Nitrox on boat with air refill

[rx7diver]

rx7diver - are you serious?

packrat12,

Yes. What, you don't/didn't plan this way when you have/had only standard air tables, an oxygen analyzer, pencil, and slate (and, maybe, a calculator)? Has it been awhile? Have you forgot how?

Or, did I make an error in one of my calculations?

Safe Diving,

rx7diver

 

[NAM001]

Perhaps I misunderstand the cns issue. it has been my understanding there is no wash out when it comes to cns like there is nitrogen loading. Long SI lowers the nitrogen loading through time (DECO on the surface). CNS is a value of a total amount of O2 exposure allowed in 24 hours represtnted in %. Any wash out or reduction in the % does not functonally start till 24 hours. If i am correct they why would any computer give credit for non exposure time related to SI through out the first 24 hours. I'm looking to learn here.

You can easily exceed your computer CNS oxygen exposure doing recreational, no stop dives, it really depends on your computer. Oceanic computers use a 24 hour rolling window and the NOAA exposure limits. They do not use a half life of elimination of 90 minutes. This allows you 5 hour in a 24 hour period with a pO2 of 1.0. Doing 4 dives per day to about 60 ft with EAN36, (pO2 just over 1.0) for about 1:15 each, easily gets you over the limit, especially since there a 5 dives in the 24 hour window. I have exceeded my O2 limit many times under these conditions. With a 90 minute elimination half life, I would never even be close. I wish Oceanic would update their O2 exposure algorithm.

 

[packrat12]

packrat12,

Yes. What, you don't/didn't plan this way when you have/had only standard air tables, an oxygen analyzer, pencil, and slate (and, maybe, a calculator)? Has it been awhile? Have you forgot how?

Or, did I make an error in one of my calculations?

Safe Diving,

rx7diver

Not at all. In fact you seemed to be so confident that your proposed profile would exceed CNS limits easily. I have yet to see you show that, more so, I have shown that at the end of your 2nd dive, your expected CNS is a paltry 23%. Still you refuse to address this. To further twist your answer you had to use a PPO2 od 1.6 for your CNS calculations. With proper training you would have known that 36% at 100' has a PPO2 of 1.45. on tables you would have used 1.5 which has an single exposure of 120. To try to make your example work you needed to twist it to a point that it no longer was your example.

The point you tried to make was that the proposed dive WOULD exceed limits, which it clearly does not!

The point every one is making is that in normal recreational dives, it is very hard to exceed CNS and OTU limits and your example most definitely did not!

 

[KenGordon]

Perhaps I misunderstand the cns issue. it has been my understanding there is no wash out when it comes to cns like there is nitrogen loading. Long SI lowers the nitrogen loading through time (DECO on the surface). CNS is a value of a total amount of O2 exposure allowed in 24 hours represtnted in %. Any wash out or reduction in the % does not functonally start till 24 hours. If i am correct they why would any computer give credit for non exposure time related to SI through out the first 24 hours. I'm looking to learn here.

CNS (%) has a 90 minute half time. Whole body (OTU numbers) are rolling 24 hours.

The CNS limit is about acute fitting, the whole body/pulmonary limit about longer term lung damage and leaving headroom for a visit to the pot.

BTW, in amongst the bickering over examples, it has not be pointed out that these limits are quite vague. People have tox'ed well below them, and have not tox'ed well past them. There are also compounding factors such as co2 which make a difference.

This is worth a read https://decodoppler.wordpress.com/2009/02/10/daily-limits-for-cns-oxygen-toxicity/ (warning discusses deco diving!)

 

[rx7diver]

...To try to make your example work you needed to twist it to a point that it no longer was your example...

packrat,

I have never felt the need to "twist" anything on anyone's on-line forum. For what gain would I do this? I try to never knowingly write nonsense. And I try to never engage in on-line pissing contests, which is what I am sensing is your intent. I've been on ScubaBoard since 2006 and have relatively few posts here given all that time. You can read all of these posts in a single evening, if you wish. If you do, you'll see that all of this is fact.

I will continue to exercise the option to dive NOAA Nitrox II at TOD = 100 fsw (which has an associated calculate P02 = 1.5 ata). And I will continue to plan such EAN36 dives using a MOD = 110 fsw for this TOD = 100 fsw dive, using the rules for CNS oxygen exposure I listed in the simple example above. All of this is what I was taught in my 1993 IANTD Nitrox course and/or subsequently learned, and all of it makes sense to me still.

