AL80 to 132ft (caution-contains math)

[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, I asked myself how much Bottom Time I would have at 132' on an AL80. I've read the arguments diving this deep on an AL80 generates, so lets just call this a mental exercise. I think I'm on the right track, but would appreciate some feedback.

I decided on 60'/min for the descent, 3000psi as beginning pressure, and my recent SAC of .79.
Descent – 60’/min. to 132’ 5.2ft3 and 225psi
(66/33+1) * 0.79 * 132/60 * 3000/78
3 * 0.79 * 2.2 * 38.46
5.2 * 38.46
225

Then, my Rock Bottom assumes a SAC of 3.0 (1.0 for me, 1.0 for my buddy, and 1.0 for a reserve), 3 minute safety stop @15', and ascent @30'/min. Also, no overheads, just straight down and back up.

Rock Bottom – SAC 3.0 (1 for me, 1 for buddy, 1 as reserve) 30’/min from 132’ 52.7ft3 and 2050psi
Ascent – (66/33+1) * 3.0 * 132/30 * 300/78
9.0 * 4.4 * 38.46
39.6 * 38.46
1525
S/S – (15/33+1) * 3.0 * 3.0 * 300/78
13.1 * 38.46
525

That leaves 20.1ft3 and 725psi available for Bottom Time.

Bottom Time - @132’ w/20.1ft3 and 725psi
(132/33+1) * 0.79 * T * 3000/78 = 725
T = 725 / (5 * 0.79 * 38.46)
T = 725 / 151.9
T = 4.77

So, by this it looks as if I would have 4:45 at 132' before I had to come back up. Am I doing this right? I'm more interested in the proceedure, but if I screwed up the math, please tell me. I'd appreciate any feedback and suggestions you all have. I'd rather get beat up here than screw this up and get hurt, or worse.

Thanks,
Jim

 

[dumpsterDiver]

I don't know, but NOBODY has a SAC of 0.79 maybe 0.8, but carrying that out is crazy.

 

[BHB ScubaTroll]

... I'm more interested in the proceedure....

Thanks,
Jim

The proceedure is to get a computer and get Nitr0x certified... follow the computer and if you need to go into deco, follow the computer again...

 

[David P]

The proceedure is to get a computer and get Nitr0x certified... follow the computer and if you need to go into deco, follow the computer again...

WHAT!?!?!?!?!?!? man I hope your joking...


hmm found another one of your posts....

Venice Beach... Single HP 149. 4 1/2 hours. The only reason I surfaced... I got hungry. I still had 1,500 PSI left in the tank.

ok so 3500-1500=2000psi used with a tank factor of 4.33 means that you used 86.6 cuft of air. 86.6/270 minutes 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!

 

[Packhorse]

I second that. I really do hope you are joking.
Everyone diver know how to do this type of gas planning.
I would look it over for you but I dont do imperial. It just makes everything so difficult.

 

[bleeb]

Another factor you may want to include is a few hundred psi for two reasons.

One is that the gauge you're using may be a bit off, especially near the bottom of its range. Gauges are a bit non-linear, and are often designed so that the largest inaccuracies are towards the range limits. OTOH if you're always diving with your own high-quality gauges AND you know how they behave, this factor may be less of an issue.

The second reason is that your regs may start becoming hard to breath as the tank pressure drops below the intermediate pressure (IP) between first and second stages, typically somewhere around 145 psi.

Other than that, in my limited experience, your rock bottom calculation doesn't seem significantly out of whack. For such a deep dive, I'd normally prefer to add a deep stop and possibly a longer safety stop, but it's debatable how important this is to include in calculations for quasi-emergency conditions.

 

[David P]

ok its late and I didn't sleep much last night.....
you want to dive at 5 ATA with a total of 77 cuft of air. Min Gas 2 divers @ 1.5 (your extra padding) coming up @ 30 fpm, 1 min @ 4ATA emergency, 1 min ascent to 3ATA(equate 3.5ATA), 1 min deep stop at 3ATA, 3 min ascent to 15' (Ill just go with 15' and not mention a min deco profile...)(equate 2 ATA), 3 min @ 1.5 ATA safety stop.
3(4+3.5+3+6+4.5)=63cuft or 2451 psi. Dont forget you should plan to have some air left (say 500 psi) on the surface for inflating your bc and incase surface conditions suck and you need to keep a reg in your mouth while fighting high seas....

or you know, just trust your handy computer...

 

[Packhorse]

SAC= 20Litres per Minute
Depth=40meters
20*((40/10)+1)=100
So at 40 meters you go thru 100LPM
Dont worry about your accent/decent time just add it to your bottom time. It makes the calculations easier and adds a little padding to your gas supply. You can calculate your deco stops air consumption seperate and add it in though
20*((5/10)+1)=30LPM

So a 10 minute dive to 40 meters including decent and assent =1000L of air + 90L for safety stop =1090L .
1090L/10Ltank =109bar of pressure used. + 50% for emergencys = 163.5 bar

 

[lazyturtle]

Descent – 60’/min. to 132’ 5.2ft3 and 225psi

Just out of curiousity..why descend this slow? The rate makes me think that you are confusing ascent and descent rates, of course that assumption is solely based on the rate you put.

 

[Packhorse]

Whats the rush?
I find 20 meters per minute (60ft) is a nice rate to decend.