Deep Diving 108 feet w/ a single AL 80 (Air.) No redundancy.

[TMHeimer]

Wow... Thread officially "Unwatched." My thanks to the members who stayed on topic, and provided their personal experiences, and views on the O.P.
Cheers.

I think it was a good reasonable question. As I posted, I would do that dive normally with pony, possibly without it if I was with a known good buddy. My brother did a dive that way in the early 80s off CA coast -- bounce dive (with buddy) to 200 feet. His eyes hurt later that day at suppertime. That story like some others posted, adds nothing to your question.

 

[boulderjohn]

The minimum gas calculation, which is the amount of gas calculated to bring 2 stressed divers to the surface (for MD dives, kind of like NDL) making all stops in a MD ascent profile.

Why make all those stops?

While doing research on NDL ascent strategies, I contacted GUE headquarters and asked for an official explanation of the MD ascent profile. I was put in contact with someone who explained it all in detail. This is a quick summary, but the explanation was that it is based on a GUE belief that the shallow part of an ascent should be done slower than the deeper part of an ascent, and doing a series of stops slows you down. This starts at half the maximum depth because it is easy to do the math to compute half. The stops are done every 10 feet because that is the way it is done in decompression diving, and it is good training to be consistent with that approach.

So are you saying that you shorten every dive considerably so that in the incredibly unlikely event that one of you highly trained divers runs out of air, you both can follow a procedure that should be unnecessary on a NDL dive?

 

[Storker]

Minimum gas calculations are much more conservative than the original rock bottom calculations, but more similar to the later rock bottom calculations, which then morphed into minimum gas.
The minimum gas calculation, which is the amount of gas calculated to bring 2 stressed divers to the surface (for MD dives, kind of like NDL) making all stops in a MD ascent profile.
So while rock bottom can change as your depth becomes shallower, MG does not. It isn't supposed to be recalculated unless it was in your plan.
The calculations for each of them are different. Hope that makes sense.

Thank you, but not quite. To me, at least.

If I'm at 30m, my minimum reserve to bring 2 stressed divers to the surface with a safety stop and a certain reserve at the surface is - given my RMV and my tank size - some 120 bar. If I'm at 20m, that reserve drops to 100-ish bar. And even less at, say, 10m. If my planned dive is a more or less triangular profile and I stay within my reserve both at 30m, at 20m and at 10m, is that min gas or rock bottom? And what's the difference?

 

[Ayisha]

Why make all those stops?

While doing research on NDL ascent strategies, I contacted GUE headquarters and asked for an official explanation of the MD ascent profile. I was put in contact with someone who explained it all in detail. This is a quick summary, but the explanation was that it is based on a GUE belief that the shallow part of an ascent should be done slower than the deeper part of an ascent, and doing a series of stops slows you down. This starts at half the maximum depth because it is easy to do the math to compute half. The stops are done every 10 feet because that is the way it is done in decompression diving, and it is good training to be consistent with that approach.

So are you saying that you shorten every dive considerably so that in the incredibly unlikely event that one of you highly trained divers runs out of air, you both can follow a procedure that should be unnecessary on a NDL dive?

I wouldn't say that they're shortened considerably. BTW, what are the PADI or DAN recommended ascent profiles now?

PMD is ascending to about 50% of *average* depth of a 100 foot or less dive, followed by 10' increments every minute (executed in a 30 second stop, and taking 30 seconds to ascend to the next stop). So it's taught and practiced like a pause and slide.

The first stop from a max depth of 100 feet or less is likely at a shallow depth, like 40 feet or less (remember, *average depth*, not max depth), and IMHO it does not considerably shorten the dive. YMMV.

Dives over 100 feet are calculated with Pragmatic Deco, ie) differently.

 

[Ayisha]

Thank you, but not quite. To me, at least.

If I'm at 30m, my minimum reserve to bring 2 stressed divers to the surface with a safety stop and a certain reserve at the surface is - given my RMV and my tank size - some 120 bar. If I'm at 20m, that reserve drops to 100-ish bar. And even less at, say, 10m. If my planned dive is a more or less triangular profile and I stay within my reserve both at 30m, at 20m and at 10m, is that min gas or rock bottom? And what's the difference?

What you're calculating is rock bottom. The difference is in the formula for each.
For a 100'/30 m dive, MG calculates as 40 cf of gas or about 1500 psi or 100 bar with a full 80 cf tank.
That's it. It doesn't change.

That gets calculated into MD or we also just use Minimum Deco on-the-Fly, much simpler.

 

[ThatDogDontHunt]

I wonder how many divers go to 100 feet with an AL 80 in Cozumel on an average day. 100? 200? I can't even guess.

I wonder how many dive sites around the world offer similar diving experiences every day. I can't even guess.

I wonder how many thousands of divers around the world go to 100 feet with an AL 80 every day. I can't even guess.

I wonder how many fatalities have occurred on an annual basis on such dives, for whatever reason. On this, I can provide a reasonable guess because of a past history of reading the DAN fatality reports. I would guess it is only a handful.

Is it a good idea to do so? That is a matter of opinion. Is it "insane" to do so? Well that is also a matter of opinion, but you will have a hard time convincing me that it is "insane."

100ft? The Blue Hole in Belize sees lots of AL80s going down to 130 feet (and sometimes a few feet deeper) on a routine basis.

 

[Dan]

So I guess you guys don't want to hear about my 220' dive on a single steel 72 in Palau in 1981?

Better than my Palau dive to 144’ on AL80 to see this guy, I bet.

 

[boulderjohn]

100ft? The Blue Hole in Belize sees lots of AL80s going down to 130 feet (and sometimes a few feet deeper) on a routine basis.

Yes, there will be several boats full every day. And a lot of them are brand new OW divers with very little experience.

 

[Storker]

What you're calculating is rock bottom. The difference is in the formula for each.

...and they are?

 

[Dan]

Now, hell no.....

With that said the first time i went deep was 103 with a AL 80.... being young and dumb as I was back then.

Another dumb ass (me) at dive # 15 in 2005, in Cayman Brac, site site called Son of Rock Monster, 126’ with AL80.