AL80 to 132ft (caution-contains math)

[shoredivr]

Originally Posted by Colliam7

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.

Bingo.

[stunaep]

correct... on the recreational perspective only, i always tell the student to subtract the 500psi then divide by 3... i don't want them not leaving enough until they regularly use the rules and correctly... (yes, that was a double-negative! LOL!!!) i've seen divers interpret the 500psi as the turning point rather than the place you should be at the return of your dive... at 500psi, one can launch a bag, ascend the line, and do a surface swim...


dive safe!





--c

[4sak3n]

Okay, my turn! GDSMan inspired me

There is a twist, I'm doing everything in metric. So those of you clued up on metric please check my working.

Assumptions:
stressed SAC of 30l/min (~ 1 cft/min) for one diver
calm SAC of 15l/min (~0.50 cft/min)
12L cylinder
10m/min (33 ft/min) ascent rate
30bar (~ 440 psi) reserve upon surfacing
dive to 30m (~100 ft)

Ascent as follows:
1 min to get one's brown, sticky stuff together at 30m.
1,5 mins to ascend to 15m (av depth of 22,5m).
1 min deep stop at 15m.
1 min to ascent to 5m (av depth 10m).
5 mins safety stop at 5m.
1 min ascent to surface (av depth 2,5m).

1min * 60l/min * 4 atm = 240l
1,5 * 60 * 3,25 = 292,5
1 * 60 * 2,5 = 150
1 * 60 * 2 = 120
5 * 60 * 1,5 = 450
1 * 60 * 1,25 = 75

Total = 240 + 292,5 + 150 + 120 + 450 + 75 + (30 * 12) = 1687,5l

1687,5/12 = 140,625bar in a 12L cylinder.

Note: there is a fair bit of conservatism built in here. e.g. would the SAC rate realistically stay up at 30l/min during the safety stop? The safety stop is 5 mins instead of 3. Etc ...

So rock bottom is 140 bar.

Assuming a fill of 210 bar this leaves 70 bar for the bottom.

70 bar * 12l = 840l
840l/ (15l/min * 4atm) = 14 mins.

So ... what'd I miss?

[4sak3n]

Yikes, thats basically doing the dive on thirds! :11:

I personally believe that I would feel safe using a rock bottom of 120 bar because the calculations I did were so conservative. This can be squeezed in by reducing the safety stop to 3 mins instead of 5.

Think I'm being reckless and stupid?

And this leaves 90 bar for the dive which, at 15l/min SAC means I have 18 mins* @ 30m. Almost exactly the NDL. How convenient!

*90bar * 12L = 1080l
1080l / (15l/min * 4 atm) = 18 mins

[ucfdiver]

Originally Posted by 4sak3n

Yikes, thats basically doing the dive on thirds! :11:

I personally believe that I would feel safe using a rock bottom of 120 bar because the calculations I did were so conservative. This can be squeezed in by reducing the safety stop to 3 mins instead of 5.

Think I'm being reckless and stupid?

And this leaves 90 bar for the dive which, at 15l/min SAC means I have 18 mins* @ 30m. Almost exactly the NDL. How convenient!

*90bar * 12L = 1080l
1080l / (15l/min * 4 atm) = 18 mins

What if a tank oring or burst disk goes and you have to share air with a buddy, increasing your sac rate?

[4sak3n]

Sharing air with a buddy is already factored in. All the calculations for the entire ascent were done assuming a 60l/min (~2 cft/min) SAC rate ... 2 divers with 30l/min (~ 1 cft/min) each.

Your point about the burst o-ring (amongst other things) is very valid and is one of the many reasons why having a redundant source of air (even if it is only something like a pony) is so valuable.

I do however think that there is a point where you can be too conservative. If you are planning for major equipment failures ... like a burst disc or o-ring then you should factor things like redundancy into your planning instead of jacking up your rock bottom gas until you need more gas to safely ascend than you have when you start the dive.

Anyway, the point about shifting rock bottom to 120bar instead of 140bar is kind of academic since I much prefer using 15L cylinders for deep dives.

[GSDMan]

Originally Posted by Colliam7

I get slightly different numbers in a couple of cases. For the descent and acsent, for example:I get ~201 and 504, respectively.

Your math was correct. So was mine, actually. Based on the fact that my SPG can't be read with that kind of accuracy, I rounded all pressures up to the nearest 25psi. I guess I should have mentioned that. Ooops.

[hudson]

The first five sections of the following page have some good stuff about deep stops.
Decompression myths and mistakes

I actually use a minimum 2 minute deep stop, not 1.

[Doc Harry]

Your calculations look good.

Your final conclusions about 4:45 are correct, too, but need some clarification.

Maybe you left this out of your discussion because it is obvious, but it is worth reviewing. Remember your descent time should be included your bottom time. Your descent time is 2.2 minutes, along with the 4:45 gives you a bottom time of about 7 minutes. I think your NDL on air is about 9 or 10 minutes, but I don't have tables in front of me. You also need to have some air remaining once you arrive at the surface, say, 500 PSI.

SO when your say you have 4:45 of bottom time - theoretically "yes." Practically, the answer may be "no."

[pengwe]

Originally Posted by ucfdiver

What if a tank oring or burst disk goes and you have to share air with a buddy, increasing your sac rate?

That's why you dive with a buddy who is following a similar gas plan.