.
1. Isolation - takes at least 8 seconds
2. Burst disc failure - takes about 54 seconds on a pumped set of 104/130's to be rendered useless. At 3500PSI you would lose 500PSI in your attempt to isolate and then have 1500PSI left in the remaining tank. But hell you'd be at the front of the cave.
3. Rule of thirds is the minimum recommended safety factor. Margins of safety are realized when a 3500PSI tank is computed at 3300 making the turn 2400PSI. Just shy of 25%
4. If forced to isolate at 2400psi you would lose 500PSI, and have half of 1900 or ~ 950PSI to exit. But now you'd be at the back of the cave.
5. Exiting in high flow causes other things to break, light cords to snag and one time a small guy leading a big guy leading through the narrow part of the lips sat there for seven minutes because the big guy couldn't go forward.
6. Solo diving with out a buddy bottle ... not me!
My 02 cents. Great thread if it gets you to plan more conservative that you normally would. /Ken
Ken, as someone who's very new, I've always adhered to, but thought the 1/3 rule was very constrictive in a higher flow system (especially for me, still at cavern), but this gives me another way to think about it. It's always nice to KNOW the rules, but appreciate when someone gives their time to explain them.Quote:
Originally Posted by Ken Hill
Sure, just keep in mind this is my opinion based mostly on a lackadaisical attitude toward diving (except the safety aspect) and gut feelings. It relies little on your fancy "math" ;)Quote:
Originally Posted by aainslie
Basically, the Rule of Thirds is farm animal simple to remember and understand. Hmmm... if it takes this much to get one person out, then it will take twice to get us both out. ba-da-bing! Now, instructors should teach and show (lights out air sharing exit is a good way) how liberal that rule really is and explain that just because you can use a third doesn't mean you always have to or even should.
You're trying to nail down, with math and logic, how to safely go inside an underground, water-filled cave (which isn't very logical in the first place). We're not constants, neither are the caves. On top of that, in the example pre-loaded on your spreadsheet you're only getting an extra 100psi! whoopie. If there's something you really want to see 100 psi further, work on your technique, if it's past that use a(nother) stage, scooter, 'breather or buddy with bigger legs then yours.
What it basically boils down to is just because you do it, doesn't mean you should talk about it on the internet. Stuff like this, how to find a hidden jump, how to get access to a closed site, etc. is best reserved for spring-side chats.
Oh, and I don't cave dive solo and don't really care to or am knowledge enough to talk about it.
Quote:
Originally Posted by OneBrightGator
My thoughts exactly. Most people have enough going on in the water already (remember task loading). It is easier to use the rule of thumb and modify it as experience dictates than to plug numbers (many are probably assumptions, right?) into a spreadsheet and remember the details the next day... unless of course you are gearing up for a trimix dive ; )
And all that trouble for almost no gain, I'll stick with thirds.
BTW - to answer the original question I do reserve more gas if the dive is more complex or if I am doing a circuit and part of it will be slower going.
However, if there are multiple bottles in the dive plan there is a reserve built in because losing one only affects a smaller and smaller portion of your total gas. (Ex - on a double stage dive losing one stage at max penetration would only kill 1000psi of my planned usable gas, I have a second stage that would cover that w/o touching backgas, and my buddy has two also.)
If I am using 1/2 +200 on stages I always reserve the ghost 1/3rd in my doubles, then calculate thirds.
I love that you're pretending to be dumb here. I've read enough of your posts to know otherwise.Quote:
Originally Posted by OneBrightGator
Do me a favour. Next time you swim something seriously high flow, take those 3 measurements nad shove them in the spreadsheet. You'll find that the difference is more than 100 psi.
Anyway, I've had my rant. It's been fun.
Oh, and jj1987, do me a favour and listen to the others. It's definitely the safer thing to do. Even I'll concede that.
I'm not pretending to be anything. I understand what you're saying and I've done plenty of dives where my egress gas was half of my ingress gas, I'm fine with that. I don't agree that it's safe to out-calculate a the basic idea of what goes in must come out. In high flow caves there's a built in safety, in low/no flow caves a diver must pad for safety. Simple, basic stuff... like me :D
RULE OF THUMB , is simplistic at it's best. such as the rule of thirds. While Andrew and Ben can disagree about GAS RULES , The basic rule of having enough gas to egress the system is the only rule that needs to be justified. Do you really need a third of your gas if you're 300 feet back ?