If you don't agree with any of this, fine. I hope some of the newer divers here can follow what I have detailed in my above simple example and what I will have detailed in my next example, your bluster notwithstanding.

Bottom line: Given the vagaries of the onset of CNS oxygen toxicity, I, personally, would NOT do the two TOD = 100 fsw EAN36 dives specified in my simple example above, even though they might individually seem safe enough, since, together, using the CNS oxygen toxicity rules I cited, the calculated Total Dive time of 40 min exceeds the 36 min single dive CNS toxicity limit. One really fun thing about scuba is, every diver can decide this type of thing for himself/herself.

EDIT: A relevant, interesting read is here: https://decodoppler.wordpress.com/2009/02/10/daily-limits-for-cns-oxygen-toxicity/.

Safe Diving,

rx7diver

 

[JohnnyC]

I will continue to exercise the option to dive NOAA Nitrox II at TOD = 100 fsw (which has an associated calculate P02 = 1.5 ata). And I will continue to plan such EAN36 dives using a MOD = 110 fsw for this TOD = 100 fsw dive, using the rules for CNS oxygen exposure I listed in the simple example above. All of this is what I was taught in my 1993 IANTD Nitrox course and/or subsequently learned, and all of it makes sense to me still.

If you don't agree with any of this, fine. I hope some of the newer divers here can follow what I have detailed in my above simple example and what I will have detailed in my next example, your bluster notwithstanding.

Bottom line: Given the vagaries of the onset of CNS oxygen toxicity, I, personally, would NOT do the two TOD = 100 fsw EAN36 dives specified in my simple example above, even though they might individually seem safe enough, since, together, using the CNS oxygen toxicity rules I cited, the calculated Total Dive time of 40 min exceeds the 36 min single dive CNS toxicity limit. One really fun thing about scuba is, every diver can decide this type of thing for himself/herself.

You did your calculations wrong.

The problem is that you're calculating your O2 exposure for a maximum PO2 of 1.6 and your dive time for 40 minutes. That is much greater (see below) than what you're actually diving (1.5 for 30 in your example). The difference is very substantial. Just to prove that, 10 minutes at a PO2 of 1.5 is 8.33 percent. At a PO2 of 1.6, that same 10 minutes is 22.22 percent. That's a BIG difference. We're basing the calculations on a normal dive, we've already established that it's possible when you start considering impractical hypotheticals. We're don't calculate for max if our PO2 is only .63 just to make our argument work.....

So that we're all on the same page here, I'm going to use IANTD OTU/CNS Tracking Table C-3201. Earliest copyright for that table is 2003. There's our common reference so we all have the same data to work with. The earliest NOAA reference I easily found is from 1991, and it aligns with the data on the C-3201 table, as well as data that the rest of us have been using. Also, I will round up for ease of math. (It makes the percentage higher so it would benefit your argument.)


Here we go!!!!!

At a PO2 of 1.5, your CNS exposure for a 30 minute dive is 25%. (At this point NDL is irrelevant, we're just getting the O2 stuff out of the way first) Single dive max is 120 minutes btw. Even if we use your 80% of max, it's still 96 minutes.

Even if you were to do two dives with zero SIT (zero 90 minute washout credit), same time at the same PO2, your exposure would still only be 50%. Still a 36 minute buffer before your 80% single dive exposure limit.

If you did 40 minutes at 1.5, your exposure would only be 34%. If you did 2 dives of 40 minutes (waaaay over NDL's but let's not that get in the way), zero SIT, your exposure would still only be 68%. And still a 16 minute buffer before your 80% single dive exposure limit. (Despite the deco obligation from your +10 minutes on the first dive, and how many minutes on the second dive since there's no SIT)

The fact that you're calling for a 2 hour surface interval decreases the CNS loading even more. Let's just use a 90 minute half life for ease of math (again in your favor), at 30 minutes you're only 12% after the first dive leaves you with 37% after 2 dives.

If we extend those dives to 40 minutes with your SIT, rounded down to 90 minutes for ease, you're still only at 51% CNS loading at the end of the day.

Regardless, if you dive to your NDL, you're single dive exposure is still only 25%. If you exceed your NDL by 10 minutes your single dive exposure is still only 34%. (Moot since we're not talking deco)

Even if you did a 30 minute dive at a PO2 of 1.6, your single dive exposure is only 67%. At 40 minutes, 89%. When you consider 90 minute half time (why wouldn't you?) you're still starting your second 30 minute dive at 34%, your 40 minute dive at 45%. In fact, the only way to actually exceed 100% CNS loading is to specifically make 2 dives for 40 minutes at a PO2 of 1.6, both of which are beyond your NDL in the first place, and thus not subject for discussion.

