Why 32% & 36%?

[Peter Guy]

I was thinking about this and decided to ask if anyone knows why NOAA came up with #1 at 32% and #2 at 36%.

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.

So, why those percentages?

 

[scubadada]

MODs of 112 and 95 ft at pO2 of 1.4?

Good diving, Craig

 

[bleeb]

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.

 

[Red Sea Shadow]

32% because it's an "all around" gas. In other words, even if the recreational diver didn't stick to the 1.4 PPO2, the MOD of 1.6 is 40m (130ft), which is the maximum recreational limit. And it's highly unlikely to spend 45min (or even 36min, which is 80% of the NOAA maximum single exposure limit) at 40m on a recreational dive. So CNS is not an issue.

 

[fdog]

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.

I was a test subject for these mixes when they were first released, and this is what we were told.


All the best, James

 

[dumpsterDiver]

I figured the 32% was because it corresponded to the max exposure of 1.6 at the limit of recreational dive depths. Guess i was wrong.

 

[Mr Carcharodon]

I believe when NOAA came up with 32 and 36 they were assuming a ppO2 limit of 1.6 ata. So 32% gives a ppO2 of 1.6 at 5 ata which is perhaps not coincidentally is the recommended recreational depth limit. 36% has depth limit of 114 fsw for 1.6 ata ppO2, which is harder to explain.

 

[Blackwood]

The only answer I could come up with was ease of mixing
...
But that can't be the answer.

Why not? In many instances (SCUBA and otherwise), things became standard for pragmatic reasons.

 

[Blackwood]

I figured the 32% was because it corresponded to the max exposure of 1.6 at the limit of recreational dive depths. Guess i was wrong.

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?

 

[MauiScubaSteve]

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