Nitrox on boat with air refill

[IVC]

No relation between NDL and CNS. CNS is related to PPO2 and time, NDL is a function of depth, time and Nitrogen content of your gas.

"NDL time limit" is time, let's call it T_0. "CNS time limit" is time, let's call it T_1.

I can compare relationship between T_0 and T_1 because they are both "time." If I keep in mind that for recreational diving T_0 << T_1, then I can say "if I stay within my NDL limit, I'll be staying within my CNS limits."

And I've just given you a like...

 

[Lorenzoid]

. . .
What I've learned from the majority of posters is that CNS toxicity due to O2 exposure at recreational depth requires times that significantly exceed NDL times for exposure to N2. Simple concept, but not something that can be determined through some ad hoc calculation. Instead, it's a table lookup.

I applaud you wanting to see it for yourself in a table. Nothing wrong with being inquisitive and skeptical. I too generally refuse to accept the word of some supposed authority, even an instructor, who tells me something is a non-issue for me. But in the case of my own Nitrox class I was willing to accept the instructor's word and the suggestion in the written materials that this is just not something a day-boat diver who stays within NDLs using 32% Nitrox needs to be concerned with. I do recall my instructor adding a caveat that it's not an exact science--even the tables are just guesses--and we might want to add some conservatism if we're doing a lot of diving on rich mixes. I believe my instructor briefly mentioned a hypothetical week-long liveaboard, maxing it out to five dives a day on 32%, suggesting we might want to dial it down in the middle of the trip. I might dust off those tables in that case. Then again, that is more diving than I care to do.

 

[Freewillow]

@ IVC. Correct. thanks for the like .

@ Lorenzoid. "even the tables are just guesses-". No they are not . Tables are based on statistics. So the tables gives you a statistical "chance" to be OK, ...........but also a lower statistical "chance" to have problems. Be also aware that individual divers have "contribution factors" that can bring them on the wrong side of the tables. These contribution factors should have been covered in any scuba education .

 

[Lorenzoid]

@ Lorenzoid. "even the tables are just guesses-". No they are not . Tables are based on statistics. So the tables gives you a statistical "chance" to be OK, ...........but also a lower statistical "chance" to have problems. Be also aware that individual divers have "contribution factors" that can bring them on the wrong side of the tables. These contribution factors should have been covered in any scuba education .

Okay, sloppy wording on my part. By "guesses" I meant "estimates based on available data." I don't claim to know much about cumulative oxygen toxicity--it really was glossed over in my basic Nitrox course as something that most recreational divers need not be concerned with--but from the little I have read I have gotten the impression that cumulative exposure tables, such as the NOAA 24-hour table, are not based on specific experimental data so much as the collective wisdom of a group of scientists. As I understand it, the little hard data that might support these tables are based on (I'm at a loss for the proper jargon) the impact on other body functions that might be related to ox-tox and maybe also the extent to which the "air breaks" that hyperbaric patients are given have been shown to prevent convulsions. (Then again, the latter may be more relevant to the NOAA single-dive table than the 24-hour table.) Like I said, I have not read much on the subject, but I haven't gotten the impression that the oxygen tables were generated based on data gathered from chamber experiments simulating multiple dives or actual incidents of divers toxing after multiple dives. My impression is that it would be a mistake to think of these cumulative oxygen tables the same way we think of decompression tables--as time-tested and as supported by as much data. In other words, if decompression tables are attempts to draw lines through "fuzzy" regions, then oxygen exposure tables are attempts to draw lines through even fuzzier regions. So, maybe that's a long-winded way of saying that the word "guess," while a little sloppy, might not be so far off.

Is my understanding incorrect? I'm here to learn.

 

[Freewillow]

@Lorenzoid.

Oxygen exposure tracking is important in technical and rebreather diving , NOT in open circuit recreational dives.

This is why these exposures risks are not covered in most agencies for recreational Nitrox.

When it comes to "Advanced Nitrox" courses, CNS and OTU are covered because divers are supposed to dive behond the NDL. The "advanced Nitrox divers" are using Nitrox @50% or event pure Oxygen in order to accelarate their deco obligations. The "problem" with such a protocole is that potentially long deco obligations are done @ a PPO2 close to 1.4 to 1.6 bars. It is this prolonged exposure to these high PP02 levels that can lead to potential problems.

If diving within the NDL, or small deco with recreational nitrox, this risk is not present.

I hope that you got it know. No problem with your questions. They are relevant.

 

[JohnnyC]

This is the only one that I still don't see how you can know/estimate unless you already know the relationship between CNS and NDL time limits.

