Nitrox on boat with air refill

[KenGordon]

It's a misunderstanding about what I'm .... <snip> .... at least one more ingredient - the data.

JohnnyC's post is a very good post. It is full of actual information.
Centrals made a good post too.

A couple of things about your posts:

You seem to be replying to these post which attempt to clarify in a style of "yeah yeah yeah I understood that, obviously! Do you think I am stupid or what?" and then poking at the poster for how they describe the knowledge, particularly when the play it down.

You apparently demonstrated a misunderstanding of cause and effect relationship of NDL and O2 exposure, but then went on to make a joke about the mechanisms used to track these as though you were fully conversant with the subject all along.

These are not ways to make for an informative thread. It does not matter whether JohnnyC misunderstood your misunderstanding. What he wrote was good and useful to you and others at your level of knowledge (note, that is proper condescension ).

Along with doing "Fundies", "get a long hose" and "BP/W are best" as standard forum answers is the common advice "read Deco for Divers by Mark Powell".

I give that as serious advice. It contains enough information for anyone wanting to learn about nitrox diving and more. It has lots of background on why stuff is done the way it is, plenty of references to the actual papers that led us to where we are today and a good number of equations on how stuff is derived. The TDI Advanced Nitrox course manual contains various parts of that book.

Beware that once you have read that book you will find the academic part of many subsequent courses rather boring.

 

[Basking Ridge Diver]

standard forum answers is the common advice "read Deco for Divers by Mark Powell".

+1 - just read the book then come back with questions...

 

[rx7diver]

Centrals,Please don't wait on my account! Have at it, if you wish. And, if you wish, show your result here. When I have time, I'll do likewise. Then we can compare our two results. And we'll get to see if my suspicions bear out. Now, won't that be fun??!!Safe Diving,rx7diver

I finally found the time to hand-calculate an example that demonstrates (to some degree) that a diver should be concerned with CNS oxygen exposure even when diving only recreational profiles--when subsequent (i.e., repetitive) dives are done using an EAN cylinder that has been topped up with air (from the dive boat's compressor). 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."

1. Initial conditions. (a) Initial and repetitive recreational dives to 100 fsw. (b) BIG cylinder (so dives are not gas-limited). (c) All dives begin with cylinder filled with NOAA Nitrox II. (d) 60 fpm descent rate. (e) 60 [sic] fpm ascent rate. (f) two-hour surface interval.

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

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

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

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

6. EAD for initial and subsequent dives to MOD = 110 fsw, is EAD = 90 fsw. Solve (0.79)[(EAD/33) + 1] = (0.64)[(110/33) + 1] for EAD.

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

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

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

10. 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, which *exceeds* the 36 min single dive CNS oxygen exposure limit.

Okay. Full example (using air-topped-up cylinders) will follow when I have time to write it out.

Safe Diving,

rx7diver

 

[packrat12]

rx7diver - are you serious? The profile you just presented does not even remotely model the plan you proposed. 100' square on 36% for 2hr SI. You said it would easily exceed CNS limits. To save you time, I will post the accurate plan based on your parameters. I even left in the deco on the dives which is due to differing models and would only increase CNS and OTU numbers. This plan gives real results that I depend on for my life. As for not giving CNS credit... where did that come from?????


DIVE PLAN #1
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 30/85

Dec to 100ft (1) Nitrox 36 60ft/min descent.
Level 100ft 29:20 (31) Nitrox 36 1.45 ppO2, 75ft ead
Asc to 30ft (33) Nitrox 36 -24ft/min ascent.
Stop at 30ft 0:05 (34) Nitrox 36 0.69 ppO2, 18ft ead
Stop at 20ft 3:00 (37) Nitrox 36 0.58 ppO2, 10ft ead
Surface (38) Nitrox 36 -20ft/min ascent.

