CNS %

[Beau640]

Am in a discussion with a buddy of mine about CNS % and risk of ox tox.

I’ve always learned through my instructors and through TDI that 80% CNS should be your upper limit at the end of a dive. My friend said she always learned from her CCR instructors that the limit wasn’t really important or proven to actually be related to risk of seizure/ox tox (just talking about pure CNS % not ppO2) and they routinely end CCR dives between 150-200% CNS clock.

I’ve never heard this before and have always adhered to the 80% standard. Just wondering what everyone out there is doing.

 

[Superlyte27]

I hate these questions. As a diver, there’s what we do. As an instructor, there’s what is written.

But, do the math on a 24 hour long dive on CCR at .9ppo2 like what happened when they connected Twin Dees to Weeki. What’s their CNS?

 

[northernone]

The normalized deviance from the training standards in our hobby regarding your question is significant.

I'm not experienced or educated enough to guess how far over the "limit" the likelihood of a fatality is significantly increased.

We all try to determine what risks we can live with and how safe is "safe enough".

In direct answer to your question, 1.2 for ~4hrs in comfortable conditions. Drop it for challenging and multi day because I simply don't know.

Cameron

 

[Superlyte27]

I very commonly do 4-6 hour dives at 1.0

 

[Blueringocto_73]

As a recreational diver with almost no knowledge on the subject, what is the recovery rate for a CNS hit in CCR diving?

Why do I hear about Po2 in what is considered over the limit for Nitrox divers?

 

[Superlyte27]

Well, for one thing, OC Nitrox divers are typically diving higher PPO2, like 1.4 and 1.6. CCR divers are typically diving at 1.2, albeit for quite a bit longer usually.

In terms of CNS hit recovery? I couldn’t say. The only person I know who took a hit didn’t survive.

 

[Blueringocto_73]

Ok, that clears up the mystery for me. Thanks

Well, for one thing, OC Nitrox divers are typically diving higher PPO2, like 1.4 and 1.6. CCR divers are typically diving at 1.2, albeit for quite a bit longer usually.

In terms of CNS hit recovery? I couldn’t say. The only person I know who took a hit didn’t survive.

 

[tomfcrist]

My opinion is that CNS% is a tolerance figure that isn’t fully understood. The “by the book” answer assumes that tolerance is fairly linear...it’s not. It’s been proven 10,000 times over to be garbage.

 

[Beau640]

So if no one is following the standards (I was able to find in both TDI and IANTD manuals 80% max CNS clock):

#1 why are you teaching something you don’t follow and don’t believe in. That to me is a disservice to your students.

#2 if it has been “proven” to be garbage, why are agencies still keeping it as a standard?

Just because many people have not had an issue above 100% clock doesn’t mean it isn’t a risk for ox tox. If keeping it below 80 mitigates the risk and potentially could have prevented the fatality mentioned above, why not do it? Why has normalization of deviance of this standard become so common? You mention deviance from certain other standards on here and people lose their damn minds. But not with this one?

Just like to figure out why people are doing what they are doing.

 

[Superlyte27]

#1 why are you teaching something you don’t follow and don’t believe in. That to me is a disservice to your students.

Maybe you should sign up and take a class and see exactly what I teach. But, in short, to answer your question, my contract with my agency says I will follow their standards. It is not my choice or position to go against those standards. By and large they have way more money, time and experience going back decades and in league with huge authorities like DAN and NOAA. Who am I to say don’t do that. So, when teaching, I follow my contract and the curriculum of my agency. However, like I said in another thread; those standards are built for the masses. Not necessarily for my individual physical makeup or experience or exposure. When working in a 200’ deep water tower, what’s your depth at the bottom of it? 200’. If we do a 2 hour dive sucking the dirt out of the bottom of that tower, what’s my CNS clock on air?

I’ll answer you another way....

A few years ago I was teaching cave diving. Our conversation was about the 5 rules of cave diving and how virtually no one had died obeying all five rules of cave diving. (Yes, I am aware of Parker). A student then asked if I obeyed all five rules all of the time. To which I hand to answer: NO. I make a conscious and calculated choice every time I choose not to run a reel. I’m not special, but I know that I have more than 2000 hours at Ginnie. I’m likely on a rebreather and I’d bet my life (and I am) that I can find my way out in zero vis and multiple issues. I’m willing to take that personal risk. Are you?

One of the members here asked Capt Jim if it was okay to not run a primary on a dive. Jim Wyatt said it best, “I’m not your mama!”