This is where the rule of thirds breaks down. It's a simplification. You have to consider a) surviving a breathing system failure, b) surviving increased gas consumption (lost, delayed, stressed, extra deco, siphon). Recalculating thirds accounts for "a", but not "b". There is no problem with recalculating thirds if you give yourself a sufficient additional reserve for needing extra gas, and have enough experience to predict correctly.
"Those who make peaceful revolution impossible will make violent revolution inevitable." --JFK
Yeah, even though we are both CAVE 1, we will still do a LOT of cavern diving (probably more actually) to work on skills.... but still like having 1k psi in my tanks when they are loaded into the truck on the way home.Originally Posted by Squirrel Girl
And like my owsi, I teach my students the same thing... 1k to start your dive, 1k to end your dive and be back on shore or at the boat with 1k in your tank... that extra 500 psi isn't going to let you see anything more on an ocean dive... unless your at 15 feet the entire time...
Tim
Tim,
Just curious who your cave 1 instructor was, and did he teach you to place a safety bottle into the cave?
Andrew Ainslie had the best suggestion, but it is also hard to calculate in your head, besides most people don't have a good handle on their SAC rate. He basically said "At any point in the dive, you should have twice the gas necessary to get out". If you look at a simple dive in no flow. When you turn at thirds, you will have 2/3 left. It took you a third to get there, so you have twice what you need to get out. The hard part is knowing just how much gas you need to get out at any given point.
If you want my opinion, don't push your luck! The big problem with Andrew's logic, and recalculating thirds in general, is the time it takes to deal with a problem. Let's say you turn at thirds, swim a little way, and get caught in the line. It takes you a couple minutes to get un-tangled, and then you are on your way out. You still have plenty of gas left to get out safely. Now consider you dilly-dally around on exit, still having twice the gas you need to get out. When you get near the entrance with only twice the necessary gas, and get tangled. That same couple of minutes you need to get un-tangled is a much larger percentage of your remaining gas than it was at turn around. I think what needs to be added to the concept of recalculating thirds is having a minimum amount of reserve. Frankly I hate to ever get below a third of a full set of reasonably sized tanks (ie 80s, or above).
There are many cave divers that make a lot of really long dives that never use any of their back/side gas. They only breathe off of stages, or rebreathers.
Kensuf,
Had two cave instructors actually... I wouldn't say either "taught" me to place a safety bottle into the cave... however they did "teach" by example... Personally I thought this was just a requirement of the NAUI standards for teaching a cave class. I won't name either of my instructors, they know who they are and both were excellent instructors, with that being said I am not currently placing a safety bottle on my dives.... BUT I have a whole whopping total of 1 (ONE) dive post class. I do plan on using a safety bottle but right now my penetration is actually not much past cavern zone anyway (made it to the chimney in little on saturday, was within 200psi of 3rds and turned dive).
Tim
Yes!In Cozumel, we would head off down various branches and do some surveying. There it was very important to know how much gas it too you to get to the start. That was what you had to reserve twice of (plus a little cushion) to get out. And there comes a limit to how low you would ever want to go. It may take me 400 psi to get to my section near where the work begins. But do I want to plan for having 800 to come out? Seems too small a margin to me. Like you say, FW, all it takes is an unplanned emergency and suddenly a 400 psi reserve seems mighty slim!
Still, those are different conditions than a typical dive in Florida where you go in, turn around and come out.
Land of Enchantment -- not so great for cave diving, but mighty scenic!
Hi Tim,
Page 215 of your NAUI S&P states the limitations of Cave 1 divers, and that you always put in a safety bottle as NAUI Cave 1 divers. The reasoning is that at the Cave 1 level you're still learning to cave dive and in the event you screw up, you might need deco gas.
I've met a few NAUI Cave 1 divers that didn't know this.
Keep diving,
Ken
that is why I don't want to name my instructors... why have my lack of memory reflect on them.... HOWEVER they did lead by example and even if that was not stated, I do plan on following through with the process.... I just need to get a dedicated regulator/tank setup for it. Have the tank, just need another DIN 1st stage.
Call me greedy, but I like my air so I've called every dive I've been on before 3'rds... and if either of my instructors wants to out themselves they would tell you that as well. But I'll say it again... They both led by example and (required to or not) and that actually set's a precedent to any student that pays attention (even when they are not listening).
can't wait for dive 2, happening this week
Tim
I know your cave 1 instructor, so it's all good.
I took Naui cave 1 a number of years ago. Never really got why you'd put in one safety bottle if the team of divers is greater than one...
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