Thread split: CNS toxicity limits

[stuartv]

Or, to put it succinctly, this statement,

Regardless of where the data came from, it’s not a useful tool.

is pure baloney.

The NOAA tables are a very useful tool for "new" divers (i.e. the ones that don't have thousands of hours of personal experience to inform their planning) to use in planning "safe" dives. Do the tables say that, if you stay within their limits you are 100% likely to NOT have a CNS hit? No. They do not give you a 100% guarantee on anything. But, when followed, they do offer a very high percentage chance of no CNS hit. 96%? 98%? Somewhere up there.

They may be of no use to YOU, but they are absolutely useful to the diving community.

 

[PfcAJ]

Or, to put it succinctly, this statement,



is pure baloney.

The NOAA tables are a very useful tool for "new" divers (i.e. the ones that don't have thousands of hours of personal experience to inform their planning) to use in planning "safe" dives. Do the tables say that, if you stay within their limits you are 100% likely to NOT have a CNS hit? No. They do not give you a 100% guarantee on anything. But, when followed, they do offer a very high percentage chance of no CNS hit. 96%? 98%? Somewhere up there.

They may be of no use to YOU, but they are absolutely useful to the diving community.

How are they useful? What information are you getting from these guidelines?

If 100% doesn’t mean anything, why avoid hitting 100%? There’s no value here.

It’s Fugazi.

 

[stuartv]

How are they useful? What information are you getting from these guidelines?

If 100% doesn’t mean anything, why avoid hitting 100%? There’s no value here.

It’s Fugazi.

I explained how they are useful in the post you quoted.

They are useful in the same way that NDLs are useful. NDLs are also not 100%. They are actually even worse in that you can somewhat pick your NDL by virtue of your choice of algorithm and algorithm parameters. NOAA O2 exposure limits are at least more "fixed" than that.

And, like I said, they are useful because, when you follow them, the data tells us that you have a very high probability of not having a CNS hit. It's not a 100% guarantee, but it's very high.

 

[PfcAJ]

I explained how they are useful in the post you quoted.

They are useful in the same way that NDLs are useful. NDLs are also not 100%. They are actually even worse in that you can somewhat pick your NDL by virtue of your choice of algorithm and algorithm parameters. NOAA O2 exposure limits are at least more "fixed" than that.

And, like I said, they are useful because, when you follow them, the data tells us that you have a very high probability of not having a CNS hit. It's not a 100% guarantee, but it's very high.

You’re not getting it.

Critically think about these guidelines.

They are NOTHING like an NDL. At all. We’re talking about going over the limit by THIRTY TIMES.

If an NDL was published as 1hr, but people routinely did dives to the same depth for 30 hours without incident, would you consider a 1hr NDL to be valid?

That NDL time would be absolutely worthless and not worth even considering because it’s objectively wrong as evidenced by all those people over all those dives completely smashing through the 1hr NDL without consequence.

Reversing the scenario just to highlight the ridiculousness of it, if I handed you a table that said the NDL for a 60’ dive is 2mins, you’d promptly deposit it in the trash because you know better. Even if I told you that it’s to keep you safe and this way you have a super low chance of DCS, there’s no way you’d use those tables.

We’re searching for accuracy and truth here, not appeal to authority because some navy manual said it’s true. It’s not true. It’s not accurate. It’s not reflective of reality. It doesn’t give you any value in making decisions. It’s just ink on paper.

 

[boulderjohn]

The NOAA CNS% attempts to quantify it but it does such a horrible job that the tool is useless.

If we take literally thousands of dives where CNS was over 1000 with zero effects by cave divers exploring the deepest longest caves in the world, when does that become empirical data instead of anecdotal?

I would really like it if this topic were split out from the rest of this thread or a new thread started. This would be a really useful thread if it were not buried hundreds of posts into a thread on a different topic.

I have retired from teaching, but one of the most difficult tap dances I had to do with each trimix class I taught was on this topic. We would discuss the CNS limits, and students would have to do precise calculations in the class. Many of my students were extremely aware of the fact that many cave divers were doing dives that blew those limits away, ignoring them completely, and they would ask me about that.

So what do I do? Do I tell them that theses divers don't believe the limits are credible, thus potentially inviting my students to ignore them themselves? What if they ignore the limits, tox, and it comes out that I supposedly gave them the green light to ignore those limits? My tap dance was that the limits are controversial, they have to make their own decisions, and nothing I said should be interpreted as endorsing violations.

If people are seriously violating the limits with no ill effects, as they evidently are, then there really are no limits, and we are all on our own. For me, the "we" does not apply personally, because I don't do those dives, but I really think divers need a lot better guidance than the collective "we" are getting now.

 

[boulderjohn]

I explained how they are useful in the post you quoted.

They are useful in the same way that NDLs are useful. NDLs are also not 100%. They are actually even worse in that you can somewhat pick your NDL by virtue of your choice of algorithm and algorithm parameters. NOAA O2 exposure limits are at least more "fixed" than that.

And, like I said, they are useful because, when you follow them, the data tells us that you have a very high probability of not having a CNS hit. It's not a 100% guarantee, but it's very high.

NDLs have been studied and studied and studied for more than a century, with much of that research published for all to see. The fact that we can change the settings on a computer to adjust how close we come to an NDL does not change the NDL any more than ascending 5 minutes before the NDL on a table changes the NDL limit on that table. NDLs are not 100%. They do vary somewhat from person to person and from dive to dive, but they are pretty accurate.

