Deep Diving - Safe bottom air pressures

[yeohcheeweng]

Thanks, Bwerb, Mikemill, Teamcasa, Thalassamania & Elan for the quick and detailed replies. I am planning such a deep dive with a buddy and was trying to work out what is the safest cylinder pressure at 50m and 60m when we should be ascending. However, the dive site where we intend to do the deep dive has only AL80 cylinders and the usual volume is 200bar (2900psi). How much extra air can an AL80 cylinder hold safely?

 

[mark00020]

how big is your single tank????

 

[Tigerman]

All things such as sac, mods, ndls and the works aside..
The fact that this question is asked would lead me to the conclusion that the op shouldnt dive to 50-60m on a single tank, much less so if that single tank is air (60m would btw be beyond the ppo2 1.4 MOD for air) as it kind of suggest to me there is limited experience with deep diving involved here.

 

[bwerb]

Thanks, Bwerb, Mikemill, Teamcasa, Thalassamania & Elan for the quick and detailed replies. I am planning such a deep dive with a buddy and was trying to work out what is the safest cylinder pressure at 50m and 60m when we should be ascending. However, the dive site where we intend to do the deep dive has only AL80 cylinders and the usual volume is 200bar (2900psi). How much extra air can an AL80 cylinder hold safely?

Would it be possible to use travel bands or stage rig an additional 80 to double your gas supply?

 

[Thalassamania]

Running the calculation for a person with a 0.5 SAC they could do it (abit without much of a safety margin)

Using the following:
SAC: 0.5 cuft/min
Decent Rate: 15 ft/min
Ascent Rate: 30 ft/min
Bottom Time: 5 min
Depth: 160 ft
Safety stop: 3 mins @ 15 ft

Gas used during the...
decent: 39.29 cuft (correct number would be 18.26 cuft)
bottom: 14.62 cuft
ascent: 9.13 cuft
stop: 2.18 cuft

Total gas used: 65.22 cuft (corrected: 44.2)

End pressure: 554.17 psi (corrected: 1342.61)


Granted, I wouldn't do it...

Edit: Going back over the numbers it seems as though my spreadsheet is doubling the amount of gas needed during the ascent and decent but I haven't figure out why yet.

Edit 2: I think I found the mistake: Over engineered the formula

Running the calculation for a person with a 0.5 SAC one could do it with (with plenty of margin).

Using the following:
SAC: 0.5 cuft/min
Decent Rate: 60 ft/min = 197 seconds for descent.
Ascent Rate: variable, 120 fpm from 197 to 90 (54 seconds), 60 fpm from 90 to 30 (60 seconds), 15 fpm from 30 to surface (120 seconds)
Bottom Time: 5 min
Depth: 197 ft
Safety stop: not needed, a complete waste of time for this profile.

Gas used during the...
decent: 6.3 cuft
bottom: 17.4 cuft
ascent: 6.0 cuft
stop: 0 cuft

Total gas used: 30.7

End pressure: 1850

Piece of cake (at least from a deco and as management perspective

While I agree in actual fact that a bounce dive can be made, I get the impression the OP is asking about turn pressures, indicating he plans to spend a little time at those depths. In that case, an AL80 is not a smart choice for plus 100 ft, much less 164+ depths.

His question is rather meaningless without his SAC, his comrades SAC and their anticipated TOB.

You also have to multiply the SAC rate by 2 or 3 as RBT should take into account emergency situations where the rate increases.

No, that's the nature of a bounce; any perturbation of the plan is an immediate thumb.

Sorry guys...I was typing a long response and pulled the trigger on my initial reply a tad prematurely. A bounce dive is one thing...doing it as a planned buddy dive of any duration and you really don't have much gas left for anything other than a reading on your depth gauge.

I can do (and have done) quite a lot of work in 5 mins.

 

[mikemill]

Hmmm, Thal your post is giving me an idea for a small app that can handle variable ascent/decent rates. Maybe even multilevel it

 

[JDMerk]

The Op wants to do deep air on a single Al80 and now is inquiring about overfilling...

You don't want to make this dive buddy.

 

[Teamcasa]

I read that for an AL80 cylinder the rock bottom pressures should be around: <snip>
What are the figures for 50m & 60m? Is there a formula underlying the calculated values?

It is, in point of fact, perfectly possible to do a bounce to 197 on a single 80 and surface with plenty of gas left over.

While I agree in actual fact that a bounce dive can be made, I get the impression the OP is asking about turn pressures, indicating he plans to spend a little time at those depths. In that case, an AL80 is not a smart choice for plus 100 ft, much less 164+ depths.

I am planning such a deep dive with a buddy and was trying to work out what is the safest cylinder pressure at 50m and 60m when we should be ascending. However, the dive site where we intend to do the deep dive has only AL80 cylinders and the usual volume is 200bar (2900psi). How much extra air can an AL80 cylinder hold safely?

It sure looks like you are planing a dive to those depths, not just a bounce down and back. Even with a great SAC rate, single AL80's are not the best choice for those depths.

However, you could double them up or have stages set up at depth with some safety divers.

 

[Blackwood]

I hate ascending without a computer (or any depth guage) but I came up following my little bubbles and kinda hurried toward the surface because I wanted to send up a smb, which only had 25 of line on it, so I didn't stop deeper. My watch read 9:36 when I reached 25 feet. I did 12 minutes of hang and hoped that was a reasonable time and ascended.

Incidentally, how did you know your depths (140, 160, 190, 25) without a working depth gauge? General knowledge of the dive site?

 

[drbill]

I have dived deep (to 200 fsw) many times on the equivalent of an Al 80, but would never do so on a single tank. At bare minimum I would carry my 19 cu ft pony and keep my max depth to 175 on that setup. I've only had to use the pony twice on such a dive.

As for turnaround time, I would never suggest one for someone whose diving capabilities, air consumption, etc. is unknown to me. I know mine under my diving conditions (although even that has changed over the last year because I no longer do dives beyond 150 fsw routinely... I've become a rather shallow fellow rather than a deep thinker these days!).