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the thing that one has to consider is, "why am i doing this dive?" if its just to hit a certain depth, don't do the dive! do your dive to see something, part of a planned recovery, part of a class, part of a team, etc... doing a dive just to see if you can is set with potential problems. don't count on sac rate as a safety factor, because there is a good chance that your sac rate will change with stress, cold, unforeseen circumstances, diver error, equipment failure/error, etc...
if you were doing a dive to sixty feet, even with a low pressure hose failure, one could do an emergency swimming ascent to the surface. at 130...??? not sure... the other thing to consider, since no one can predict when narcosis will hit and to what level, what if you get narced and blow your schedule? what if you enter deco? where is your extra gas? is it worth a case of the bends for a "look what i can do dive"...??? my answer is still no.
i've been adding a little more to my knowledge base for the last two years by taking more classes, doing dives, and thinking! i've seen divers die by taking themselves too far.
just be careful... maybe sling an 80 for your bailout...
--c
__________________
"My life has no purpose, no direction, no aim, no meaning, and yet, I'm happy. I can't figure it out. What am I doing right?" --Snoopy
you are very fortunate to have that kind of sac rate... my experience has been that the average person breathes about 1 cu ft per min on the surface! more when they are new to diving and more when they are stressed. small framed individuals that are thin have lower sac rates, too...
the best i've ever gotten is in the .4 range... don't remember the circumstances. i'm usually averaging on the low side of .6 and the high side of 1.75 (just depends on the level of work... deco is usually near .6 while all out kicking pushes me above 1)
i use sac rate as only a virtual referrence... i use a 1/3 or 1/6 rule for planned dives. gas that i have to breath is more important than how fast or slow i consume it.
cheers,
--c
Quote:
Originally Posted by NorthWoodsDiver
.79 for a SAC seems way way high. on a bad day I might hit .55 and generally amin in the .3 to .35 range.
on a shallow dive an al80 will last me over 2 hrs with 750 left at the surface. A deeper dive to 140ft on a standard 80 I take about 9-10 minutes bottom time before starting back and always get back in the boat with well over 1000psi even after some minor deco obligations and a multiple stop ascent.
this particular profile seems far to conservative for even me and I generally fall on the overly conservative side. Just follow thirds and tables or a computer (or better yet use both). take the limiting factor to turn on. If you hit thirds before a suunto calls your dive or before even a standard table limits you there is an issue somewhere.
__________________
"My life has no purpose, no direction, no aim, no meaning, and yet, I'm happy. I can't figure it out. What am I doing right?" --Snoopy
Care to share that understanding with me? It appears to me that PADI considers their OW students too stupid to comprehend.
My thoughts are that they(SDI-My course) don't teach this aspect because they are teaching strictly no deco diving. SDI teaches with computers and focuses very little on the tables. If you stay within recreational limits (above 60' to them) and your dive ends with 500psi in your tank..you are good for go. I personally felt it was a catch 22 to trying to plan your dive...Fortunately, my brother in law is a techie...he explained it all to me. We were also never taught anything about SAC rates..so it was even more difficult to understand.
Presumably, 95%? of all divers never probably never do anything more with their open water cert...
I could be wrong in the reasoning..but that is my understanding.
My thoughts are that they(SDI-My course) don't teach this aspect because they are teaching strictly no deco diving. SDI teaches with computers and focuses very little on the tables. If you stay within recreational limits (above 60' to them) and your dive ends with 500psi in your tank..you are good for go. I personally felt it was a catch 22 to trying to plan your dive...Fortunately, my brother in law is a techie...he explained it all to me. We were also never taught anything about SAC rates..so it was even more difficult to understand.
Presumably, 95%? of all divers never probably never do anything more with their open water cert...
I could be wrong in the reasoning..but that is my understanding.
But to ensure that you surface with that 500psi reserve you need to know a few things:
-Your SAC.
-Your planned bottom time.
-Your depth(s) (at different depths/avg. if you´re going to multilevel).
-Your tank volume at the start of the dive and what gas you´re using.
No OW-course I´ve ever heard of teaches that.
To be able to determine if 500psi is enough for a particular dive (or any) you also need to think about:
-Your buddy.
-Your gas consumption when stressed.
-Your gas and your planned profile (to determine a desired ascent-plan)
-The inherent risks in the dive you´re planning
etc...
__________________
"Minds are like parachutes - they only function when open."
On the darker, dark side because they have the cookies now...
...
-Your SAC.
-Your planned bottom time.
-Your depth(s) (at different depths/avg. if you´re going to multilevel).
-Your tank volume at the start of the dive and what gas you´re using.
No OW-course I´ve ever heard of teaches that...
