Nitrox on boat with air refill

[tursiops]

I have some charts in front of me.

What tables did you use that allowed you to go to 5 ft depth intervals?

 

[Freewillow]

The original NOAA manual made no mention of CNS toxicity having a half-like with the exception of exposures to oxygen partial pressures of 1.6 bar... AND ONLY THOSE. The new manual mentions that technical divers do use the half-life concept in their calculations. I tend to agree with Bill Hamilton when he suggested that practice was bull****.
.

Can you explain why CNS toxicity @ 1.6 bars would have a half life and CNS toxicity @ a lower level would not. This seems very strange at a first glance.

 

[NAM001]

I pushed the tables a bit because ie 40 ft had a 120 ndl abnd a 50 ft had a 60 +/-. It is the same thing when i pushed the ndl by a few minutes. All for the purpose of getting the highest possible PPO2 to work with, to achieve the highest otu's to try to reach the limit.. I came close. What is perhaps more important is the things that i put that is not so blatently apparent. IE 4 x 1 hour SI's for a total dive day of 9 hours. dive depth of 50-60 ft where many would not even use nitrox. In short the errors that may draw fire from posters from using so many tables was,,,,, i hope more than off set by these 2 factors. IN conclusion I just dont know divers that make 9 hour dive's, day after day after day. I know there are some, somewhere, but certainly not enough to suggest htat if i can be done, it is a problem. Its not what the normal rec diver does.
The table i used was a padi or a tdi i think. The otu's were calculated with (.5/(ppo2-.5)) ^-.83333 to get per minute otu value.

What tables did you use that allowed you to go to 5 ft depth intervals?

 

[Doppler]

Can you explain why CNS toxicity @ 1.6 bars would have a half life and CNS toxicity @ a lower level would not. This seems very strange at a first glance.

You'd have to ask NOAA... When Bill Hamilton was alive, I had the opportunity to speak to him about CNS 24-hour limit. I did not ask why 1.6 had a half-life and lower PPs did not. His opinion on the CNS clock and using half-lives for CNS "working exposures" (his term) were unnecessary and unsupported by NOAA's research. He explained himself by saying something to the effect: "That stuff is bull****... track CNS loading using the 24-hour limit... it's easier and supported by science."

You might also try asking Dick Rutkowsi... the other godfather of Nitrox and a past NOAA diving officer.

 

[scubadada]

Scubadada, can you explain further? To get in 5h at PPO2=1.0 requires (using 32% for an example) (say) 70 ft for 60 mins....5 times. But to get an NDL of 60 mins requires a 6h surface interval. How do I fit all that bottom time and surface interval into 24h? I understand this stuff pretty well...I'm just looking for a concrete example.

Hi Turisops,

I do most of my diving in Boynton Beach. I most frequently dive EAN 36 because that is what is often available, banked. I dive EAN 32 when I can. My average reef dives (start outside, East, and then crossover and end inside, West) have an average depth of about 60 feet. With EAN 36, the pO2 is just over 1.0. If I dive entirely inside, West, average depth is about 53 feet, pO2 0.94. There are deeper reefs and the Castor where my average depths are more like 70-75 feet. My average reef dives are between 65 and 75 minutes, the deep reefs and the Castor are more like an hour. It's not difficult to see that 4 or 5 dives in a 24 hour period could quite easily exceed the NOAA 24 hour limit of 5 hrs at a pO2 of 1.0. It is what it is. I dive Oceanic computers, they use a rolling 24 hour rolling period with no credit for surface interval (O2 half life). I know exactly when I approach or exceed my 24 hour exposure. The issue of an O2 half life is an interesting one and some computers, for example Shearwater Petrel, utilize a 90 minute half life. This would result in never, or extremely rarely being able to exceed one's daily O2 exposure doing mostly no-stop, OC dives

Good diving, Craig

 

[scagrotto]

Can you explain why CNS toxicity @ 1.6 bars would have a half life and CNS toxicity @ a lower level would not. This seems very strange at a first glance.

