How deep can you theoretically go on single tank?

[Charlie99]

all the limits aside, how deep can you go on a single cylinder?

Here's a theoretical answer to the hypothetical question of how deep can one go on a single tank.

Assumptions: Al80
SAC: 0.5cfm
Descent rate 100fpm or 3atm/min (faster than normal, but roughly what I used for bombing a wreck)
ignoring narcosis.

The AL80 would last about 10 minutes, you'd get down to 1,000'.

Oh! You wanted to make it a ROUND TRIP ??????!!!!!!!!

 

[Charlie99]

I've been at the "dragged" end of an OOA who wanted to bolt and I have no desire to repeat it.

That sounds like another empirical confirmation of Rick's observation on the relationship between breathing rate and ascent rate

 

[jonnythan]

I dare you to keep a 0.5 cfm breathing rate at 500 feet on an Al 80

 

[Charlie99]

I dare you to keep a 0.5 cfm breathing rate at 500 feet on an Al 80

You first!

 

[RiverRat]

An aluminum 80 has no place deeper than 100' and is in it's prime around 30'.

I don't know if I'd agree with that.....
Thousands of tropical dives are done daily to 100-120 fsw on 80's. Usually for a short period on a wall and the rest of the dive is multi-level up to a shallower depth. Granted it would be nice to have 100's on these dives but that's just the way it is. Some of these places you can't even get Nitrox. So if you want to dive you just have to adjust you plan accordingly.

 

[Soggy]

Ok, guys...stop nit-picking. Yes, there are circumstances where a dive *can* be done down below 100 ft. Yes, you can drop down and do a multi-level dive coming up. What I am referring to is dropping to 130 ft and doing a square-dive profile at or near that depth. Single tanks, Al80 or no, have no place in this type of diving. You can argue all day about it, but when you run the numbers, sit down and really analyze it, there is no room for contingency. NDL at 130 ft on air is 5 minutes...yes, you can do a bounce dive and come right up and probably have enough gas to survive it should something blow at the bottom. Is it smart to do so? No.

I'd also argue that many of those thousands of divers doing 120 ft dives in the caribbean are just lucky and if any actual problem occurred, they'd probably be dead. A lot of stuff is done daily that is stucking fupid....a lot of people drive drunk, too without getting caught or injured...doesn't make it a good idea. Just because the dive is there and you only have air to do it on, doesn't mean you should do it, or that it's smart to do it....if you don't have the right tools for the job, *DON'T DO IT.* I don't understand why that is such a difficult concept for some people to grasp.

If you don't care about contingency planning at all, then be my guest and dive the Doria on a single al80. Can it be done? Sure. Will you survive if you *or your buddy* has even the slightest problem? Probably not.

 

[miketsp]

Since we've got such a bunch of nit-pickers the next logical step would be to calculate the optimal group size for this exercise.
Since rock-bottom for a buddy pair depends on one member having enough gas to get the other member back up, there should be room for improvement by increasing the group size.
In a large group 1 catastrophic failure would have no impact on gas planning. OTOH depending on the MTBF for catastrophic events, as the group grows the risk of having multiple catastrophic events also increases.
However, catastrophic events do seem to be pretty rare - based on other threads discussing o-ring blowouts and the large number of divers doing stupidly deep dives every day with a very low casualty rate.
So the logical conclusion would seem to be - only do stupid dives in very large groups.

PS. We should also discuss the algorithm for air-sharing when a large number of donors are available. I suppose it would be a form of Weighted Round Robin, taking more air from those that have a higher residual pressure than the group average as obviously they have a better SAC and need less air to get back.

 

[jonnythan]

The only thing better than having no one watch your back and be responsible is having 10 people not watching your back or being responsible.

 

[Soggy]

So the logical conclusion would seem to be - only do stupid dives in very large groups.

This would be one of the reasons why dives (especially technical ones, but it works well for recreational also) are best conducted in groups of 3. Not only do you have 2 sources of redundancy, but you also have one level headed guy in the event someone fubars the dive and runs out of gas. Now you have one guy who can make the decisions while the other two handle the emergency.

However, regardless of group size, rock bottom is still calculated by determining the amount of gas required to get 2 stressed divers to the surface at a safe ascent rate and with all decompression/safety stops. While it can be done, there is no reason to require a buddy switch for the OOG diver mid-ascent.

Basically, under no circumstances should an out of gas situation be a true emergency...it should simply be an annoyance/inconvenience.

 

[Diver0001]

Since we've got such a bunch of nit-pickers the next logical step would be to calculate the optimal group size for this exercise.
Since rock-bottom for a buddy pair depends on one member having enough gas to get the other member back up, there should be room for improvement by increasing the group size.
In a large group 1 catastrophic failure would have no impact on gas planning. OTOH depending on the MTBF for catastrophic events, as the group grows the risk of having multiple catastrophic events also increases.
However, catastrophic events do seem to be pretty rare - based on other threads discussing o-ring blowouts and the large number of divers doing stupidly deep dives every day with a very low casualty rate.
So the logical conclusion would seem to be - only do stupid dives in very large groups.

PS. We should also discuss the algorithm for air-sharing when a large number of donors are available. I suppose it would be a form of Weighted Round Robin, taking more air from those that have a higher residual pressure than the group average as obviously they have a better SAC and need less air to get back.

Since we're started, the optimal support team needed for one buddy pair to make a safe dive to 100' using AL80's would be this:

1 buddy pair (2 divers) both equipped with AL 80's

2 support divers equipped with dual AL 80's and 2 AL 80 stage bottles per diver, one of which will be carried as a redundant air supply for buddies #1 and #2 and one of which serves as a Murphy bottle to account for the possibility that one of the other stage bottles packs it in or that one of the support divers needs help. these two support divers will initially be stationed at 100' but will be relieved by 2 other similarly equipped support divers after 10 minutes, after which the first two support divers will take up station at the stop depth and wait for our risk taking buddy pair.

You need 1....no make that 2 DIR divers to offer bitter criticism and 2....no make that 4 other divers (call them redundant support divers -- After all redundancy is good) to follow around the DIR divers screaming "we don't care" through their regs during the entire dive. The DIR divers are, of course, not wearing computers, but their 4 support divers can keep them safe anyway.

Oh, and we need Leigh Cunningham. We just can't make a dive like this without Leigh Cunningham. (if you don't know who this is, see the attachment)

Finally, we need at least 2 divers on rebreathers to make the video.

So, according to my calcualtions, a team of 13 support divers will be sufficient to allow our buddy pair to make the 100' dive safely.

Within the NDL's of course. If our buddy pair wants to put their computers 2 minutes into deco then we have a whole other rat's nest of problems that have to be solved.

R..