Provocative talk yesterday at NSS-CDS: Toss out the rule of thirds?

[mr_v]

The chances that all of one diver's gas will evaporate at your greatest penetration is pretty remote.

Imagine the largest new tank you want to carry with you. Then imagine filling it with the most expensive gas mix. All of that together is still less than funeral cost. In fact, good technical gear is so wildly available at decent prices, it makes no sense to go overhead with optimistic gas planning.

 

[The Chairman]

it makes no sense to go overhead with optimistic gas planning.

You're missing the point. That's OK. I can't remember the last time I hit thirds. Thirds is only a tool to make a rough plan with. If you want to push that tool to absurd limits, you'll often get the dive you deserve, but not the one you want.

 

[DiveLikeAMuppet]

Straight rule of thirds is not conservative at all - but I think thirds come from a time when people dived halves so thirds were seen as progress, especially in a cave with some flow?

It would be interesting to see the math - even if you plan fourths and do a bit of rounding, you are not getting out with x5 gas consumption. That's CO2 hit level.

 

[PfcAJ]

What condition is causing a sudden OOG at max pen?

If something goes wrong on a cave dive (anything) and you don’t do anything about it, you’re going to drown. If your tanks are leaking and you just don’t address it, you’re cooked regardless of what your gas reserves are.

 

[dray_gnv]

Is there a video of the event/talk maybe?

I believe it was recorded but unsure when or where NSS-CDS will post it.

 

[LFMarm]

This was discussed recently here No-Flow Safety Factor / Margin where you can also find a simple excel to calculate the safety factor for a given reserve and flow.

 

[rjack321]

Yesterday, at the annual NSS-CDS conference in Alachua, Stratis Kas gave a talk in which he questioned the supposed conservatism of the sacred rule of thirds. His argument is that with a two-diver team, if one diver is out of gas at the farthest point on thirds, there is no realistic hope of team survival. His research shows that RMV increases for both divers, but particularly for the out-of-gas diver, by a factor of as much as 6-10 times normal average.

His math showed that even a three-person team, with two sharing, wouldn't make it. On fourths, a four-person team, which included Ed Sorenson and Wes Skiles among the donors, had an acceptable margin.

He didn't offer a clear solution to the dilemma he posed. He suggested it likely that in a two-diver team, on traditional thirds, in the extreme scenario, the out-of-gas diver would have to be abandoned for anyone to survive. He acknowledged that the extreme scenario is also extremely unlikely.

Thoughts?

Except if you look at the OOG fatalities that have happened in the last two+ decades one diver doesn't just spontaneously go OOA at max penetration with 2/3rds left for the buddy(ies). The whole group swims around lost until they are all low on gas and finally the first diver goes OOA and they all rapidly perish shortly thereafter.

So this whole discussion has limited applicability to how a group of 2,3,4 divers is actually running OOA

 

[Alekseolsen]

Coming from Europe we were always taught that rule of third for 2 persons developed in Florida high flow caves, and that third for 2 person teams in a no-flow cave is insufficient - so I sort of dont see the "news" here?

 

[Mobulai]

Correction we had 40 not 4 bars

What do you mean by 'back'? Why not surface before? Draining the tanks to the last bar doesnt sound logic of its a calm open water spot?

Yes absolutely right, that makes more sense, it was a show of an insta-buddy situation and alignment of bad decisions on both of us; I don’t wanna derail the thread by it but I need to provide context so here goes: my (just assigned for this dive) buddy was not OOG because of any leaks, he didn’t change tanks or bother to check his spgs before going in (I learned that after the fact)
Just as we got to the 30m drop I saw one of his SPGs from the side so I asked him about it, surprise!
We then argued shortly that he wanted to go back alone so I can complete the dive tagging to others in the big group and that I wouldn’t trust him to do that — and I’d join him back, it’s still a long way back in the shallows. He even tried tried asking the group leader (who had dil-out tanks for CC) to switch with him so that he can continue diving who swore at him through the loop some gibberish (never had I seen body language so eloquently say “are you F’in mad!?”)
Having 2 fresh tanks I had 150bars and some peace of mind.. (and this is where I F’ed up) I passed him one of my tanks so we can both be comfortable..
Now it’s not a short swim back to shore, and it’s “calm” but there’s a constant rip tide that’s not ideal for swimming to the exit — so we decided to take our tanks as far as they can carry us
So we dove back (mistake 2 on my side) fullfilling this prophecy to the letter

The whole group swims around lost until they are all low on gas and finally the first diver goes OOA and they all rapidly perish shortly thereafter.

About 100-200m from shore exit we had 15 bars on both my tanks and 10+0 on my buddies; that’s when we surfaced and swam out.
I did get a deserved strong word from the fill guy at the shop about draining the tanks that low.

This was my 3rd SM OCdive after training, and I did acquire some complacency after finishing dives with 100+ bars (still not like my buddy) and had a “relaxed” attitude towards the OOG situation.. lesson(s) learned

 

[Underwater Tourist]

Coming from Europe we were always taught that rule of third for 2 persons developed in Florida high flow caves, and that third for 2 person teams in a no-flow cave is insufficient - so I sort of dont see the "news" here?

Same here (trained in MX, not in EU), isn't it cave 101 that thirds is as aggressive as you can possibly dive

If it is indeed "news" to a lot of people, then its pretty concerning