Aluminum 80s versus larger steel tanks

[Mossman]

I used to use a fatty 100AL on air. As my air consumption improved, I found that I constantly bumped into deco on air, because I was staying longer at depth. I could not use the entire tank on the profile without adding deco stops or going up. The bigger tank ran me into deco and, I would assume, raised my risk of DCS. As 100AL nitrox was not available, I moved to 80 nitrox and never get very close to deco. Additionally the fillers never fully fill 100AL. So I was already 10% short. On a 120 air, I would bust the crap outta NDL, I would guess. So in some fashion, bigger tanks would increase my DCS risk on the same profile. So apples to apples, yea maybe the risk is greater?

Is the "increased risk" quantifiable?

As far as I know, as long as you stay within your NDL, the risk of DCS is pretty much the same - "undeserved" hits happen at the same statistical frequency, whether you push your profile to 1 min of your NDL or 10 min.

The "increased risk" here comes from the potential to unwittingly exceed your NDL and then fail to do mandatory deco stops. As long as the diver is aware, the "risk" shouldn't increase at all.

On the other hand, with more air, there is a less of a risk of running OOA. Again, this is the same type of risk. Aware divers will start their ascent well before they run too low on air. As long as the diver is aware, the "risk" shouldn't increase at all with less air.

So we're really only talking about unaware divers here: divers on 120s who don't watch their NDL versus divers on 80s who don't watch their gauges.

The question is, what "risk" is greater? The risk of exceeding one's NDL, missing mandatory stop(s), and getting DCS as a result versus the risk of running OOA, not having a buddy close by, and doing an OOA ascent or simply breathing water and drowning.

I was always taught that it's better to risk DCS, which may be highly treatable on the surface, versus running OOA and drowning underwater, but YMMV.

 

[MMM]

From the DAN website, a research paper on scuba causes of death. Don't even see DCS mentioned in here: http://www.diversalertnetwork.org/files/DivingFatalityCauses.pdf OTOH, running out of air is right up there. Yes, it is possible to run out of air on a tank of any size...but less likely when you have more of it.

I was always taught that it's better to risk DCS, which may be highly treatable on the surface, versus running OOA and drowning underwater, but YMMV.

 

[boulderjohn]

From the DAN website, a research paper on scuba causes of death. Don't even see DCS mentioned in here: http://www.diversalertnetwork.org/files/DivingFatalityCauses.pdf OTOH, running out of air is right up there. Yes, it is possible to run out of air on a tank of any size...but less likely when you have more of it.

A joint PADI/DAN study of scuba fatalities was conducted a couple of years ago, and many of the new changes to PADI standards and instructional procedures are based on that study. As has been noted in this thread, DCS is extremely rare to begin with. Fatalities from DCS are even more rare. The number one cause of death is health-related, especially cardiac events. The next most important cause of death is drowning caused by an air embolism preceded by a rapid, panicked ascent to the surface following an out-of-air incident.

 

[Skittl1321]

From the DAN website, a research paper on scuba causes of death. Don't even see DCS mentioned in here: http://www.diversalertnetwork.org/files/DivingFatalityCauses.pdf OTOH, running out of air is right up there. Yes, it is possible to run out of air on a tank of any size...but less likely when you have more of it.

Wouldn't you just run out at the 95 minute mark instead of the 65 minute mark? I mean, ops with 120s are still planning to do a safety stop at 700 and on the boat at 500 right? That extra air isn't being planned to be saved for an emergency. So the people who don't pay attention to their gauges are still not going to pay attention to their gauges. They'll just see more fish before they run out.

More air increase bottom time. It probably increases safety when doing well planned deeper dives, because you do have more air. And maybe if your OOA emergency is from a blown o-ring, you have more air to lose, so you might not end up OOA; but if you just aren't paying attention (which is every OOA story I've heard), more air doesn't fix stupid. If you plan your dive to end at 500; starting with more doesn't add any safety margin. The added safety margin of having more air is when you plan the dive so that extra air is extra; not used.