Since the entire oxtox thing is about unnaturally elevated partial pressures it strikes me as completely plausible that there could be a sound reason that a half-life might apply at some particular point. Exposure to a ppO2 of roughly 0.21 won't kill for CNS related issues you even after dozens of years, but even a brief exposure to a ppO2 of more than 1.6 can cause significant issues (even aside from depending on keeping a reg in your mouth). AFAIK, the latter is (virtually) always resolved quickly by reducing the ppO2 below 1.6 , with no half-life reduction required. Despite the already-excessive exposure all you need to do for an instant cure is reduce the continued exposure. It's not a big leap to figure that waiting for a relatively modest period of time after a cumulative exposure may reduce the effect of that cumulative exposure. In fact, the entire idea of a 24 hour exposure that resets itself can only be predicated on time at natural ppO2 eliminating the effects of the elevated exposure.

That begs the question of why you'd only apply a half-life to exposures of 1.6 or greater, but we're definitely dealing with something that doesn't behave in a linear fashion and being cautious and conservative is the safe approach. I'll also note that 1.6 is the threshold at which we expect the rapid onset of problems. Of course it's also an exposure at which it's much easier for people doing recreational profiles to exceed the limits, and who doesn't want a get out of jail free card?. FWIW, I'll note that applying a half-life reduction to an exposure of 1.6 may result in a lower calculated exposure than you'd have from exposures at 1.4 with no credit for a half-life reduction. That strongly suggests that applying a half-life at 1.6 but not 1.5 isn't the best approach.

All that said, until I see something that convinces me that there really is a sound reason to apply a half-life reduction and that it's backed up by reliable evidence I'm not going to complain that my Oceanic doesn't give me credit for the SI. That I don't normally do square profiles and typically use 32% means it's very unlikely that it will matter to me, anyway.

 

[scubadada]

I would call you an unusual diver, even in SE FL. Most divers I see down there get a couple of dives a day for a couple of days at best. You are definitely a hardcore diver. As a result, you do enter the realm where CNS/OTU matters. Most recreational divers are really just occasional tourists. You are not.

Could it be that you are not the average diver, unless average is defined by YOUR diveing habits. The knife cuts both ways. And of course those I dive with can do 5 hours a day also but not at the depth that is the problem issue with cns and otu's as it is a combined result.

I currently live in Basel, Switzerland, my primary residence is in Philadelphia, I've had a townhouse in Boynton Beach for 4 years. I'm only able to spend 2 weeks in the US out of every 2 months, I get in as much diving as possible when I'm in FL. I've been diving like this for many years, btw, I'm a fit 61 years old. I dive my way, you dive yours.

Very best, Craig

 

[NAM001]

First i am no expert but i will comment on what i have observed. when you do hte otu calculations the results are pretty linear to about 1.5 to 1.6 from that point they sky rocket. look at the O2 permissable limits and see what happens to otu's and cns when you hit about 1.55. for sake of comment the change from .5 to 1.5 is the same as the change from 1.6 to 1.7 or 1.8. Here is a chart that may or may not help. look at teh hour dive column and see what happens at the 1.6 area for cns value. I would think tht knowinf where you stand at those high levels is significant to ones safety and therefor one should be very aware as to what halflives are doing. use excel and plot eh cns values and make a graph of it and you will see the exponential rise at 1.6. these numbers alone makes a darn good argument for 1.4 PPO2 and less for recrerational diving.

http://wetlands.simplyaquatics.com/d/14029-1/otu_cns.pdf

Since the entire oxtox thing is about unnaturally elevated partial pressures it strikes me as completely plausible that there could be a sound reason that a half-life might apply at some particular point. Exposure to a ppO2 of roughly 0.21 won't kill for CNS related issues you even after dozens of years, but even a brief exposure to a ppO2 of more than 1.6 can cause significant issues (even aside from depending on keeping a reg in your mouth). AFAIK, the latter is (virtually) always resolved quickly by reducing the ppO2 below 1.6 , with no half-life reduction required. Despite the already-excessive exposure all you need to do for an instant cure is reduce the continued exposure. It's not a big leap to figure that waiting for a relatively modest period of time after a cumulative exposure may reduce the effect of that cumulative exposure. In fact, the entire idea of a 24 hour exposure that resets itself can only be predicated on time at natural ppO2 eliminating the effects of the elevated exposure.

