Point of no return on air, what is it?

[MXGratefulDiver]

I wouldn't go much below 40 or 50 feet on an AL80 with air ... mostly because if I go to all the trouble of going deeper than that, I'd want to stay for a while ... so I'll take a bigger tank(s) and a gas appropriate for the depth I wanted to go ...

... Bob (Grateful Diver)

 

[AzAtty]

Why do I have 3500PSI in an Al 80?

Ghetto cave fill, dude.

 

[fnfalman]

Kevin,

We know how much gas we have assuming that the tank was filled all the way.

Now we need to know is the Surface Air Consumption (SAC) rate. That way we'd know how long the tank would last. Then look at the dive plan at a certain depth (say, 100-ft) and see how long one can remain down there until reaching No Decompression Limit. Then calculate to see how long it would take one to surface (what ascension rate does the diver want to use; 20FPM, 30FPM, 60FPM), take into account of a minimum of 3-minutes safety stop and also take into account of leaving around 500-psi in the tank for safety reserve.

Check out this website Determining SAC Rates

The formula for determining the gas requirement is as follow:

Cubic feet required = (minutes X SAC rate) X [(depth in feet + 33) ÷ 33]

Let's use my own example. As you know, I use the HP100 at 3150-HP. So assuming that the tank is fully filled and I have 100-cuft of air available to me. With 500-psi for reserve, that equals to 16-cuft (rounded up from 15-something). So that means I actually would have 84-cuft to work with. This is a simple ratio equation because everything is pretty much linear: 500psi/3150psi = X-cuft/100cuft. calc it out and you get X = 16cuft.

100cuft - 16cuft air reserve = 84cuft to play with.

I'm going to a depth of 100-ft and the PADI air table gives me 20-minutes until I hit NDL. My SAC rate from my dive history is around .75cuft/minute - not spectacular, but hey, I'm an air hog and a fat dude. Using the formula above shows:

CuFT required at 100ft = (20min x .80cuft/min) X [(100ft + 33ft) / 33ft] = 64.50cuft
CuFT required at 15ft safety stop = (3min x .80cuft/min) x [(15ft + 33ft) / 33ft] = 3.50cuft

Thus far my gas requirements are already at 68cuft out of 84-cuft available to me (if I were to want to have the 500-psi reserve at the surface). That means I have 16-cuft available for descend and ascend. 16cuft/.8cuft/min = about 20-minutes for descend and ascend.

Or just buy an air integrated computer and go for it. Haha.

 

[Teamcasa]

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:

Depth is always a subject when diving is talked about, particularly when talking to non-divers. Depth will continue to attract those who need an ego boost as well.

In my opinion, I'm not taking a single AL80 past 130' for any more than a few minutes before I'm headed back up to where all the cool stuff is.

 

[AfterDark]

There is a wall a few miles from where I'm presently sitting where several people have successfully done bounce dives to beyond 200 fsw on single AL80's. However, in the past few years, two of those who attempted it didn't surface. One was found 10 months later, half buried in the silt at 205 fsw. The other one's still out there somewhere ...

... Bob (Grateful Diver)

They found their personal limit.

 

[Garth]

so... can I have a trauma doc on the boat with chest tubes ready to go with a baro unit on standby, helicopter hovering, and 10 million dollars in my bank account for my wife and daughter if one of those idiots at the surface forget what their job is? okay at least 20 million dollars. .

 

[cloudswinger]

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.

It seems that some people use the fudge factor all right, as in "oh fudge, I'm out of air!"

 

[dwhthediver]

As a DM candidate, I'd have to tow the line and say to dive within your training and to not exceed the max depth of your training.

As for minimum depth, that would be the deepest point you've been trained to dive to, which for PADI would be 60ft for OW, 130ft for AOW.

It's 100 for AOW, 130 if deep certified, but that is off subject. I have to agree with those who have said 1 foot per cubic foot, but since everyone's sac rate is different, you have to know your consumption and comfort level. I have done dives to 130 on an AL80, but diving isn't about depth, some of my most memorable dives never got deeper than 30'

 

[Hank49]

I don't know what the max is. I do know a few people that have done 300 feet on a single 80. A lot more who've done 250 on the same. Don't personally know of any who died doing it.

 

[redacted]

I don't know other than I'm sure it is much deeper than I would ever contemplate diving regardless of my gear configuration. I think I would be were more curious about the relative benefits of a 15 foot rest stop vs. a 20 ft rest stop or something else more applicable to basic recreational diving.