Why 32% & 36%?

[lamont]

Back when NOAA named Nx1 and Nx2 was there even a "recreational limit?"

And if there was, would NOAA (a scientific, not recreational outfit) care?

Scientific diving has similar kinds of limits. Really they're both just no-deco limits with a max depth floor beyond where the tables get silly.

 

[rongoodman]

The only answer I could come up with was ease of mixing -- 15% O2 and 85% air = 32%; 20% O2 and 80% air = 36% -- or for an AL80 450 psi of O2 or 600 psi of O2. All of which are easy numbers to remember!

But that can't be the answer.

Except for 30/30, all the GUE standard mixes can be obtained by topping He off with pre-banked 32%.

 

[Peter Guy]

As I was taught, it was for planning convenience since PN2s are such that they're the equivalent of diving 10' or 20' shallower than air, allowing people to use the standard NOAA air table and just read one or two rows up, without having to do any additional calculations.

Thank you (and James) for this.

In playing with EADs here, NOAA then became very conservative with this method -- especially regarding 32%. (EAD of 32% at 100' = 81' which "rounds up" to 90' for a table, i.e., one row up.) In playing with the EADs and rounding up to the next highest depth, 32% is one row up from 100 to 60' (that's all I played with) while 36% is two rows up from 100 to 70'.

HOWEVER, 31% and 30% yield the same results and probably so does 33% -- at least doing EAD calculations and rounding to the deeper depth. So I guess I'm still left with the question of why 32%? Or am I just not understanding the NOAA tables which is a distinct possibility!

 

[boulderjohn]

Except for 30/30, all the GUE standard mixes can be obtained by topping He off with pre-banked 32%.

But the cause and effect are reversed. The standard mixes came about because dive shops were banking 32%. That percentage already existed; it was not created to make it easy to make standard mixes.

 

[lamont]

As I was taught, it was for planning convenience since PN2s are such that they're the equivalent of diving 10' or 20' shallower than air, allowing people to use the standard NOAA air table and just read one or two rows up, without having to do any additional calculations.

That's sort of how GUE teaches to use them (20% advantage for 32% actually), I don't think that was the rationale, though, I think that observation came after the fact.

I'm pretty sure some of the early 1990s-era nitrox articles in rubicon reference the 1.6 ppO2 @ 130 foot rationale for 32%. I don't know where 36% comes from.

 

[dumpsterDiver]

....I'm pretty sure some of the early 1990s-era nitrox articles in rubicon reference the 1.6 ppO2 @ 130 foot rationale for 32%. I don't know where 36% comes from.

It sounds better when you say it.

 

[Mr Carcharodon]

OK I know it's dangerous but I cracked open a book. Section 15.7.2 of the NOAA Dive Manual says the NOAA tables for 32% and 36% support all no-deco dives, and dives requiring a 10 fsw stop. So their tables support slightly non-rec dives. They were working to 1.6 ppO2 as a limit. They confirmed that 1.6 ppO2 at 5 ata was why there is 32%. Why 36%, well you could dive a richer mix shallower, and it's a nice round number. Does there need to be more of a reason?

 

[Peter Guy]

Mr. C -- I'm sorry, and I may be a bit dense, but I don't understand your statements.

a. NOAA 1 & 2 "support all no-deco dives?" What does that mean? As long as the dive time is short enough, even air will be a no-deco dive, right? Please explain to this poor guppy.

b. 36% is a "nice round number?" What? So is 35 (even rounder) and so is 40.

Since 32% DOES have an MOD of 40 meters at a PPO2 of 1.6, having that gas makes a sort of sense. But 36%, OK, Why?

 

[Mr Carcharodon]

Sorry that should have been clearer. They picked the gasses and then built a EAN32 table and a EAN36 table. The times on the tables correspond to NDL plus a bit of time. The bit of time being the length of a 10 fsw stop. Which is to say exposures long enough to require a 20 fsw stop were out of bounds. NOAA alluded to an "extended" NOAA table that addresses dives requiring stops deeper than 10 fsw. The "extended" table is not included in the NOAA manual, but they go onto to say that the “extended” NOAA table is an exact copy of the US Navy (nitrox?) tables.

Regarding 36% it seems there is a lower limit of 32% set by ppO2 considerations, and an upper limit of 40% set by conflagration avoidance. 36% is conveniently half way in between. The NOAA tables I have were published in 2000 prior to decompression software being widely available so most people were planning on tables. Could they have done a 35, 37, 39, ect% table -- sure. But they would not have been very different than the 36% table, and they would potentially overload the gas blenders and divers with a multitude of choices. Anyway the last bit is speculation.

 

[Peter Guy]

Mr. C -- Thank you.

So it seems that the best guess answer is the 1.6 PPO2 at 40 meters = 32%. Hmmmm.