Where do the CNS limits come from? How were they tested? How believable are they? I personally don't know, and I have tried to find out.

 

[PfcAJ]

I would really like it if this topic were split out from the rest of this thread or a new thread started. This would be a really useful thread if it were not buried hundreds of posts into a thread on a different topic.

I have retired from teaching, but one of the most difficult tap dances I had to do with each trimix class I taught was on this topic. We would discuss the CNS limits, and students would have to do precise calculations in the class. Many of my students were extremely aware of the fact that many cave divers were doing dives that blew those limits away, ignoring them completely, and they would ask me about that.

So what do I do? Do I tell them that theses divers don't believe the limits are credible, thus potentially inviting my students to ignore them themselves? What if they ignore the limits, tox, and it comes out that I supposedly gave them the green light to ignore those limits? My tap dance was that the limits are controversial, they have to make their own decisions, and nothing I said should be interpreted as endorsing violations.

If people are seriously violating the limits with no ill effects, as they evidently are, then there really are no limits, and we are all on our own. For me, the "we" does not apply personally, because I don't do those dives, but I really think divers need a lot better guidance than the collective "we" are getting now.

This is why I don’t teach. I’m not at all interested in party line liability driven scuba instruction that it seems everyone is unfortunately bound to.

Not a dig at you, just at the situation we’re all in. There are plenty of other examples of this within scuba instruction.

 

[stuartv]

You’re not getting it.

Critically think about these guidelines.

They are NOTHING like an NDL. At all. We’re talking about going over the limit by THIRTY TIMES.

If an NDL was published as 1hr, but people routinely did dives to the same depth for 30 hours without incident, would you consider a 1hr NDL to be valid?

That NDL time would be absolutely worthless and not worth even considering because it’s objectively wrong as evidenced by all those people over all those dives completely smashing through the 1hr NDL without consequence.

Reversing the scenario just to highlight the ridiculousness of it, if I handed you a table that said the NDL for a 60’ dive is 2mins, you’d promptly deposit it in the trash because you know better. Even if I told you that it’s to keep you safe and this way you have a super low chance of DCS, there’s no way you’d use those tables.

We’re searching for accuracy and truth here, not appeal to authority because some navy manual said it’s true. It’s not true. It’s not accurate. It’s not reflective of reality. It doesn’t give you any value in making decisions. It’s just ink on paper.

I am getting exactly what you are saying. You are choosing to ignore what I am saying. And making up ridiculously extreme examples to try and support your argument.

Yes, if the tables said a 60' dive had an NDL of 2 minutes, I would ignore it. But, that is an example of a table putting such a huge limitation on a dive that it's not worth doing.

And that is not what the NOAA CNS tables do. Diving within the limits prescribed by the CNS tables still afford us adequate bottom time to have very enjoyable dives.

I am approaching it from the standpoint of "what gives the vast majority of the people the ability to dive without getting hurt the vast majority of the time?"

You are approaching it from the opposite perspective. You are taking some examples of some extreme violations of the table that have not produced CNS hits and saying that just because there are occasions where you can commit a major violation without a CNS hit means that the tables have no value.

And that is just rubbish. Millions of dives happen within the bounds of the tables and without CNS hits. Millions of very fun dives with very enjoyable amounts of bottom time. Those tables give a very safe baseline that MOST divers can and do use for enjoyable dives without incident.

The fact that there are extreme examples of table violation with no "hit" is one way we know that we really do not understand all the parameters of a CNS hit. Those examples do NOT mean that the tables are useless. There are also plenty of examples of divers taking a CNS hit even though they were within the limits prescribed by the tables. That is one way we know that we are not at the "2 minute NDL" level of ridiculous overcautiousness.

There is a percentage of dives that happen and get "undeserved" CNS OxTox hits. When that percentage is at some level that the experts have deemed "safe" - which seems to usually run somewhere around 2%-ish or less - then we know that the tables are conservative in the "right" amount (based on our current level of understanding of the issue).

Those tables are the basis of a LOT of dives that happen without incident - and that is their value.

You may as well also preach that recreational dive computers are useless because some get bent anyway and some people surface with 45 minutes of omitted deco and don't get bent.

 

[steinbil]

There is a percentage of dives that happen and get "undeserved" CNS OxTox hits. When that percentage is at some level that the experts have deemed "safe" - which seems to usually run somewhere around 2%-ish or less - then we know that the tables are conservative in the "right" amount (based on our current level of understanding of the issue).

I hope/think you're very wrong with that 2% of dives estimate. 1/50 dives for a risk of drowning by seizure is not what I call safe.

 

[stuartv]

I hope/think you're very wrong with that 2% of dives estimate. 1/50 dives for a risk of drowning by seizure is not what I call safe.

Yeah, well, did you see this post?

Cave diver drowns - Jackson Blue Springs, Florida

Obviously I do not dive 100% oxygen at all times at all depth. It is known that high po2s are bad. What is not known with ANY certainty is the relationship between higher po2 and time. The NOAA CNS% attempts to quantify it but it does such a horrible job that the tool is useless. Hitting 100%...

scubaboard.com

I don't know what the real percentage target is intended to be. But, I suspect it is less than 2%. I.e. at LEAST 98% of the time, when the tables are followed, you won't have a hit. Keywords: "At LEAST".