Nitrox math problems?

[lairdb]

I've been staring at the RAID Nitrox course material for a while now, and I'm still stuck. Tell me if I've got it wrong, but:

On p.22 of the 4-Jan-18 edition, it calculates a 40 minute ABT dive. In the analysis, it shows that the ppO2 0.9 exposure limit is 360 minutes, and then subtracts 90 minutes.

Where does the 90 come from? Is it just... wrong? That's... potentially concerning for an engineering/life-safety text.

On the "wrong" theory, I pulled an updated copy of the manual. In the current (17-Oct-19) edition, the calculation has been ...changed:

Now, 90 has been changed to 40, but 360-40 is not 270. That's also... concerning.

Further,
-This is labeled CNS, but it's not the CNS % (which they don't calculate, for no reason I can see), and
-40 is still not the right number; NOAA is clear, and RAID reproduces their text: CNS and OTU are calculated surface-to-surface, not surface-to-ascent, so it should be 40+ascent+safetystop, i.e. at least 45.

Very confused. I think:
-It should be 360-45=315 remaining daily exposure minutes.
-That's actually the exposure limit minutes, which is more relevant to pulmonary tox, not CNS tox.
-They should have shown a CNS% calculation, which is the more likely material calculation for typical recreational diving.
-The CNS% should be 0.28*45=12.6%.
-Should they have calculated actual OTUs, at least for the learning exercise? (0.85*45=37.35 OTU.)

Thanks.

(And edited to add Shearwater's take, which I would summarize as "OTUs are Not Useful".)

 

[inquis]

40 is still not the right number; NOAA is clear, and RAID reproduces their text: CNS and OTU are calculated surface-to-surface, not surface-to-ascent,

(Not a RAID instructor, but my thoughts...)

Using surface-to-surface at the bottom PO2 will over estimate exposure. Perhaps using time to ascent is a coarse attempt at improving accuracy? If I had to guess, though, I'd say it was an error. FWIW, a more accurate approach would break up the surface-to-surface timeline into smaller chunks.

Your "I think..." bullets seem mostly correct to me. The only thing I would disagree with is the "it's relevant to pulmonary tox, not CNS tox". NOAA tracks pulmonary-related exposure (reduction in vital capacity) with OTUs. There are 2 CNS "clocks" or "percent" metrics (current dive and 24 hr exposure) that are intended to address seizure risk. (Hopefully, you understand that percent and minutes are just two ways of looking at the same limit.)

In this case of 0.9 PO2/single dive, the two limits are the same. While they didn't actually divide the time by 360 mins to get a %, it's clear this dive is a long ways from the limit (either in mins or %).

 

[inquis]

They should have shown a CNS% calculation, which is the more likely material calculation for typical recreational diving.
-The CNS% should be 0.28*45=12.6%.

Explicitly on this comment, again, it's the same limit (360 minutes = 100%). It's no coincidence that the value of 0.28 in your calculation is merely 100%/360min.

Arguably, the more important takeaway is neither CNS(%) or CNS(minutes) are very relevant to recreational diving because it's practically impossible to reach those limits. It's really only a consideration for technical diving, and even there, applicability is far from unanimous (as you point out with the Shearwater link).

 

[lairdb]

(Hopefully, you understand that percent and minutes are just two ways of looking at the same limit.)

This is probably the most important reminder for me -- I had that at one point, and it got away. That makes the labeling of the minutes calculation make sense, and the percent/minute now clicks.

Using surface-to-surface at the bottom PO2 will over estimate exposure. Perhaps using time to ascent is a coarse attempt at improving accuracy? If I had to guess, though, I'd say it was an error. FWIW, a more accurate approach would break up the surface-to-surface timeline into smaller chunks.

Well, sure, but chunks vs square profile is really "just" computer vs. tables. That said, the inherited guidance from NOAA is clear: ppO2 calculations are to be done using surface-to-surface time, and if the textbook is showing how to do it by the book, it should do it by the book.

Generally, the actually disturbing thing for me is that the textbook appears to not only have lookup errors but also can't do basic subtraction.

Your "I think..." bullets seem mostly correct to me. The only thing I would disagree with is the "it's relevant to pulmonary tox, not CNS tox". NOAA tracks pulmonary-related exposure (reduction in vital capacity) with OTUs.

Right -- as discussed and much thanks for unfogging that.

Arguably, the more important takeaway is neither CNS(%) or CNS(minutes) are very relevant to recreational diving because it's practically impossible to reach those limits. It's really only a consideration for technical diving.

I think I'm going to have to build a spreadsheet to prove that to myself, assuming a repetitive mission with negligible intervals and a square profile, but a few quick mental doodles sure suggest you are correct for realistic scenarios.

 

[inquis]

FWIW, the NOAA 24 hr exposure window is only one approach to the multi-dive characterization, and it is relatively conservative. I can see bumping up against these limits fairly easily with 4 rec dives a day.

Other entities use a 2 hr or 1.5 hr half-time reduction (Shearwater the latter). For example, finish a dive at 80%, wait 90 minutes, and you're at 40%. That leaves room for a 60% dive. (Although, some would argue using a max of 80%.) It's much harder to trigger a CNS warning on a Shearwater with a series of recreational dives. The OTU limits are even harder to approach and are not even tracked by SW.

 

[GF99/99]

Honestly I would not stress about it too much. This magical 80% is just a theoretical number

In a recreational diving scenario it is really really really really hard to get to 80% CNS. I would actually be really impressed if you could even do it.

Even as a technical diver unless you are doing sub 100m diving long bottom times or super long cave dives you still have to put in a pretty good effort to get over 80% CNS. I have done lots of sub 150m dives with 6hr+ deco or lots of super long cave dives and even then I only get up to about 150%-170% CNS. But at this point you start doing air brakes and even then 80%CNS is just a theoretical number everyone's body will react different to high CNS levels so its something you need to discuss with your team as to what protocalls your going to follow and what is an acceptable risk.