Point of no return on air, what is it?

[Rhone Man]

I would have thought that an experienced diver with a good reason to have to go could safely go to 150 or 160 feet on a single Aluminium 80 (comment: my opinion - not interested in a "deep air" argument). Any deeper than that and your potential problems start to stack up, not least getting back to the surface in the event of a failure of gas supply. Coincidentally or no, that is about the depth limit that BSAC applies (as opposed to the conventional North American limit of 130 feet for recreational diving).

I did the wreck of the St George in DR (144 feet) on air with Al 80 and I didn't really give it too much thought (although I was diving with two experienced technical divers). The Zenobia in Cyprus (130 feet) on any given day is packed with divers on single tanks not much bigger than Al 80s.

But anything deeper than about 120 feet, you really should be asking yourself how smart your dive plan is and whether you are experienced enough to execute it. I am told (and I believe) that in Cayman it is not uncommon for young and dumb dive pros do "bounce" dives to 300 feet on Al 80s for kicks. I am also told (and I believe) that every so often they don't come back. On any level (redundancy, narcosis, O2 toxicity, DCS risk), that is really, really stupid.

 

[liuk3]

Why do I have 3500PSI in an Al 80?

Weird. Well, assuming nothing goes wrong and I don't give a rat's ass about deco, if I descend at 60FPM for 6 minutes turn around and ascend at the same rate, I'll use just about all the gas in an 80 that was overfilled to 3500PSI and I'll reach 360feet.

Well, you could probaby plan to go deeper too if you are willing to suck the tank dry on the way back up at about 75 ft. and exhale all the way up to the surface on an empty tank. LOL.

Disclaimer: This is a joke. Please do not do your gas planning like this.

 

[k374]

yeah, I think the original question may have been a bit confusing. I wanted to get a feel for your own personal limit given your own experience, for instance, "if I was given an 80 with a full tank of air I would not go below xxx feet because beyond this I feel I would not be able to get back alive".

 

[Rhone Man]

Well, you could probaby plan to go deeper too if you are willing to suck the tank dry on the way back up at about 75 ft. and exhale all the way up to the surface on an empty tank. LOL.

Disclaimer: This is a joke. Please do not do your gas planning like this.

Sorry, that disclaimer really tickled my funny bone...

 

[liuk3]

Sorry, that disclaimer really tickled my funny bone...

Yeah, I thought with my luck some idiot would try it, and I'd get stuck in litigation for making such a moronic comment.

 

[Cave Diver]

yeah, I think the original question may have been a bit confusing. I wanted to get a feel for your own personal limit given your own experience, for instance, "if I was given an 80 with a full tank of air I would not go below xxx feet because beyond this I feel I would not be able to get back alive".

This is still a huge "it depends" question for me. Conditions, vizibility, how I feel on any given day, etc. all play a part in what my limit would be. Is it a quick bounce just to see something? Is it a planned depth? Is someone in trouble and I'm going after them?

If we want to talk in generalities, I generally stay shallower than 160' on air and single tanks. Typically if I'm planning anything below 150', I prefer double tanks and mixed gas if I'm on OC.

 

[DCBC]

What, in your personal opinion, would be the minimum depth before you think it would be unlikely to return alive to the surface given a single AL80 with air at 3500psi at the surface. This is just for curiousity of course, I'm not planning to do my own version of Mythbusters with this data :grinjester:

This depends upon several factors including: the mixture, personal effect of narcosis, fitness, BMI and air consumption.

I've done bounce dives on air to 200' on a single tank, decompressing on back-air. It does however depend upon the individual, as some people who go to 150' never know enough to come up.

Have a good idea how narcosis will affect you before beginning the dive. Go for a dry dive in a decompression chamber where it's safe to get an idea and make sure your buddy does the same (200' is no place to run out of gas for either of you).

When you go deep on OC, most divers are surprised how fast the mixture will be used. Know your consumption rate and project it for the depth you will be diving. Be sure to add a fudge factor to ensure you are safely out of the water before you're out of gas.

The key to safe diving is to not push the limits. Plan your dive to have more than enough gas and then take more.

 

[OldNSalty]

Well, in light of the re-phrased question I would say that my theoretical limit is somewhere around 150-160 feet but I can't think of a single, reasonable and realistic reason why I would ever do it.

 

[GrumpyOldGuy]

yeah, I think the original question may have been a bit confusing. I wanted to get a feel for your own personal limit given your own experience, for instance, "if I was given an 80 with a full tank of air I would not go below xxx feet because beyond this I feel I would not be able to get back alive".

There is a difference bewteen absolute limit of what one can do, and a reasonable depth where you can enjoy the dive and be virtually certain to bring yourself (and your buddy) back alive. There should be a good amount of difference between the 2 depths to give you a safety tolerance.

So, the initial and direct answer to comming back alive, for me and my moderate experience is probably 175 feet.

In real life for me, I stay significantly shallower:

60' preferred depth - can dive a long time and enjoy it, best bang for the buck

80' max normal - still completely safe, just a little shorter dive

100' personal max - I have been beyond, its not worth it for me. I start to worry about things too much and I am not geared up for it. I just do not enjoy the dive much below this depth.

Others will have different limits, each diver is different and dive conditions vary.

 

[Blackwood]

yeah, I think the original question may have been a bit confusing. I wanted to get a feel for your own personal limit given your own experience, for instance, "if I was given an 80 with a full tank of air I would not go below xxx feet because beyond this I feel I would not be able to get back alive".

I wouldn't plan much below 80-90 feet on an Al80 because I don't feel that I have enough gas to A. provide sufficient bottom time to make it worth the effort and B. get my buddy team back alive if it's that .1% of the time that something goes wrong.