Nitrox on boat with air refill

[Centrals]

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

He is the founding father of IANTD. One of his famous quote: Science Always Wins over BS!!
Anyway, dug out couple of IANTD old student manuals such as Adv Nitrox and Intermediate Trimix. There was no mention on the half-live of CNS Toxicity. But there is a statement: CNS% is believed to be reset after 12hrs.
When I checked my old tec log books, the CNS% and OTU were carried over to the following dive(same day).
Oxygen Toxicity Calculation by E.C. Baker: "A common practice in technical diving to calculate a decay in the CNS fraction during Surface Intervals using exponential decay equation with a half-time of 90mins."

 

[NAM001]

Ken I agree with you . If i read the data correctly It supports what i said. compartment 1 is the fastest to change. It is possible to get say compartment 2-3 to be the one to hit ndl before compartment 1.... but you would have had to exceed ndl of the fastest compartments to get those slower compartments to a condition on prior dives to have that happen. And that goes beyond the rec limits of not exceeding ndl. There is no doubt that on repeditive dives that middle compartments have higher levels than compartment 1 at start of dive. The first dive provides that ski ramp shape of tissue saturation levels. After that they try to take on the look of bell curves. Compartment one still rules unless some slower ones are already close to being full. Technical dives really brings this bell curve out when the SI gets its hands into things.

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.

---------- Post added November 20th, 2015 at 04:30 AM ----------

That is what my booKs show also when they discuss the daily limits. I also perhaps extrapulated from the reading that there is a per dive on gas limit and daily on gas limt (pretty straight forward) and a current residual level tracking going on. the latter will take into consideration lowering of levels on a surface interval half life basis. I get that impression because of the functioning of the O2BG. You hit your dive limit when the first of,,,,, per dive limit or the O2BG limit is achieved. No instruction is provided in regards to the O2BG levels in training manuals. That made sense to me when learning my integrated t3 aeris computer and the time remaining function being determined by gas psi consumption rate, N2BG or O2BG as being the controlling factor. The O2BG did not appear to function like a gage to reflect how much of the per dive limit was used (resetting to 0 at start of dive) , rather it functioned like the N2BG showing a continuing reduced residual level as the SI proceeded. Repeditive dives showed both the N2 and O2 BG's showing residual levels at the start of the dive. Whether the O2bg daily or perdive data being affected by SI i dont know. I assume it is per dive data. Use of a 40% mix puts the O2bg in over drive.

He is the founding father of IANTD. One of his famous quote: Science Always Wins over BS!!
Anyway, dug out couple of IANTD old student manuals such as Adv Nitrox and Intermediate Trimix. There was no mention on the half-live of CNS Toxicity. But there is a statement: CNS% is believed to be reset after 12hrs.
When I checked my old tec log books, the CNS% and OTU were carried over to the following dive(same day).
Oxygen Toxicity Calculation by E.C. Baker: "A common practice in technical diving to calculate a decay in the CNS fraction during Surface Intervals using exponential decay equation with a half-time of 90mins."

 

[Lorenzoid]

. . .

(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. . . .)

Me, too. What I am getting from this is that the oxygen toxicity models are even less "settled science" than the decompression models, which are far from settled. Fair to say?

 

[Doppler]

Me, too. What I am getting from this is that the oxygen toxicity models are even less "settled science" than the decompression models, which are far from settled. Fair to say?

Safest viewpoint... that and keep current with developments.

For the record, most of the divers doing serious exploration are also comfortable "doing research" on CNS limits. The body has too many variables to make many blanket statements. For example, on long dives, many of us have greatly exceeded the published guideline for single dive exposure with no ill-effects. A diving doctor buddy suggests research is needed to explain this. Perhaps CNS uptake/stress is altered as ambient depth increases and other gases diluting the oxygen have an influence on the body's reaction to oxygen levels... best option, know the limits and stick to them.

 

[KenGordon]

but you would have had to exceed ndl of the fastest compartments to get those slower compartments to a condition on prior dives to have that happen. And that goes beyond the rec limits of not exceeding ndl.