The rule of thirds is a built in safety margin , not really a gas plan that pushes the envelope.
Do you really want to push the envelope ? Maybe that's not a good Idea. Maybe penetrating to thirds and goofing off on egress is a better plan , while penetrating to thirds and surveying on egress would suck.
The plan makes all the difference.
Isn't the rule still 1/6s for cavern and intro.?Quote:
Originally Posted by jj1987
jj is probably talking single tank which would be 1/3Quote:
Originally Posted by Serota
It varies from agency to agency from what I understand. According to the intro class this past weekend, we're allowed to dive single tank ONLY, with turn pressure being when you hit 2/3. It was repeated several times during lecture that we can not under any circumstances dive doubles. I'm not an instructor, and don't have access to the nacd workbook, so my information could be off here. I believe that TDI, NSS-CDS, GUE, and maybe others I'm unaware of have provisions for intro students (or equivalent) to dive doubles.Quote:
Originally Posted by Serota
I have my own opinions about this, as I've spent several hours reading, discussing, and thinking about this exact topic, but since this is a public forum, and I don't have enough experience to make my opinion worth much anyways, I'm not going to go into it here. I know there's a thread on TDS about this, so no need to start another. For those of you who disagree, I'd encourage you to write an email in a professional manner to the people at the following link, rather than starting it on a message board that won't change anything.
http://www.safecavediving.com/training_committee.shtml
I would like to say that Ginnie springs is the only place I know of that enforces this. The thread on TDS was specifically attacking them. Please remember when going there that it's not their rule, so don't get mad at them when they enforce it, they do so much for the diving community, I'd hate to hear them taking crap for simply enforcing a rule that isn't theirs.
Not to hijack, but NACD does allow for doubles at the Intro level. The student has to take the class in doubles and show mastery of the required skills in doubles, and the instructor has to provide a "waiver" for lack of a better term, that is normally predicated on the fact that the student will continue with at least Apprentice level within a certain period of time. In your situation, you had an instructor who chose not to provide that option, but it doesn't mean that the NACD does not do so.Quote:
Originally Posted by jj1987
Isn't this only good for 6 months, or am I off base here?Quote:
Originally Posted by inkdiver
BTW- forgot to add, if my above post was off, anyone please feel free to chime in and correct me, that was my personal understanding, and like I said, I'm new to this stuff.
Actually, at one time it was good for 1 year....like you, I may not be aware of any changes that may have been made since then. I guess I should be tarred and feathered for not keeping up.....Quote:
Originally Posted by jj1987
It's called a discretionary apprentice waiver and it's good for 90 days. Some instructors will reissue the waiver for another 90 days in some circumstances.Quote:
Originally Posted by jj1987
Andrew, I had a huge message with examples and stuff all typed out, but I lost interest in making the point.
Anyway, thanks for the thread, as this will quite likely get people thinking a little more about how they calculate their gas consumption, and perhaps think a little more about what they are doing before they commit themselves to a large dive.
Let me narrow my whole previous paragraph down to this-
You are not letting Murphy have a large enough portion of your gas supply. You can (and should) calculate gas consumption for large dives, but always add a fat portion for stuff you will never see coming, and by all rights should not happen. I've had enough completely stupid stuff happen that almost killed me, that I never could have planned for, and a fair number of "completely astronomical odds" events.
Anyway, something to think about, it used to be, (and its been a few years since I've dove with any) that the WKPP dove 1/2+2 on their stages- leaving backgas untouched. Which at first, didn't make sense, until someone clued me in that they also had SAFETY bottles, in addition to, (and actually in replacement of, in the case of emergencies) their regular gas supplies. Then it clicked- they didn't calculate that gas, but if needed, it could replace or supplement a threatened gas supply (and add additional regulators and bottles) so, in fact, they dove with large safety margins.
Anyway, already too long. Lets have more arguments like this.