That begs the question of why you'd only apply a half-life to exposures of 1.6 or greater, but we're definitely dealing with something that doesn't behave in a linear fashion and being cautious and conservative is the safe approach. I'll also note that 1.6 is the threshold at which we expect the rapid onset of problems. Of course it's also an exposure at which it's much easier for people doing recreational profiles to exceed the limits, and who doesn't want a get out of jail free card?. FWIW, I'll note that applying a half-life reduction to an exposure of 1.6 may result in a lower calculated exposure than you'd have from exposures at 1.4 with no credit for a half-life reduction. That strongly suggests that applying a half-life at 1.6 but not 1.5 isn't the best approach.

All that said, until I see something that convinces me that there really is a sound reason to apply a half-life reduction and that it's backed up by reliable evidence I'm not going to complain that my Oceanic doesn't give me credit for the SI. That I don't normally do square profiles and typically use 32% means it's very unlikely that it will matter to me, anyway.

 

[KenGordon]

I don't see that happening. Remember I am referring to NDL and you cant get a slower compartment to fill before a faster one. Of course you can fill say 3 compartments and then do a SI and have the slowest not full and the slower ones still full,,,, but you have left the NDL criteria presumed to be associated with rec diving. full means you are no longer diving NDL and the slowest fills first. No way around it.

Here are a couple of non deco multi level dives and a couple of square dives to the edge of deco.

They demonstrate that the limiting factor is only the fast tissues for short deep dives while repeat multi level dives and long shallow dives are limited by the slower tissues. They are calculated using ZHL16A coefficients with about a 80% surfacing GF (gradient factor). 100% GF is too liberal to demonstrate this as it is hard to get into deco without running out of air. 80% leads these to be just on the edge. A more conservative GF would show the same thing with shorter times. This deco calcs are very crude and assume instant movement between levels. The gas calcs assume a 20l/min RMV/SAC and mostly leave an adequate reserve in a 15l.

The columns are the compartment half time, the nitrogen partial pressure in that compartment, the ratio of that to 79% (which can be compared to ambient to see how close to equilibrium the compartment is). Last is the ceiling depth imposed by that compartment. Negative values => above the surface so not limiting an ascent at sea level, positive values => a deco ceiling. The most 'saturated' compartments have the most positive/least negative values.

The two multi level dives show the state of the compartments at the end of each level and the surface interval.


First dive
8 minutes at 3.0 atm (20m, 66 ish feet)
compartment: ppInert (ratio) min depth


1: 4 1.9750 (2.5) -4.6
2: 8 1.5800 (2.0) -4.5
3: 13 1.3561 (1.7) -4.9
4: 19 1.1992 (1.5) -5.2
5: 27 1.0833 (1.4) -5.4
6: 38 1.0030 (1.3) -5.4
7: 54 0.9434 (1.2) -5.4
8: 77 0.8998 (1.1) -5.2
9: 109 0.8684 (1.1) -5.1
10: 146 0.8489 (1.1) -4.9
11: 187 0.8362 (1.1) -4.7
12: 239 0.8262 (1.0) -4.6
13: 305 0.8185 (1.0) -4.4
14: 390 0.8123 (1.0) -4.3
15: 498 0.8075 (1.0) -4.1
16: 635 0.8037 (1.0) -4.0
ceiling -4.00m due to compartment 16 gas used 480