Again, there isn't a relationship between them other than time being a component used to derive the limit. That is the only way in which they are related. Quit trying to tie them together. You can far exceed your NDL without coming close to your CNS clock. Let's say you were to do a deco dive to 12m. On air your PO2 is .46. That's not even on a tracking table. In the example I posted prior, you can exceed your CNS while not exceeding your NDL (although as shown, it's not really feasible, and you wouldn't be running that type of profile in a recreational dive). Beyond the scope of a recreational dive, either one can exceed the other, ideally, you never exceed your CNS clock. At that point you are not only concerned about pulmonary oxygen toxicity, the percentage we've been talking about, but you also have to consider daily OTU's for repetitive diving. Again not recreational.

It's when you are trying to tie them together that you are confusing yourself. View them as completely independent limiting factors. You cannot derive a relationship between the two.

Like freewillow said, CNS toxicity is purely a function of the partial pressure of oxygen and time. This is why you can exceed your CNS limit in 45 minutes if you are breathing oxygen at 1.6 PO2. However, if your PO2 is 1.4, your time becomes 150 minutes. Your FO2 (fraction of inspired oxygen) and your depth are what derive PO2. It doesn't matter if you exceed your CNS by breathing 100% O2 at 6m, or some other FO2 at whatever depth that makes the gas have a PO2 of 1.6, your maximum exposure is still 45 minutes. Your NDL for that depth is irrelevant. Remember that it is a dose. Imagine that it is an oxygen pill. You can either take one large pill, or 4 small pills, but either way you're taking 100% of the dose. That's your CNS clock.

NDL is a function of depth, time, and inert gas loading. It's the inert gas loading that defines your decompression obligation. It is completely irrelevant when determining your dose of oxygen, you just want to make sure that your dose is less than 100%.

Your questions most definitely have real ramifications, however the conditions during which they would present themselves are far beyond recreational diving. And realistically at that point, there's a good component of voodoo involved as well. Daily limits are designed so that you can take a chamber ride, repetitive limits get lower and lower, yet people still exceed these on big dives, some divers easily take a PO2 of 2.0 without toxing. In the end, there is still a lot of unknown about oxygen's effects on the body, and current standards are really just best guesses based on empirical research, however there are always outliers that break the table.

In the end it keeps coming back to what was said earlier, at the recreational level, you are not doing anything that would bring you any risk from oxygen-related issues beyond exceeding your MOD, which you can do regardless of mixture. Hell, the MOD of air is 66m/218ft.

 

[IVC]

Again, there isn't a relationship between them other than time being a component used to derive the limit. That is the only way in which they are related. Quit trying to tie them together.

It's a misunderstanding about what I'm saying - I'm agreeing that it's just a different time limit and I'm not trying to tie them in any other way except to notice that in recreational diving I have to worry about NDL and not CNS because dive profiles are such that NDL will kick in way before any CNS limit.

What my (last) observation was is that in order to know the order of magnitude for different time limits one has to have the tables to look up those limits (alternatively, you can call it "knowledge," but it's still derived from learning limits from tables.) So, while I can rig together a solution for NDL and MOD using just common sense and basic DC, I cannot do the same with CNS. It requires at least one more ingredient - the data.

 

[Centrals]

What my (last) observation was is that in order to know the order of magnitude for different time limits one has to have the tables to look up those limits (alternatively, you can call it "knowledge," but it's still derived from learning limits from tables.) So, while I can rig together a solution for NDL and MOD using just common sense and basic DC, I cannot do the same with CNS. It requires at least one more ingredient - the data.

You can do the same for CNS% and OTU. Tables are readily available and all you need is the dive plan ie. breathing gas, time spend at each depth from the deepest all the way to the 3m. Soft wares are also available if you want to use your pc. This is not rocket science.

 

[IVC]

Tables are readily available and all you need is the dive plan ...

I didn't have tables at the time, hence the problem. I have them now, so it won't happen again.

This is not rocket science.

It is rocket science - you didn't figure it out, someone else figured out what happens in the tissues, someone else figured out how to model exposure and gas dissipation, someone else figured out what happens to a human body and under which conditions... You know it because someone else did all the hard work.

We can pretend it's a simple matter, but that's just ignoring the value R&D brings to the society.

 

[Centrals]

I didn't have tables at the time, hence the problem. I have them now, so it won't happen again.



It is rocket science - you didn't figure it out, someone else figured out what happens in the tissues, someone else figured out how to model exposure and gas dissipation, someone else figured out what happens to a human body and under which conditions... You know it because someone else did all the hard work.

We can pretend it's a simple matter, but that's just ignoring the value R&D brings to the society.

1. I am talking about the CNS and OTU tables not the dive table.
2. I am talking about the calculation on the exposure on CNS and OTU NOT the theorem behind them.