Oops- I completely lost track of the original point of this whole argument.Quote:
Originally Posted by allen
Like Andrew, let me draw some fire.
First, I will no longer solo backmount. All other commments are based on this.
I do not modify the rule of thirds solely based on being solo. Being sidemount, you cannot completely lose your gas due to one tank failure, as you can backmount (again, has that ever actually happened?) and ALL OTHER THINGS ACCOUNTED FOR you should barely have enough gas to exit with the loss of one bottle.
Actually, the only thing I modify the rule of thirds for (never more than 1/3) are siphon systems. I will turn early based on the perceived flow. Every other contingency (scooter loss is most common) I will plan for a swimming exit at maximum penetration. If the dive is very long I will usually drop a safety bottle somewhere at the equivalent penetration of 1/2 to 2/3 of that bottle.
Please keep in mind I am talking exploration here. If I am at Ginnie (not in the last 8 years), It's usually 1/3s plus 100 for peace of mind, or about the same at Peacock. It doesnt hurt my feelings to exit with 2100 lbs.
Jason
First let me say, that I will continue to solo backmount, unlike jason.
However, here's the rub. When diving backmount and solo, I dive only the stage or stages, saving all, yes all, backmount gas for emergencies. I dive 1/2 AL80, in and 1/2 out. if carrying two, then half of one, drop it, half of second, and then turn the dive. backmount is for emergencies only, and since I backmount 120's or 130's there's more than enough reserve gas.
I also carry a small hand mirror so I can do my own bubble checks.
-skip
That's what I do, whether solo or not.Quote:
Originally Posted by skip
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
People often ask me why my isolator points down. My answer is that I expect my chances of isolating during a dive are approximately zero. In the first place, we dive the rule of thirds. If you have a gas failure, you share air. Period.
In the second place, in about 99.9% of all the times that people have isolated during a dive, isolating was the wrong thing to do.
My point in a nutshell. If you have a reg failure, you turn off that post. Don't isolate!Quote:
Originally Posted by addexdiver
There are two reasons to isolate during a dive: burst disk failure and cylinder neck o-ring failure. Otherwise, don't close the isolator.
Actually I have had this happen and had no trouble feathering the valve (which I only needed for buoyancy off that reg anyway since I switched to the good reg for exit). But that still gave me adequate gas to easily leave the cave on the one tank - plus the entire other tank at some minor inconvenience - and a buddy with full reserves...Quote:
Originally Posted by addexdiver
Yes the manifold provides the "best" solution in the "most likely" failure of an isolable leak in that you have access to both tanks once isolated. On the other hand it also gives the possibility of loosing all gas from both tanks if isolation is delayed or impossible.
Either independent tank configuration gives the best result in a "limiting" air accident - meaning you don't typically need your full reserve (both tanks - manifold) to safely exit but you dang sure want at least half that reserve (independents). Gas isolation between the tanks is automatic. In an isolated leak the gas is still available but probably unnecessary - in an unisolable leak you breath it down if possible and then have the full minimum reserve still remaining. Sidemount is slightly better in that it does provide an "improved" ability to swap regs between tanks and better access to the hoses - but as you said this is generally not needed anyway and could be done in backmount if needed (which it shouldn't be).
Also sidemount gives the option to swap tanks between divers but this also may not always be advisable - but is a nicer option to have then having to share air between divers (especially since many sidemount diver's ability to share air between them is done only by swapping tanks since they don't generally carry a long hose on either tank). In BMI not only does the diver have the ability to independently exit with adequate reserves - plus less convenient reserves in an potentially isolated leak tank - but still may have a buddy with sufficient reserves for them both to exit in tandem.
I know many people who find traveling in sidemount to be more efficient. I have only 50-100 dives in sidemount and, for me, sidemount is clumsy and inefficient. I'd much rather deal with any problem (other then a bottle flooded with water) in BMI. :)
[And for swapping regs: bringing an extra reg along (shrink-rapped or on a 3rd bottle) would simplify swapping regs between tanks.]