 

[Mike]

Wouldn't you just run out at the 95 minute mark instead of the 65 minute mark? I mean, ops with 120s are still planning to do a safety stop at 700 and on the boat at 500 right? That extra air isn't being planned to be saved for an emergency. So the people who don't pay attention to their gauges are still not going to pay attention to their gauges. They'll just see more fish before they run out.

More air increase bottom time. It probably increases safety when doing well planned deeper dives, because you do have more air. And maybe if your OOA emergency is from a blown o-ring, you have more air to lose, so you might not end up OOA; but if you just aren't paying attention (which is every OOA story I've heard), more air doesn't fix stupid. If you plan your dive to end at 500; starting with more doesn't add any safety margin. The added safety margin of having more air is when you plan the dive so that extra air is extra; not used.

Well said, and exactly why tank size isn't the issue. As you said - stupid is as stupid does, no matter what's on their back.

 

[KevinG58]

Wouldn't you just run out at the 95 minute mark instead of the 65 minute mark? I mean, ops with 120s are still planning to do a safety stop at 700 and on the boat at 500 right? That extra air isn't being planned to be saved for an emergency. So the people who don't pay attention to their gauges are still not going to pay attention to their gauges. They'll just see more fish before they run out.

More air increase bottom time. It probably increases safety when doing well planned deeper dives, because you do have more air. And maybe if your OOA emergency is from a blown o-ring, you have more air to lose, so you might not end up OOA; but if you just aren't paying attention (which is every OOA story I've heard), more air doesn't fix stupid. If you plan your dive to end at 500; starting with more doesn't add any safety margin. The added safety margin of having more air is when you plan the dive so that extra air is extra; not used.

Well, kind'a sort'a. But at any given pressure, excepting zero you have more air available using a 120, than a 80. It's the proverbial bigger tank after all.

 

[Mossman]

A joint PADI/DAN study of scuba fatalities was conducted a couple of years ago, and many of the new changes to PADI standards and instructional procedures are based on that study. As has been noted in this thread, DCS is extremely rare to begin with. Fatalities from DCS are even more rare. The number one cause of death is health-related, especially cardiac events. The next most important cause of death is drowning caused by an air embolism preceded by a rapid, panicked ascent to the surface following an out-of-air incident.

That's why I choose a dive op that provides healthy fresh fruit and does the heavy lifting so I don't have to risk a coronary climbing up the ladder with the heavy 120.

Post-dive wetsuit washing can be very taxing on the heart as well, much better to let the dive shop handle it and relax with a cold beer and cigarette instead

---------- Post added May 22nd, 2014 at 12:25 PM ----------

Wouldn't you just run out at the 95 minute mark instead of the 65 minute mark?

Maybe. The longest dive I've done in Cozumel was only 91 minutes and that wasn't called due to LOA.

 

[cvchief]

Well said, and exactly why tank size isn't the issue. As you said - stupid is as stupid does, no matter what's on their back.

Right, so big tanks and small tanks aren't safer. Glad we cleared that up. And most of those poor SOBs in the chamber (and there are a few) don't really need a ride.

So maybe we missed the wrong argument: Is there a greater risk of taking a chamber ride without DCS on either the big or the small tank.

Clearly DD has the safest system with his 'extra' air in a second bottle and with a complete system.

We had a special request today: Got to do 104 minutes on CS and 84 on Pardiso. Even on 36, I am feeling a little nitrogen loading. Or maybe it was the chicken mole on the comida de dia at Casa Cozumel?

 

[Dave Dillehay]

Hey Chief, is this a challenge? If so, my longest as a dive guide with paying clients was at Paso De Cedral, ending in San Francisco—2 hours 2 minutes, max depth 55 ft but multi level following our Oceanic computers. It was I that ended up calling the dive as I was OOA!. However time of a dive means nothing unless depth and profile are considered. But it is not reasonably questioned that more air can provide more dive time.

Dave Dillehay
Aldora Divers

PS You still in town? When are we going to get that Margarita?

 

[Mike]

Well, just so we keep this thread from getting too huggy...

Right, so big tanks and small tanks aren't safer. Glad we cleared that up.

To clarify :Big tanks are safer than smaller tanks (more air = more air). Smaller tanks are not safer than bigger tanks. (less air = less air)

More is betterer then less.

(Whew) that was a close one!