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

[dray_gnv]

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 Edd 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?

 

[mr_v]

Have you ever been out of air? Have dealt with an emergency in overhead? If so, the talk makes sense. 1/3 is very optimistic in my book.

 

[Wibble]

Thirds is the absolute minimum gas required. Generally a more conservative gas plan is recommended: quarter/forths, sixths, etc.

Diving with backmount and the possibility of one person loosing all their gas would be a damn fine reason for using a more conservative turn pressure.

Or ditch the doubles and use sidemount which is inherently more resilient. Even CCR is more resilient with two sources of breathing gas (CCR + bailout) which has been sized for that dive.

 

[Mobulai]

I mean.. makes lots of sense that a 1/3 of reserve would be barely enough if an oog diver breathed exactly the same way they did in the way in — now add panicked high RMV… falotes X fans

Makes sense that an additional margin of safety is applied beyond the 1/3s; No-Flow Safety Factor / Margin

For an open water only diver (no overhead env) 1/4th on sidemount is where I can find peace
I had the experience of having an OOG buddy (sidemount) way before getting close to the turn point (I had more than 3/4s on each SM tank) in a calm open water spot — we made it back with a total of 4 bars in 4 tanks (I also used my foldable snorkel the last 2~5mins)

Thirds is like minimum wage, and only barely works in a perfect world where there’s no friction and all bodies are perfect spheres

 

[Manatee Diver]

This has been a topic that has been thrown around for a while.

The rule of thirds should say, "Always plan to have twice as much gas as you need to exit" for a two man team in a low flow system that means that hard thirds can leave you running out of gas before you exit due to stress and the fact that the last hundred or so psi is unavailable. The most common means to back away from thirds it to subtract 100-200 psi from your turn pressure.

So for example instead of using a 1,000psi turn you use a 900psi turn.

So you swim, both your and your buddy have identical breathing rates and at 2,100psi just as you are signaling to turn, both your regs decide that it would be fun to spontaneously dissemble. So you need 1,800psi to exit, your buddy needs 1,800psi to exit, but since both your buddy's tanks have 2,100psi, you have 300psi in tank to spare.

If you want more conservatism, subtract 200psi and now you have 600psi to spare.

You can add more conservatism, but eventually you will be mathing yourself out of the water. At a certain point you have to accept you can only plan for so many failures, as this already assumes that two complete tanks failures at the same time at turn pressure.

 

[pisauron]

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?

Is there a video of the event/talk maybe?

 

[CG43]

An interesting topic.

How were these factors determined?
What is considered normal average?
Out-of-gas corresponds to OC. Are there also values for O2 consumption in rebreathers?

 

[Redtwo]

I mean.. makes lots of sense that a 1/3 of reserve would be barely enough if an oog diver breathed exactly the same way they did in the way in — now add panicked high RMV… falotes X fans

Makes sense that an additional margin of safety is applied beyond the 1/3s; No-Flow Safety Factor / Margin

For an open water only diver (no overhead env) 1/4th on sidemount is where I can find peace
I had the experience of having an OOG buddy (sidemount) way before getting close to the turn point (I had more than 3/4s on each SM tank) in a calm open water spot — we made it back with a total of 4 bars in 4 tanks (I also used my foldable snorkel the last 2~5mins)

Thirds is like minimum wage, and only barely works in a perfect world where there’s no friction and all bodies are perfect spheres

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?

 

[crofrog]

Thirds is the absolute minimum gas required. Generally a more conservative gas plan is recommended: quarter/forths, sixths, etc.

Diving with backmount and the possibility of one person loosing all their gas would be a damn fine reason for using a more conservative turn pressure.

Or ditch the doubles and use sidemount which is inherently more resilient. Even CCR is more resilient with two sources of breathing gas (CCR + bailout) which has been sized for that dive.

This is malarkey. Trade 1 person loosing potentially over half their remaining gas for the theoretical risk of catastrophic manifold failure which results in complete gas loss quickly enough to not provide any movement to the exit.

And in a total loss of gas the full volume of the remaining gas isn’t capable of being donated.

If you really want to mitigate do stage only dives reserving the full volume of the back gas for exit and stage the cave with safety bottles.

 

[The Chairman]

Have you ever been out of air? Have dealt with an emergency in overhead? If so, the talk makes sense. 1/3 is very optimistic in my book.

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