You've gotta use the right math to get the right numbers. You can't arbitrarily choose conditions just to try and make it work in your favor. Remember, we're not talking random hypotheticals, we're talking an actual planned dive that goes to plan. Your planned dive in fact.

Basically, you showed us a dive plan of 30 min at a PO2 of 1.5 with a 2 hour SIT and a second dive of 10 min at a PO2 of 1.5, but you want to use the math for 40 min at 1.6 (w/no SIT/single dive) to prove your point. Can't do that amigo. Play it straight or don't play it at all. Making up rules to suit your internet argument is SUPER lame.

 

[rx7diver]

The example in detail winds on for several hand-written pages. To simplify the presentation, I will first post a simple example. Then I will post the full example. The simple example actually suggests the critical elements that make the full example "work."

Continuing.

Inspection of the first, simple example that "works" suggests what's needed for a EAN-cylinder-topped-up-with-air example (i.e., a *full* example) to "work": If (i) the initial dive is begun with EAN having a sufficiently high FO2, and (ii) the diver's RMV is sufficiently low to NOT deplete too much from a sufficiently large cylinder, then we'll have a full example that "works," too. Okay, here's the full example:

1. Initial conditions. (a) Initial and repetitive recreational dives to 100 fsw. (b) Use OMS LP 121 (125 ft^3 @ 2,640 psig) containing 3,500 psig of NOAA Nitrox II to start initial dive. (c) Subsequent air top-ups will be to only 2,640 psig, the rated service pressure of the cylinder. (d) Diver's RMV = 0.5 ft^3 per min. (e) Descent rate = 60 fpm. (f) Ascent rate = 60 [sic] fpm. (g) Safety stop = 3 min @ 15 fsw. (h) Surface interval = 2 hrs.


STUFF FROM FIRST, SIMPLE EXAMPLE:


2. Initial dive starts with 165.7 ft^3 EAN36. Solve 125/2,640 = x/3,500 for x.

3. For dive planning, use MOD = 110 fsw (rather than TOD = 100 fsw).

4. At MOD = 110 fsw, EAN36 has P02 = 1.6 ata. Compute: (0.36)[(110/33) + 1)].

5. Single dive CNS oxygen limit = (80%)(NOAA CNS single dive oxygen limit) = (80%)(45 min) = 36 min.

6. Single day CNS oxygen limit = (80%)(NOAA CNS 24-hour oxygen limit) = (80%)(150 min) = 120 min.

7. EAD for initial dive to MOD = 110 fsw using EAN36 is EAD = 90 fsw. Solve (0.79)[(EAD/33) + 1] = (0.64)[(110/33) + 1] for EAD.

8. Initial dive has NDL = 30 min. Use "1984 US Navy Standard Air Decompression tables."


NEW STUFF:


9. Initial dive has run-time = 34.8 min ...

10. ... and (since diver's RMV = 0.5 ft^3 per min) EAN36 gas consumption = 68.1 ft^3. (See attached scan of hand-computed profile.)

11. EAN36 remaining at end of initial dive = 165.7 - 68.1 = 97.6 ft^3, ...

12. ... and psig at end of initial dive = 1,438.3 psig. Solve 2,640/125 = x/(165.7 - 68.1) for x.

13. Need to add 125 - 97.6 = 27.4 ft^3 of *air* to top up to 2,640 psig.

14. Content of cylinder after topping up is EAN33. Compute FO2 = (27.4/125)(0,209) = (97.6/125)(0.36) = 0.3269 = 33%.

15. At MOD = 110 fsw, PO2 of EAN 33 is PO2 = 1.5 ata. Compute PO2 = (0.3269)[(110/33) + 1] = 1.417 ata.

16. EAD for second dive to MOD = 110 fsw using EAN33 is EAD = 90 fsw. Solve (0.79)[EAD/33 + 1] = (0.6731)[(110/33) + 1] for EAD.


CONTINUING STUFF FROM FIRST, SIMPLE EXAMPLE:


17. For 1:42 <= Surface Interval <= 2:33, NDL for 2nd dive (to EAD = 90 fsw) is NDL = 10 min.

18. For dive planning re CNS oxygen exposure, do NOT give credit for oxygen half-life.

19. For dive planning re CNS oxygen exposure, for Surface Intervals that do NOT exceed 2 hours, treat the two dives as a single dive. So, here the Total Dive Time = 30 + 10 = 40 min.


