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

[crofrog]

This is a good fairly recent example of OOG, it also directly highlights what I was talking about. Diver 3 exited the cave with 1500 in one cylinder and 0 in the other. Even if he would have gone back to get Diver 4, he'd have had no practical way to share gas with him, given they were ~100ft from the exit, 43cuft likely would have been enough to get them back to the door, they did a lot wrong that put them in that situation to start with...

It also highlights some of the task loading issues with side mount which is why some organizations view it as a tool for specific missions and not a general purpose configuration specially for newer cave divers.

https://nsscds.org/wp-content/uploads/2022/10/Manatee-2022.5.14.pdf

 

[Wibble]

It also highlights some of the task loading issues with side mount which is why some organizations view it as a tool for specific missions and not a general purpose configuration specially for newer cave divers.

Sidemount is inherently more resilient and compact when diving in tighter environments that backmount simply cannot access. It's flexible in that you can unhook or remove one side for passing restrictions. The valves are far more accessible for shutdowns than backmount, with all regs & hoses in front of you so tracing leaks is more simple.

Remembering to periodically switch sides is absolutely not a problem except in the minds of the Z-mount designer (FFS!! ). Donating the long hose is very simple too: if you're breathing from it, then donate as normal; if using the necklaced shorthose, then you'd notice the 90 degree elbow and simply unclip the longhose or just pull and break the boltsnap "fuse".

Backmount is of course deeper/thicker than sidemount. It is notoriously simple to do shutdowns and carry to the site and picking a twinset/doubles off the floor to get on your back never causes injury....

Sidemount can be split to halve the load, important when caving to the dive base or moving between sumps.

Sidemount is a bit of a pain when using stage cylinders as you've effectively got two stages attached to yourself, whereas backmount has all the bulk behind you and frees up your chest area for more stages.

If caught up in line, sidemount can be disassembled on the fly to untangle things. Backmount isn't so simple and may need someone else to unpick the knots.

 

[jadairiii]

YHis 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.

IF this is correct, I would say the issue isn't with the Rule of Thirds, but horrifyingly out of shape divers. Or....it's more of people violating Rule #1.

 

[Rilelen]

IF this is correct, I would say the issue isn't with the Rule of Thirds, but horrifyingly out of shape divers. Or....it's more of people violating Rule #1.

Stratis Kas addressed this at length in the talk - this is NOT a physiological problem of being "in shape," it's a psychological stress response. I'm a research psychologist, and he is right. We use physiological measures such as heart rate and respiratory response as dependent measures when studying stress and anxiety, because they skyrocket under stress - even if the person reports feeling calm. That is, you can feel calm and in control, and still find your respiratory rate increasing by upwards of (at the highest end) 700%.

Kas and colleagues have been modeling this and that was a large part of his talk, that you need to account for not only a stressed recipient, but also a stressed donor, both of whom are likely to be using far more than twice the gas they used to enter the cave on exit.

There's an excellent old post by Andrew Ainslie about the massive amount of gas he blew through during an incident at the back of Ginnie: How Much Gas is Enough? Running on Empty

 

[Wibble]

IF this is correct, I would say the issue isn't with the Rule of Thirds, but horrifyingly out of shape divers. Or....it's more of people violating Rule #1.

Isn't it more to do with taking a simplified "rule" literally?

When doing cave courses on open circuit, a load of time and effort goes into gas planning especially where there's discrepancies between divers, their kit, their SAC, their skills.

The classic lardy unfit gas hog with big tanks diving with the petite gas sipper on small tanks is a nightmare to calculate the turn pressure for each diver. For the petite diver it could be a rule of 10ths!

 

[crofrog]

IF this is correct, I would say the issue isn't with the Rule of Thirds, but horrifyingly out of shape divers. Or....it's more of people violating Rule #1.

In the incident report I posted they noted that one of the divers 2 reported he had a SAC of ~3cfm one they realized they where lost.

Sidemount is inherently more resilient

There is more to resiliency in a system than "these 2 cylinders are completely independent of each other"

Remembering to periodically switch sides is absolutely not a problem

Except for when it is. If they all started with 3800psi as stated in the incident report

Diver 1: exited with 1200psi (34cuft) / 1500psi (43cuft), 35% of his total gas left
Diver 2: exited with 1500psi (43cuft ) / 850psi (24cuft), 30% of his total gas left
Diver 3: exited with 1500psi (43cuft) / 0psi (0 cuft), 19% of his total gas left
Diver 4: didn't exit.

Diver 3 and 4 were together, diver 3 would have been unable to donate.

Can you find me an example of manifold actually suffering a catastrophic failure that would have resulted in a catastrophic gas loss. I've personally seen one bend from falling off a bench and landing directly on the manifold, it still wasn't leaking.

 

[rjack321]

This is a good fairly recent example of OOG, it also directly highlights what I was talking about. Diver 3 exited the cave with 1500 in one cylinder and 0 in the other. Even if he would have gone back to get Diver 4, he'd have had no practical way to share gas with him, given they were ~100ft from the exit, 43cuft likely would have been enough to get them back to the door, they did a lot wrong that put them in that situation to start with...

It also highlights some of the task loading issues with side mount which is why some organizations view it as a tool for specific missions and not a general purpose configuration specially for newer cave divers.

https://nsscds.org/wp-content/uploads/2022/10/Manatee-2022.5.14.pdf

This was a downstream traverse, in a strong siphon, by intro divers, carrying a camera, who had never been in the system before. Being in sidemount certainly didn't help, but really was not directly related to this OOA accident. And *ehemmmm* as I previously pointed out, they all swam around lost until one diver was OOA. it was only by luck that the first 3 managed to exit before going OOA themselves. Nobody went OOA at the point of maximum penetration with the remainder of the buddies having 2/3rds of their gas.

 

[crofrog]

The classic lardy unfit gas hog with big tanks diving with the petite gas sipper on small tanks is a nightmare to calculate the turn pressure for each diver. For the petite diver it could be a rule of 10ths!

What is the difficulty in this?

 

[crofrog]

This was a downstream traverse, in a strong siphon, by intro divers, carrying a camera, who had never been in the system before. Being in sidemount certainly didn't help, but really was not directly related to this OOA accident. And *ehemmmm* as I previously pointed out, they all swam around lost until one diver was OOA. it was only by luck that the first 3 managed to exit before going OOA themselves. Nobody went OOA at the point of maximum penetration with the remainder of the buddies having 2/3rds of their gas.

Agreed, it is exactly what you were talking about how it doesn't happen. I was using it as both an example of how OOG actually happens, and as an example of how side mount isn't this panacea as Wibble seems to think.

 

[rddvet]

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?

This is literally nothing new. I learned 3rds was flawed 15 years ago. I didn’t see the lecture. So maybe it’s innovative he has some data, but the underlying thought that thirds isn’t all it’s cracked up to be is far from new.