CNS %

[Beau640]

I’m sorry I shouldn’t of said you. I didn’t actually mean you I meant instructors in general.

I don’t think the comparison to a primary reel at Ginnie is equivalent though. A new cave diver can’t find his way out like someone who has dove years at Ginnie. It makes sense the risk is different in that scenario with experience.

However a diver with 1 dive under his belt theoretically will have the same risk of ox tox with 1000 dives under his belt. Experience isn’t mitigating the risk you have when above 100%.

I guess the bigger question is where does the number come from? Is it actually backed by science? Are we actually drastically increasing our risk (even if it’s still a small risk could be greatly increased from the risk below 100%)? Do we have any clue at all?

I don’t do dives big enough to violate this clock and so that’s why I’m very curious what everyone is doing and why they are doing what they are doing. Is there actually any data to support the practice or are people just violating the clock because their mentors did as well and it seems like everyone gets away with it without a hit.

 

[tursiops]

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.

Ha. Much garbage is still edible....

Just because something is not well understood or exactly right doesn't mean you can throw it out and ignore it completely. There are gradations of wrongness.

I assume you monitor your blood pressure now and then? it is not well understood what those numbers really are telling you, or what your normal is. But it is still a helpful measurement.

I assume you measure the air pressure in your tires? Do you really think that little cheap pressure guage is giving you edactly the right info?

Do you watch your CNS clock? So you ignore it completely simply;y because it is not exactly right? No? I thought not. So why do you call it garbage and disparage it so strongly?

Do you actually understand how the Bluetooth on your cell phone works? No? Then throw it out, it's garbage.

 

[Superlyte27]

So yes, it is backed by science. And frankly, the science should have been covered in Nitrox or Advanced Nitrox and Deco.

You have to remember, this stuff is calculated by “what’s safe for the majority of divers”. So, just like diving a 30/85GF is fine by me, other people are diving straight navy tables. And that’ll work for them. I’m a middle age fat guy. I want to mitigate some risk.

Let’s say that 20 people make a dive that by the tables should bend all 20... How many do you think will actually get bent? Probably none, if the violation wasn’t significant. But, if you increase the number to 100 people... how many will get bent on a single dive? Now, increase that to 2 million divers making 10’s of millions of divers per year. So, the math is such that it if you do everything right every single time, you PROBABLY won’t get bent. LOL

Navy divers use to dive a PPO2 of 2.8. It worked for most of them. But a lot of them were getting tox’d. So the bean counters said “we better reduce this” and they did. People were still getting tox’d. Not many, but some. So they reduced it again. Now, instead of it being 1000 navy divers in peak physical condition, it’s 2 million middle age fat guys (for the most part). Sure, a giant portion of people in navy diver physique will probably be okay. But I’m middle age 225lb fat guy.

When I went to commercial diver school in 1998 they were still doing an OX TOX test the first day of class. 30 minutes at 60’ on pure oxygen. 30 people every month for 6 months. I ran the chamber on these dives. Guess how many tox’d? None. But, it’s in a controlled environment with no stress and divers in peak shape. Spread the sample size to the 2 million active divers, how many THOUSANDS are going to die? So.... it has been reduced to be safe for the masses.

Can you imagine is 1% of 1% of divers were taking an O2 hit every year with 2 million active divers worldwide? I suck at math, but that’s a lot of friggin people.

 

[Beau640]

As a Middle aged fat guy don’t you want to keep your clock low then?

 

[boulderjohn]

I think this says it nicely.

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.

I think that in terms of CNS%, we all know the science is very inexact. We all know that some people violate those guidelines regularly, and we may do it ourselves. Those guidelines, though, are what we have.

So what happens if we tell our students they are free to ignore those guidelines? What if we provide our own guidelines for our students based on our own personal opinions? What happens if one of our students follows our guidelines, toxes, and dies? What do you think the questioning would be like in the ensuing trial?

 

[Superlyte27]

As a Middle aged fat guy don’t you want to keep your clock low then?

If I told you that I’ve had dives at 1000% CNS, wouldn’t you think 200% is low?

 

[Superlyte27]

And Beau, if you’re ever in Florida looking for some advanced training, look me up. I’d love to work with you and tell you what is taught and tell you how and why I choose to do the things I do.

I truly believe that if you teach from the stance of, “Don’t do this because I said so” and never state the WHY then you’ve kinda failed as an instructor. One of my best mentors was Jim Wyatt because he was very good at explaining “why” to his students.

 

[lv2dive]

If you told me you had a non-commercial dive at 1000% CNS, I’d say show me a pic of of petrel, bc I’m pretty sure even Matt, Andy and Brett aren’t seeing those kind of numbers at TD.

If I told you that I’ve had dives at 1000% CNS, wouldn’t you think 200% is low?

 

[lv2dive]

I think the highest CNS %age I’ve hit was right above 200% - not very high compared to some of my friends/aquaintences. On a few of the longer open circuit dives, I’ve have the experience where I have suddenly gotten incredibly irritated / irritable on deco. In those cases, I immediately switched to a low O2 mix for a decent period of time. I can’t know for sure, but I would not be surprised if I was starting to experience CNS toxicity in those instances and managed to hold it off. CNS toxicity is definite something about which I am concerned, but I definitely started pushing my exposure up very gradually over time. I think that is the best way to understand how ones body deals with all of the different types of stress coming from extended decompression diving - to gradually build up exposure.

 

[tomfcrist]

Ha. Much garbage is still edible....

It’s not an accurate representation of tolerance limits for the VAST majority of divers. Is it an Uber conservative limit? Yes....which is the only thing it has going for it.