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Basically. recreational dive planning involves your dive buddy and deciding your max depth based on your training and no decompression limits (NDL) for your chosen depth.
For example, when I lead open water divers for the first dive of the day, I would take them down to maximum of 60 feet until the first person (fp) gets down to 1500psi; I do a check at 15 minutes, we then ascend to 40 feet & cruise there until my fp gets to 1000psi, ascend to 15 feet for the 3 minute safety stop then surface; This should be a 45 minute dive. My divers are also instructed to let me know if they get down to 1500psi before 15 minutes.
I can dive to 165ft for 20 minutes with full double 80's based on my crappy working SAC of .76cfm , this includes a 1/3 reserve in case I need to deco with it. As I recall, 23 minutes would be my max bottom time.
Diving deeper than your training or doing deco diving with just a computer is foolish and could kill you!!
If you are interested in tec diving, google IANTD or TDI and check out their courses...
Quote:
Originally Posted by GSDMan
Since I got next to no instruction in my OW class on actual dive planning, I've been teaching myself with this board as my text book. As an exercise in planning...
What are you going to do when/if you hit deco obligation and have a 1st stage failure?
Wait, which thingy is the 1st stage again?
For me, doing a dive like this would be plain stupid. It's the perpetual arguing about it that got me to wondering just how much bottom time I might have.
Basically. recreational dive planning involves your dive buddy and deciding your max depth based on your training and no decompression limits (NDL) for your chosen depth....
Whats wrong with adding proper gas management to that?
Im a bit confused by your math. Probably as I use metric but..
if its 149CF @3500psi then at 1500 psi its 63cf no??
149/3500*1500=63.8cf
63.8cf/270minutes= 0.236 CF per minute which is indeed pretty low at the surface.
And shouldnt it be @ 1ata? or was this in orbit?
close, he stated he had 1500 psi left, so it would be ((149/3500)2000) and you're right it is 1 ata, no 0 calculations are right, I just misspoke.
Quote:
Originally Posted by CoolTech
Not to mention the 10% overfill we usually recieve in the southern USA...
lol ok fine (164/3850)2350=100cuft used/270 minutes= .37 SCR, Ill even go out on a limb and say that your tank was filled in ice water, it is painted black so when you were diving right on he surface on a really hot day the sun heated it up and gave you even more gas... and it was a drift dive where you only floated and never had to kick your legs. Now you are saying that your dive was at 15 feet, if the entire dive was that deep then you would have one of the best SCR of .246 If you told me you did the dive at even close to 33 feet or deeper I would have to say you are either one of a primordial dwarf that uses almost no air to breath, a world class athlete, or just a liar.
Quote:
Originally Posted by CoolTech
So... your math is off, because your calculations are off, because everyone breathes the same... and every dive should be the same.. and every diver should be the same... etc... etc... etc... worthless waste of time...Welcome to planning your dives in South Florida, USA....
Um, no my math is not off, no not everyone breathes the same, that is why we calculate our SCR, so that we can perform "gas planning". I don't know any diver with half a brain who would think that planning a dive and making sure you have enough gas is a "waste of time"
So what is this "real diving" as you mention it? I don't know what VB stands for or where that is, but I am guessing it must be close to the Bermuda Triangle where basic math and laws of physics don't apply to all people equally. I could be totally wrong, you have under sized lungs that are super efficient, you developed the skill of only breathing with one lung, you actually snorkeled for a good part of the dive or something that you did not state. But if it isn't something like that then I would have to guess that this dive is just a chest thumping exaggeration or total fabrication of your imagination. And then you are going around giving out dangerous recommendations to a new diver? What does nitrox have to do with the dive anyhow? True if you had doubles, you could have longer bottom time, but the dive plan is for an aluminum 80, as an instructor surely you can "do the math" to plan a dive...
GSDMan, kudos to you for working through this kind of problem.
I'm just back from a trip and a little short on sleep, but going through your math, I see that you are using 3 cfm as your gas consumption for your rock bottom. Why is this? It looks as though you are mixing up rock bottom with rule of thirds. We were taught to use a stressed gas consumption rate for calculating rock bottom, on the theory that someone who has gone out of gas and is having to ascend on someone else's supply may be blowing through gas as a result of adrenaline. What we use is 1 cfm for each diver; you might, at this point, want to use a bit more, simply because your unstressed SAC rate is so high, but I doubt you need to go to 3 cfm. However, what you have learned from the exercise is that rock bottom for significant depths in a small tank is a HUGE proportion of the gas you are carrying.
The bottom line is that both safe gas management and staying within decompression limits result in very short bottom times in the 100 to 130 foot range. As Mike Ferrara has often said, such dives are better planned and executed as staged decompression dives, with proper equipment and training, so that you can actually get a chance to enjoy what you went down there to see.
...means that you have a surface gas consumption rate of .32, and that is assuming that you are actually sucking air out of your tank while floating on top of the water... HIGHLY SUSPECT!
I was diving Invade The Keys last year and my worst SAC rate was 0.40, best was 0.33, and that was doing wreck and reef dives and quite a lot of moving around.
I'd say 0.32 is certainly do-able for an experienced diver with little movement in warm water.
By the tables with EAN 28% (on a flat table...) This diver has a bottom time of 15 minutes... not 4:45 minutes... The idea of calculating descent rate and ascent rate with a computer is rediculous. The computer calculates these on a minute, by minute basis and supplies the diver with correct data based on thier actual dive, not on a flat table...
This guy is looking for a method to plan dives based on specific conditions and his own rate of air use. Seems to me he has addressed it pretty well. He's got some good suggestions on padding it a little from others so hopefully this thread is of value to him.
Sure, the 28% tables say you can be at that depth for longer but if your air use and cylinder size prevent it, either you won't be that deep for that long or you'll be there a whole lot longer waiting on the recovery team. He's trying to plan for the former rather than the latter and good for him.
If you're in the water doing the dive while planning it, trusting the computer is about all you got left.
, this includes a 1/3 reserve in case I need to deco with it. As I recall, 23 minutes would be my max bottom time.