No, on the 15m single dive example the limit is 4th compartment. The faster compartments have not yet reached their limit. This is because the over pressure allowed (the supersaturation) is lower for those slower compartments, so they do not need to 'fill up' as much as a fast compartment to become the limiting compartment.

This has a particular impact on multi level dives and repeat diving, even within NDL.

 

[Centrals]

After a more thorough reading on the various IANTD study books and the log books that I have. I realized in Adv Nitrox course the CNS% and OTU numbers were carried over to the next same day dive. But in more "advanced" tec training, the half-live is introduced. The concept is mentioned in IANTD's Encyclopedia which is being used in conjunction with the corresponding tec study books.

I dive pretty conservatively especially on tec dive. Pushing the personal limit is one thing that I have no desire for.

 

[NAM001]

That looks like the condition at start of a repeditive dive after a SI. Are you suggesting that as the dive progresses the fast compartment will not take over as controlling as it fills and exceeds eh compartment 4 levels???

No, on the 15m single dive example the limit is 4th compartment. The faster compartments have not yet reached their limit. This is because the over pressure allowed (the supersaturation) is lower for those slower compartments, so they do not need to 'fill up' as much as a fast compartment to become the limiting compartment.

This has a particular impact on multi level dives and repeat diving, even within NDL.

 

[KenGordon]

That looks like the condition at start of a repeditive dive after a SI. Are you suggesting that as the dive progresses the fast compartment will not take over as controlling as it fills and exceeds eh compartment 4 levels???

Sorry about the verbose presentation. This would be nicer as a proper graph...

Below the 60 minute dive has the ceiling/compartment printed every 15 minutes. Then an extra 15 minute as per a deco dive and then a further 45 minutes for a 2 hour bottom time to demonstrate the extreme end.

What happens is that the fast compartments reach equilibrium quickly but that they can take more over pressure than the slower ones so they never become limiting. The ratio column is the tissue pressure/the fractional N2 pressure. Compare that to the absolute pressure at the depth and you see that after just 15 minute the first compartment is nearly at equilibrium and isn't going to take on much more gas. A longer dive makes a difference to the slow tissues, not the fast ones. Eventually they become limiting.



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


1: 4 1.8869 (2.4) -5.1
2: 8 1.6519 (2.1) -4.0
3: 13 1.4592 (1.8) -4.1
4: 19 1.2995 (1.6) -4.4
5: 27 1.1687 (1.5) -4.6
6: 38 1.0717 (1.4) -4.8
7: 54 0.9965 (1.3) -4.9
8: 77 0.9397 (1.2) -4.9
9: 109 0.8978 (1.1) -4.8
10: 146 0.8715 (1.1) -4.7
11: 187 0.8541 (1.1) -4.6
12: 239 0.8404 (1.1) -4.5
13: 305 0.8297 (1.1) -4.3
14: 390 0.8212 (1.0) -4.2
15: 498 0.8145 (1.0) -4.1
16: 635 0.8092 (1.0) -3.9
ceiling -3.94m due to compartment 16 gas used 750


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


1: 4 1.9685 (2.5) -4.6
2: 8 1.8869 (2.4) -2.4
3: 13 1.7505 (2.2) -1.9
4: 19 1.5899 (2.0) -2.0
5: 27 1.4264 (1.8) -2.5
6: 38 1.2865 (1.6) -2.9
7: 54 1.1670 (1.5) -3.4
8: 77 1.0704 (1.4) -3.7
9: 109 0.9958 (1.3) -3.9
10: 146 0.9473 (1.2) -4.0
11: 187 0.9147 (1.2) -4.0
12: 239 0.8887 (1.1) -4.0
13: 305 0.8681 (1.1) -4.0
14: 390 0.8515 (1.1) -3.9
15: 498 0.8385 (1.1) -3.8
16: 635 0.8282 (1.0) -3.8
ceiling -1.89m due to compartment 3 gas used 1500