TOO LONG. GOTTA JUMP TO A NEW POST. (***Jump to post #129***)

 

[NAM001]

Ill pull my books and read them again. I don't remember the problems doing anything but adding the new dive % to the last dive total. I may have missed it though. With out a wash out aspect then the cns and otu limit has a great amount of safety it by just adding the numbers and running on total acquired. I knew the O2 wash our was 90 min half life and nitrogen was something like 45 in half life and that runs the bar graphs on the computer I had sssumed that by only adding new dive acquisition you set a limit for how many times you can load and unload your self in one day. I also assumed the bar graphs was trying to show current loading and not the days acquisition. Hence the separate calculation and totaling of cns and out's. One other thing I thnk that my shearwater is the first computer that gives me cns and out's, although they all have the O2BG and those figures do not match. Another thing is that the O2BG is showing O2 for the dive and the CNS and OTU is for the day. The charts I have viewed have 2 figures for these things and they are per dive and per day.

CNS (%) has a 90 minute half time. Whole body (OTU numbers) are rolling 24 hours.

The CNS limit is about acute fitting, the whole body/pulmonary limit about longer term lung damage and leaving headroom for a visit to the pot.

BTW, in amongst the bickering over examples, it has not be pointed out that these limits are quite vague. People have tox'ed well below them, and have not tox'ed well past them. There are also compounding factors such as co2 which make a difference.

This is worth a read https://decodoppler.wordpress.com/2009/02/10/daily-limits-for-cns-oxygen-toxicity/ (warning discusses deco diving!)

 

[rx7diver]

Continuing.

19. For dive planning re CNS oxygen exposure, for Surface Intervals that do NOT exceed 2 hours, treat the two dives as a single dive. So, here the Total Dive Time = 30 + 10 = 40 min.


TOO LONG. GOTTA JUMP TO A NEW POST.

Continuing.


NEW STUFF:

20. If, as before, you consider entire 40 minutes to be spent at PO2 = 1.6 ata, then, as before, the 40 minutes *exceeds* the 36 min (= 80% * 45 min) single dive CNS oxygen toxicity limit.

21. However, if you consider the first 30 minutes to be spent at PO2 = 1.6 ata and the last 10 minutes to be spent at PO2 = 1.5 ata, then the CNS oxygen exposure is 30/45 + 10/120 = 0.75 = 75%, which *almost* exceeds the 80% limit, but doesn't quite. I imagine that for some divers, this would be close enough to decide to NOT do these two dives. And others would see this as sufficient evidence to go ahead with these two dives.


END FULL EXAMPLE.


A couple of final comments: I used my intuition to "reverse engineer" things to construct an example showing that, divers doing recreational dives and topping up their EAN cylinder with air, can NOT always assume that their CNS oxygen exposure will be okay. Clearly, for this last full example, we could specify a diver's RMV to be sufficiently small (smaller than 0.5 ft^3 per min) so that this example will produce a CNS oxygen exposure that exceeds 80%. Alternately, we can specify a larger capacity cylinder (or a higher initial EAN36 fill pressure) to this same end. Finally, I worked here with only an initial dive and one repetitive dive. More repetitive dives, under the right initial conditions, can, too, produce a CNS oxygen exposure that exceeds the 80% limit.

Some of you will have found this exercise useful.

It was a fun exercise for me, dusting off basic nitrox computational skills I haven't used in quite a while. There's nothing like getting back to first principles every once in a while!

Safe Diving,

rx7diver

---------- Post added November 14th, 2015 at 10:44 PM ----------

So that we're all on the same page here, I'm going to use IANTD OTU/CNS Tracking Table C-3201. Earliest copyright for that table is 2003. There's our common reference so we all have the same data to work with. The earliest NOAA reference I easily found is from 1991, and it aligns with the data on the C-3201 table, as well as data that the rest of us have been using. Also, I will round up for ease of math. (It makes the percentage higher so it would benefit your argument.).

JohnnyC,

I used the NOAA CNS oxygen exposure table seen in the linked article (by Doppler). This is the table I committed to memory a very long time ago, before my hair and beard turned completely grey, in fact.

Safe Diving,

Ronald

 

[tursiops]

rx7diver, PPO2=1.6 may have been "recreational" in 1993 when you first learned about Nitrox, but it is not anymore.
You need to recognize that we have gotten smarter since 1993.