OTU's this dive: 56
CNS Total: 23.1%

69.2 cu ft Nitrox 36
69.2 cu ft TOTAL


DIVE PLAN #2
Surface interval = 0 day 2 hr 0 min.
Elevation = 0ft
Conservatism = GF 30/85

Dec to 100ft (1) Nitrox 35 60ft/min descent.
Level 100ft 19:20 (21) Nitrox 35 1.41 ppO2, 76ft ead
Asc to 30ft (23) Nitrox 35 -24ft/min ascent.
Stop at 30ft 0:05 (24) Nitrox 35 0.67 ppO2, 19ft ead
Stop at 20ft 1:00 (25) Nitrox 35 0.56 ppO2, 11ft ead
Surface (26) Nitrox 35 -20ft/min ascent.

OTU's this dive: 37
CNS Total: 23.7%

47.2 cu ft Nitrox 35
47.2 cu ft TOTAL


DIVE PLAN COMPLETE

 

[Basking Ridge Diver]

Wait! What? False information given on the internet?!? Who would have thought?

 

[IVC]

You apparently demonstrated a misunderstanding of cause and effect relationship of NDL and O2 exposure, ...

I misspoke in post #67 and corrected it as soon as it was pointed out. If you want to be fair, read the rest of the thread and you'll see that in post #14 (way before the one you are using) there is absolutely no "demonstrated misunderstanding" that the NDL is due to the N2 exposure (so I don't get nailed on a technicality again, I understand it's the bubbles that can form from any gas, but in this case by far the primary effect comes from N2, which is what I pointed out in the post #14.)

More importantly, if you read the OP you will notice that my concern was with effects of elevated O2 concentration (MOD and CNS toxicity,) not N2 saturation (NDL.) Further, the parameters of the problem were "no tables, no computer and no sensor" because it was the situation I was in. I have the tables now, so no further problem (to avoid getting nailed on technicality again, the "tables" includes Nitrox NDL *and* "DSAT Oxygen Exposure Table for Enriched Air".)

JonnyC made some excellent points and I both acknowledged and thanked him for it. I especially liked when he was being precise about "FO2" vs. loose notation I was using "PPO2 at sea level" when describing gas mixture.

As for the rest, just because I am new here doesn't mean I am new to specialized discussion boards (far from it,) or that I'm new in life in general. Some of us *do* know a thing or two from other aspects of our lives and it's just a matter of fine tuning terminology and getting accustomed to the idiosyncrasies of each forum. Not everything is "pretending to know things."

 

[KenGordon]

My use of 'apparently' was deliberately chosen.

You seem very keen to be seen as understanding the subject around your question. If you did not keep taking offence at people posing their answers as if to someone who does not understand then your 'misspeaking' would be forgotten.

 

[scubadada]

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]

I have not dove my Oceanic PP3 in a while.. :blinking: I forgot that was the default handling of CNS for Oceanic. As you also stated, we still may not have exceeded CNS but rather the Oceanic handling of it. Even with Oceanic's handling, it takes a while to get there, especially in rec limits. I prefer a real prediction of my CNS level and would also like to see Oceanic correct it.

https://www.shearwater.com/news/shearwater-and-the-cns-oxygen-clock/

 

[IVC]

You seem very keen to be seen as understanding the subject around your question. If you did not keep taking offence...

Projection on both counts.

I am very explicit about what I do and do not know so I can get meaningful answers (and there were many in this thread.) I know the principles, I don't know the actual models. What you perceive as "offense" is nothing of the kind - we just keep on spinning in circles about what the initial problem was. The initial problem was whether there was a method to figure out CNS exposure using basic DC, or whether there was a method to figure out that "close enough to air" is sufficient as a method to handle CNS O2 exposure. Saying "don't worry," "it's a non-problem," "calculations show...," "you'll reach NDL limit way before CNS limit," etc. all involve knowing the actual models, so they don't address the OP. On the other hand, saying "empty the tank before refill," "calculate the actual O2 content based on pressures," "get the tables and carry them," etc. do address the OP, even though they are much less technical. It's not about "taking offense," it's about getting back on topic.