At least one agency requires it - some of that is on the IANTD OW test and all of it is supossed to have been taught during the classroom time.
For the O.P. Great job thinking this thru - very few divers stop to consider the effects of all those variables, and as you noted it can leave you with very little gas available for the bottom portion of your dive.
Another article you might find helpful is one written by Bob Bailey Gas Management - Many of us have benefited from his well written article on Gas management.
BTW - you don't need a class to learn this stuff, just some basic math and common sense.
1/6 rule (recreational)
3000-500=2500
2500/2=1250
turn at 1250 used... to return around 2500
1/3 rule (technical)
3000-1000=2000
2000/2=1000
turn at 1000 used...
(on the spg... turn at 1750 and 2000, respectively to return with the correct gas left on the spg)
i've never used sac rate, bottom time, etc. when diving this way... we use tank factoring when using diss-similar tank volumes, too, when one is diving with a buddy with a smaller tank size or fill... sure, if you freak out or your buddy does after the descent, your gas will go quicker... use the force!
even if you start with less gas
2700-900=1800
1800/2=900
turn at 900
one would have to watch their time and depth if they are staying in "no-stop" diving... a dive computer, tables and timer could do that... when i did cave, the only time we ever went into deco was when we specifically did a 90ft + dive and wanted to go in... deco gas was waiting for us, but because of the slow ascent and profile, we were out of deco by the time we were out, anyway... i've usually turned dives for using up my gas rather than approaching my no-stop limits (while rec diving)...
cheers,
--c
Quote:
Originally Posted by grazie42
But to ensure that you surface with that 500psi reserve you need to know a few things:
-Your SAC.
-Your planned bottom time.
-Your depth(s) (at different depths/avg. if you´re going to multilevel).
-Your tank volume at the start of the dive and what gas you´re using.
No OW-course I´ve ever heard of teaches that.
To be able to determine if 500psi is enough for a particular dive (or any) you also need to think about:
-Your buddy.
-Your gas consumption when stressed.
-Your gas and your planned profile (to determine a desired ascent-plan)
-The inherent risks in the dive you´re planning
etc...
__________________
"My life has no purpose, no direction, no aim, no meaning, and yet, I'm happy. I can't figure it out. What am I doing right?" --Snoopy
Last edited by stunaep; May 12th, 2008 at 02:56 PM.
Reason: clarified and corrected math...
Four comments: 1. The math approach appears correct (as I worked through your equations), and reasonable. Your SAC 'fudge factor' for the Rock Bottom estimation (1 vs .79) is conservative, etc. 2. For some reason when I do the algebra, I get slightly different numbers in a couple of cases. For the descent and acsent, for example:
Quote:
Originally Posted by GSDMan
3 * 0.79 * 2.2 * 38.46
5.2 * 38.46
225
13.1 * 38.46
525
I get ~201 and 504, respectively. Not a big deal, but you asked about the math. It could easily be my math. 3. This thread in some ways is classic SB. The OP asks specifically about the math calculations on a 'mental exercise' and responses range from 'use a computer', to essentially 'why would you do such a crazy dive', to 'your SAC rate couldn't be that high', to full frontal assaults on other posters, and NOT just because their comments were extraneous to the OP. Good reading. 4. I am not sure that PADI, or any other agency considers their OW students too stupid to comprehend math. But, there seems to be a considerable level of apparent innumeracy among the populace, including among some very extensively educated individuals. So, perhaps, the more detailed math that GSDMan is asking about is omitted from most OW instruction because it is considered not essential (i.e. instead use tables, watch your pressure, use rules of thumb, etc. for dive planning) and represents more of a distraction than a benefit at the initial OW level. That is surmise on my part, not intimate knowledge of any agency doctrine with regard to calculations. Personally, I like the math, and I commend the OP for the level of interest that generated the question.
i've never used sac rate, bottom time, etc. when diving this way... we use tank factoring when using diss-similar tank volumes, too, when one is diving with a buddy with a smaller tank size or fill... sure, if you freak out or your buddy does after the descent, your gas will go quicker... use the force!
even if you start with less gas
2700-900=1800
1800/2=900
turn at 900
one would have to watch their time and depth if they are staying in "no-stop" diving... a dive computer, tables and timer could do that... when i did cave, the only time we ever went into deco was when we specifically did a 90ft + dive and wanted to go in... deco gas was waiting for us, but because of the slow ascent and profile, we were out of deco by the time we were out, anyway... i've usually turned dives for using up my gas rather than approaching my no-stop limits (while rec diving)...
cheers,
--c
You have your turns wrong, if your on 1/3s then you turn after the first 1/3 is consumed, the second 1/3 is for return and the thrid or last 1/3 is emergency reserve.
There’s nothing like a trail of blood to find your way back home.