I only carry a buddy bottle solo when I'm scootering. Otherwise, if it's a swim dive, the buddy bottle isn't needed. I have 2 completely independent air sources. I plan even more conservative air management. If I lose one bottle completely, I will still have plenty to get me out. If I lose both (not likely), then it's just my time. With backmount you don't have completely independent air sources. Sure having a failure that loses all your gas isn't likely, but it's still possible.Quote:
Originally Posted by skip
Some folks have a H valve on one of the tank with just a small reg for that exact purpose.Quote:
Originally Posted by Gary
When solo, I'm usually using a RB, so thirds isn't the issue. It's "can I get out, if multiple things go wrong". I'll carry an extra primary, and "80" for primary bailout, and if I'm scootering or doing a long swim, I'll drop a "40" somewhere along the way.
One day I just "went for a swim" in devil's. Lovely dive, right up until I realized I was at the Hinkel w/o enough bailout to exit if the 'breather took a vacation. Talk about a paranoid exit...
Sorry for the confusion. When I said isolate, it was isolation of the leak using the post isolator. English is so imprecise.
If you aim the x-over valve down and don't want to use it, why don't you get rid of it use a x-over no valve configuration to take advantage of the reliability gains through fewer failure points? I understand it is protect the valve thing.
I agree on not going to the x-over valve first. Shut the post - it is much more likely to solve the problem, and save on loss of gas. So restating your conclusion - "My point in a nutshell. If you have a reg failure", you isolate that post. Don't shut the x-over valve.
Quote:
Originally Posted by Slüdge
BMI or SM?Quote:
Originally Posted by Gary
Points all well taken. But in a "limiting" air accident, which I take to mean one which was isolable "using a post valve", you will lose a roughly given amount of gas which does not depend much on whether you are independent or connected, so the total volume remaining after problem isolation (post valve is shut) in any configuration whether independent (SM, BMI) or manifolded (BM) is roughly the same. I do agree it is somewhat available in SM(requiring concentration and added task loading) and with more difficulty in BMI, but not near as easily as in manifolded (nothing to do other than breathe)...thus manifold wins due to ease of gas access. Independents only win (as I pointed out) when the leak cannot be isolated using a post valve.Quote:
Originally Posted by Gary
Pausing to swap tanks or regs is slowing your exit - NOT what you ideally want to do. I do agree swapping tanks (if an option) makes sense at a certain point when total gas loss on the person of the stricken diver is near. Here, the tank swap saves time ultimately because the diver can swim faster on the remaining egress and is not slowed by a buddy air share. Swapping regs on the other hand, makes no sense to gain access to the "bad" side (except as a last resort when both you and your buddy are otherwise done) and only reinforces my point on benfit of the x-over. It will definitely delay your exit. In this case, I certainly agree the buddy provides additional available reserves that now make sense to use rather than adding time to exit.
Come to think of it - it was in Mexico so it was manifolded AL80 doubles (eek!) but was still pretty easy. (So the reg failed free-flow but I had full gas plus a stage and feather the valve for the power inflator.)Quote:
Originally Posted by addexdiver
Being able to reach valves easily should be something every backmount diver shoud be able to do. In BMI my tanks are turned inward slightly so the knobs are even more accessable (and out of the way).
Hey Jason,Quote:
Originally Posted by rchrds
Thanks for the feedback. I THINK I agree with you... but why exactly do you want more Murphy gas?? For me it sort of depends on the dive. Big dives I am way over conservative. But what that spreadsheet helps with is knowing the exact level of conservativeness ofr a specific situation. Rules of thumb result in varialble levels of risk.
Jason is heading for Afganistan, so he may not answer very soon.
Essentially he is saying the *minimum* safe reserve is 1/3 of your total gas supply, on a short dive. IE 1/3 of your backgas or sidegas if SM.
On a complex, multi stage dive, save *all* of your back/side gas, and only breathe off the stage bottles. Also plant extra *safety* bottles along the way.
One of the few things I ever agreed with GI3 is having lots of spare gas around. It doesn't have to be 1/3 of your total gas supply on a big dive, but should never fall below 1/3 of back/sidegas.
I have had too many friends cut corners on reserve gas, and pay for it with their lives :(
Quote:
Originally Posted by aainslie