Re: Volume matching

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Posted by mike on September 01, 2001 at 01:53:30:

In Reply to: Volume matching posted by New Tech Guy on August 31, 2001 at 15:04:11:

well, what the heck, i'll give it a try;

suppose you (who can last an hour on a steel 72) are diving with a buddy who sucks air like a hoover. You have a steel 72 cuz that's all you need, and your buddy has a LP 120, and that will barely last him 45 minutes as it is.

OK, your steel 72 filled to 2250 psi contains 71.4 cu ft and your buddy's LP 120 really is filled to 2640 psi and really does contain 120 cu ft of air.

In your steel 72 (71.4 really), each cubic foot of air exerts roughly 31.5 psi (and by example, if you've gone through 3 cu ft while fooling around on the surface, your SPG should read 2250-(31.5 x 3) = 2250-94.5 = roughly 2150 psi. So using up 100 psi according to your gauge really means that you've drained a little over 3 cu ft from your steel 72 and now you have approximately 68 cu ft get the idea from there.

Your airhog buddy however, has a bigger tank, under higher pressure (120 cu ft at 2640 psi) so each cu ft of air in his tank is roughly equivalent to 22 psi (if he had a HP120, then each cu ft of air would be closer to 29 psi on his gauge (3500 psi/120 cu ft)

Let's say your SAC is .4 cu ft/min and your buddy's SAC is .9 cu ft/min. That means, that while fooling around at the surface, you are consuming (.4)(31.5 psi)=12.6 psi per minute and your buddy is consuming (.9)(22 psi)=19.8 psi per minute.

Go down to 100 fsw, your RMV is (.4)(4 ATA)= 1.6 cu ft/min, and you are going through your air at (1.6)(31.5) = 50.4 psi per minute while your buddy is going through his tank at (.9)(4 ATA) = (3.6 cu ft/min RMV) x (22psi/cu ft)= 79.2 psi /min.

Using these figures, your tank should in theory, last you 44 minutes at depth and your buddy's should last him 33 minutes. If you turn the dive on thirds, using disproportionate tank reln'ships, you'd plan the dive as if your buddy had the steel '72. If your airhog friend had your '72, he'd only be able to stay at 100 fsw for 19.8 minutes, and again, that doesnt give any time for ascent or descent. It's no fun being with a buddy like that. You by the way, could last 75 minutes at 100fsw, assuming no time for ascent/descent. Of course, always double chack the math.

Now, just to play the devil's advocate, what if your buddy's primary suffers a catastrophic failure or his primary second stage freeflows and empties his tank before you can even come close to meeting your deco obligations, getting out of the cave, fighting off the great white shark, whatever......lets say that this happens 20 minutes into the dive, when you've both accumulated deco time from RNT from earlier dives (maybe its your second dive of the day on the Yukon or the Olympic or Farnsworth) and bolting to the surface is not a desirable thing.

using the above figures, after 20 minutes, you should have used 1000 psi and have 1250 remaining according to your gauge. Your buddy has used around 1600 psi and has a little over 1000 psi of air remaining in his tank. Your buddy's reg takes a s*** and now both of you are sucking your tank dry faster than you can say "rule o' 3rds". Can you get to the surface w/o risking getting bent? Do you have enough air to manage 3 min at 20 or 3 min at 15 fsw or 5 min at 10 fsw or what ever your deco obligation is?

Your tank's 1250 psi pencils out to less than 40 cu ft of air for the both of you. The two of you are sucking down a combined 5.2 cu ft per minute, so the two of you have less than 8 minutes left before blowin and goin from 100 fsw and shining on a 5 min at 10 fsw deco obligation. And that's assuning that you are both calm and relaxed, and not the least bit bothered over the situation.

Both of your RMVs double from anxiety over the blown reg, and the impending consequences. You take 2 1/2 min to get to the deco stop (lets say 15 fsw at 5 min for the sake of argument), at an average depth of 58 fsw, thats (2.7 ATA x (.9 + .4)(2) = 3.6 cu ft /min)(2.5 min) = 18 cu ft lost just ascending to the safety stop. The two of you have just 21 cu ft of air for your safety stop/deco obligation at say, 10 or 15 fsw for 3-5 min. Lets say, you have to do a deco at 15 fsw for 5 min. That means that you'll collectively consume (.9 + .4)( x 2, cuz yer still freakin over what just happened & you're still not sure that one of you ain't gonna get to see the inside of that chamber)=2.6 cu ft /min...2.6 x 1.45 ATA at 15 fsw = 3.8 cu ft /min on deco. 5 minutes x 3.8= 19 cu ft used up on deco. You & your buddy surface with just 2 cu ft in your steel 72, or as your gauge would say, 63 psi. Oooh, good thing you weren't in a cave, or deeper, or working against a current, or cold, or tired, or dehydrated!

Ok that's it. If someone finds errors in the math, have Chris or Seahunt delete the post so no one gets misled on account of math errors.

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