12 minutes at 2.8 atm
compartment: ppInert (ratio) min depth


1: 4 2.1824 (2.8) -3.4
2: 8 1.9886 (2.5) -1.7
3: 13 1.7720 (2.2) -1.7
4: 19 1.5660 (2.0) -2.2
5: 27 1.3826 (1.8) -2.8
6: 38 1.2390 (1.6) -3.3
7: 54 1.1236 (1.4) -3.7
8: 77 1.0341 (1.3) -4.0
9: 109 0.9671 (1.2) -4.1
10: 146 0.9244 (1.2) -4.2
11: 187 0.8960 (1.1) -4.2
12: 239 0.8736 (1.1) -4.1
13: 305 0.8560 (1.1) -4.1
14: 390 0.8418 (1.1) -4.0
15: 498 0.8308 (1.1) -3.9
16: 635 0.8221 (1.0) -3.8
ceiling -1.68m due to compartment 2 gas used 1152


28 minutes at 2.5 atm
compartment: ppInert (ratio) min depth


1: 4 1.9766 (2.5) -4.6
2: 8 1.9762 (2.5) -1.8
3: 13 1.9320 (2.4) -0.5
4: 19 1.8317 (2.3) -0.1
5: 27 1.6863 (2.1) -0.3
6: 38 1.5316 (1.9) -0.8
7: 54 1.3794 (1.7) -1.5
8: 77 1.2438 (1.6) -2.1
9: 109 1.1315 (1.4) -2.6
10: 146 1.0551 (1.3) -3.0
11: 187 1.0024 (1.3) -3.2
12: 239 0.9595 (1.2) -3.3
13: 305 0.9249 (1.2) -3.4
14: 390 0.8969 (1.1) -3.5
15: 498 0.8745 (1.1) -3.5
16: 635 0.8568 (1.1) -3.5
ceiling -0.07m due to compartment 4 gas used 2552


Surface interval
60 minutes at 1.0 atm
compartment: ppInert (ratio) min depth


1: 4 0.7900 (1.0) -11.2
2: 8 0.7966 (1.0) -10.0
3: 13 0.8310 (1.1) -8.9
4: 19 0.9000 (1.1) -7.6
5: 27 0.9821 (1.2) -6.2
6: 38 1.0404 (1.3) -5.1
7: 54 1.0640 (1.3) -4.3
8: 77 1.0544 (1.3) -3.8
9: 109 1.0232 (1.3) -3.6
10: 146 0.9894 (1.3) -3.6
11: 187 0.9600 (1.2) -3.6
12: 239 0.9325 (1.2) -3.6
13: 305 0.9077 (1.1) -3.6
14: 390 0.8860 (1.1) -3.6
15: 498 0.8677 (1.1) -3.6
16: 635 0.8525 (1.1) -3.5
ceiling -3.52m due to compartment 16 gas used 0


Second dive
8 minutes at 3.0 atm
compartment: ppInert (ratio) min depth


1: 4 1.9750 (2.5) -4.6
2: 8 1.5833 (2.0) -4.5
3: 13 1.3824 (1.7) -4.7
4: 19 1.2807 (1.6) -4.5
5: 27 1.2398 (1.6) -4.0
6: 38 1.2196 (1.5) -3.5
7: 54 1.1908 (1.5) -3.1
8: 77 1.1458 (1.5) -3.0
9: 109 1.0900 (1.4) -3.0
10: 146 1.0409 (1.3) -3.1
11: 187 1.0012 (1.3) -3.2
12: 239 0.9654 (1.2) -3.3
13: 305 0.9341 (1.2) -3.3
14: 390 0.9070 (1.1) -3.4
15: 498 0.8844 (1.1) -3.4
16: 635 0.8657 (1.1) -3.4
ceiling -2.99m due to compartment 8 gas used 480


12 minutes at 2.8 atm
compartment: ppInert (ratio) min depth


1: 4 2.1824 (2.8) -3.4
2: 8 1.9897 (2.5) -1.7
3: 13 1.7855 (2.3) -1.6
4: 19 1.6180 (2.0) -1.8
5: 27 1.4975 (1.9) -1.9
6: 38 1.4133 (1.8) -1.8
7: 54 1.3359 (1.7) -1.8
8: 77 1.2550 (1.6) -2.0
9: 109 1.1724 (1.5) -2.2
10: 146 1.1057 (1.4) -2.5
11: 187 1.0539 (1.3) -2.7
12: 239 1.0081 (1.3) -2.9
13: 305 0.9685 (1.2) -3.0
14: 390 0.9345 (1.2) -3.1
15: 498 0.9064 (1.1) -3.2
16: 635 0.8832 (1.1) -3.2
ceiling -1.62m due to compartment 3 gas used 1152