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


1: 4 1.9745 (2.5) -4.6
2: 8 1.9510 (2.5) -1.9
3: 13 1.8773 (2.4) -0.9
4: 19 1.7555 (2.2) -0.7
5: 27 1.6017 (2.0) -1.0
6: 38 1.4502 (1.8) -1.5
7: 54 1.3078 (1.7) -2.1
8: 77 1.1847 (1.5) -2.6
9: 109 1.0849 (1.4) -3.1
10: 146 1.0179 (1.3) -3.3
11: 187 0.9721 (1.2) -3.5
12: 239 0.9350 (1.2) -3.6
13: 305 0.9052 (1.1) -3.6
14: 390 0.8811 (1.1) -3.6
15: 498 0.8619 (1.1) -3.6
16: 635 0.8468 (1.1) -3.6
ceiling -0.69m due to compartment 4 gas used 2250


15 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


The NDL (ish) dive above continuing into deco
15 minutes at 2.5 atm
compartment: ppInert (ratio) min depth


1: 4 1.9750 (2.5) -4.6
2: 8 1.9732 (2.5) -1.8
3: 13 1.9565 (2.5) -0.3
4: 19 1.9037 (2.4) 0.5
5: 27 1.8022 (2.3) 0.7
6: 38 1.6700 (2.1) 0.4
7: 54 1.5201 (1.9) -0.2
8: 77 1.3717 (1.7) -0.9
9: 109 1.2395 (1.6) -1.6
10: 146 1.1450 (1.4) -2.1
11: 187 1.0776 (1.4) -2.5
12: 239 1.0216 (1.3) -2.7
13: 305 0.9757 (1.2) -2.9
14: 390 0.9379 (1.2) -3.1
15: 498 0.9075 (1.1) -3.2
16: 635 0.8831 (1.1) -3.2
ceiling 0.71m due to compartment 5 gas used 3750


And then after a much longer time (2hrs total)
45 minutes at 2.5 atm
compartment: ppInert (ratio) min depth


1: 4 1.9750 (2.5) -4.6
2: 8 1.9750 (2.5) -1.8
3: 13 1.9735 (2.5) -0.2
4: 19 1.9618 (2.5) 1.0
5: 27 1.9206 (2.4) 1.7
6: 38 1.8399 (2.3) 1.9
7: 54 1.7189 (2.2) 1.6
8: 77 1.5727 (2.0) 0.9
9: 109 1.4225 (1.8) 0.1
10: 146 1.3047 (1.7) -0.6
11: 187 1.2155 (1.5) -1.2
12: 239 1.1383 (1.4) -1.6
13: 305 1.0728 (1.4) -2.0
14: 390 1.0176 (1.3) -2.3
15: 498 0.9723 (1.2) -2.5
16: 635 0.9355 (1.2) -2.7
ceiling 1.89m due to compartment 6 gas used 6000

 

[NAM001]

Ken i think we are both on the same page. Clearly you can leave me in the dust when it comes to the detailed science of this. Perhaps the question i need to ask is What is NDL??? When the fist compartment in a group of 16 no matter which one it is hits full. ,,,,, when 3 compartments get min 80% full??? It just hit me that NDL is more a term every one uses but perhaps does not really know what it is. You know like when you say something is 3 miles away and after 2 blocks we ask are we there. To be honest i would guess that most all know the definition of NDL as the time you can stay down with out having to do deco STOPS during ascent. And most do not think all dives are deco dives, and dont understand the ascent rate is the deco for the dive.

Sorry about the verbose presentation. This would be nicer as a proper graph...

Below the 60 minute dive has the ceiling/compartment printed every 15 minutes. Then an extra 15 minute as per a deco dive and then a further 45 minutes for a 2 hour bottom time to demonstrate the extreme end.

What happens is that the fast compartments reach equilibrium quickly but that they can take more over pressure than the slower ones so they never become limiting. The ratio column is the tissue pressure/the fractional N2 pressure. Compare that to the absolute pressure at the depth and you see that after just 15 minute the first compartment is nearly at equilibrium and isn't going to take on much more gas. A longer dive makes a difference to the slow tissues, not the fast ones. Eventually they become limiting.

 

[KenGordon]

NDL is the time at which you can no longer directly ascend to the surface. In the dissolved gas models that is when ANY compartment reaches a inert gas partial pressure which would (probably) cause DCS at the surface given an ascent at however m/s the model designer chooses.

I am trying to show that which compartment is first to limit is non trivial and no general intuitive rule covers it.