28 minutes at 2.5 atm
compartment: ppInert (ratio) min depth


1: 4 1.9766 (2.5) -4.6
2: 8 1.9763 (2.5) -1.8
3: 13 1.9349 (2.4) -0.5
4: 19 1.8499 (2.3) 0.1
5: 27 1.7423 (2.2) 0.2
6: 38 1.6366 (2.1) 0.1
7: 54 1.5279 (1.9) -0.1
8: 77 1.4154 (1.8) -0.5
9: 109 1.3033 (1.6) -1.0
10: 146 1.2139 (1.5) -1.5
11: 187 1.1447 (1.4) -1.8
12: 239 1.0835 (1.4) -2.1
13: 305 1.0305 (1.3) -2.4
14: 390 0.9850 (1.2) -2.6
15: 498 0.9472 (1.2) -2.8
16: 635 0.9161 (1.2) -2.9
ceiling 0.21m due to compartment 5 gas used 2552


Exmaple square 30m dive
16 minutes at 4.0 atm
compartment: ppInert (ratio) min depth


1: 4 3.0119 (3.8) 1.2
2: 8 2.5675 (3.2) 2.4
3: 13 2.1840 (2.8) 1.4
4: 19 1.8586 (2.4) 0.1
5: 27 1.5883 (2.0) -1.1
6: 38 1.3858 (1.8) -2.1
7: 54 1.2278 (1.6) -2.8
8: 77 1.1079 (1.4) -3.3
9: 109 1.0193 (1.3) -3.7
10: 146 0.9634 (1.2) -3.8
11: 187 0.9265 (1.2) -3.9
12: 239 0.8975 (1.1) -3.9
13: 305 0.8746 (1.1) -3.9
14: 390 0.8564 (1.1) -3.9
15: 498 0.8422 (1.1) -3.8
16: 635 0.8310 (1.1) -3.7
ceiling 2.38m due to compartment 2 gas used 1280


Exmaple square 15m dive
60 minutes at 2.5 atm
compartment: ppInert (ratio) min depth


1: 4 1.9750 (2.5) -4.6
2: 8 1.9685 (2.5) -1.8
3: 13 1.9325 (2.4) -0.5
4: 19 1.8499 (2.3) 0.1
5: 27 1.7210 (2.2) 0.0
6: 38 1.5749 (2.0) -0.4
7: 54 1.4241 (1.8) -1.1
8: 77 1.2845 (1.6) -1.7
9: 109 1.1659 (1.5) -2.3
10: 146 1.0837 (1.4) -2.7
11: 187 1.0263 (1.3) -2.9
12: 239 0.9793 (1.2) -3.1
13: 305 0.9411 (1.2) -3.3
14: 390 0.9099 (1.2) -3.3
15: 498 0.8849 (1.1) -3.4
16: 635 0.8651 (1.1) -3.4
ceiling 0.08m due to compartment 4 gas used 3000

 

[IVC]

Our response is STILL accurate, you can, in recreational mode, ignore the CNS/OTU risks, especially in the real world!

This is accurate (provided you sort out how to properly account for SI and whether half-life credit is good model or not for lower PPO2,) but the real life constraints in the OP didn't allow for retroactively looking up tables and/or coming up with an answer based on additional knowledge. The situation was "basic DC, no tables, no extra knowledge, no sensor."

(For the record, I am learning quite a lot from all the discussion here, particularly on how some models are not as "settled science" as some would believe. It's just that I'd like to keep the OP in the proper perspective for what it was - a first Nitrox dive with an unexpected quirk.)