What % Nitrox settting on dive computer?

[MikeS]

It illustrates that there is no way to round the O2 percentage “conservatively” when it is used to calculate nitrogen loading, MOD, and O2 exposure for CNS limits.

Rounding the percentage of O2 down, while making the nitrogen loading calculations more conservative, makes the MOD and O2 exposure for CNS limits less conservative.

What I do is round the O2 percentage down resulting in more conservative nitrogen loading calculations. Then I round the percentage up and use a table to calculate the MOD, I typically use a 1.5 O2 PP. Then I adjust the computer PP down to get the same MOD.

This results in more conservatism for both nitrogen loading calculations and MOD calculation. As far as calculating O2 exposure for CNS limits, I don’t think it is practical to reach the limit if you maintain the O2 PP under 1.5. But I keep an eye on it and don’t get anywhere near the limit.

Mike

 

[Atticus]

First, I assume this is a recreational discussion that we're having...

Who's teaching ppO2 of 1.5? I believe PADI and NAUI both use 1.4.

Admittedly I frequently use 1.6 for tech diving, during deco (never bottom mix).

-Merlin

 

[Big-t-2538]

Atticus once bubbled...

Who's teaching ppO2 of 1.5? I believe PADI and NAUI both use 1.4.

When I was taught Nitrox...I was taught 1.6, that was with IANTD...but it also came with the disclaimer that 1.4 is the standard and unless we have good reason to push the limit...very good reason mind you, don't do it.

 

[Atticus]

I should clarify my question - who's teaching to plan a dive to 1.6 ppO2 in the recreational context? Most everyone agrees on 1.6 for contigency, but teaches 1.4 planned.

 

[chipster]

Glad I started this thread


Still lots of conflicting asnwers here, except maybe for the computer setting at 1.4 instead of 1.5. Yes Iantd excepts the standard as 1.5 then scale down under either a working dive or cold conditons. Cold being your are going to be cold not just the water temp. Because if I am wearing a dry suit I will not be cold.

Why does everyone here think calculating your gas is rocket science.

Ex. Start with a 30% mix at 3000# end with 1000# we will have 300# of O2 left . No topp it up and lets just it is now 3000# for ease of calculation. At 21% O2 we will have 420#. Thats 21% of the top up of 2000#. now we add the two and we get a total of 720#. Divide this by our total PSI which is 3000# and you get 24%. This is fact and you cannot change that unless you are diving on the summit of a mountain somewhere .

 

[MikeFerrara]

chipster,
Calculating what you should have after a top off is easy. It's still best to analyze before using it.

As to the original quaestion, one percent either way makes no difference, IMO.

 

[MikeS]

Atticus once bubbled...
Who's teaching ppO2 of 1.5? I believe PADI and NAUI both use 1.4.

TDI teaches 1.6. I choose 1.5 to add some conservatism.

Mike

 

[DeepScuba]

Hey Mike:

TDI teaches 1.6 as a MAX, But teaches 1.4 for Bottom use, just like all the rest.

Using 1.4, CNS loading is a mute point when Rec diving.

 

[cornfed]

DeepTechScuba once bubbled...
You should have bought a Suunto, then you could dial in your EXACT nitrox fraction.

I'm pretty sure that Suunto's won't let you enter fractional oxygen percentages. What model are you talking about?

 

[Big-t-2538]

chipster once bubbled...
Why does everyone here think calculating your gas is rocket science.

Ex. Start with a 30% mix at 3000# end with 1000# we will have 300# of O2 left . No topp it up and lets just it is now 3000# for ease of calculation. At 21% O2 we will have 420#. Thats 21% of the top up of 2000#. now we add the two and we get a total of 720#. Divide this by our total PSI which is 3000# and you get 24%. This is fact and you cannot change that unless you are diving on the summit of a mountain somewhere .

It isn't rocket sciencce as you point out...I am merely stating that unless you have a good pressure gauge your "guesstimation" is ball-park at best. Granted the whole diving thing is not an exact science by any means, but the diver has to be aware of that...and getting really fussy about 27% vs. 27.5% is just not a big deal....it matters, but with all the other variants involved (temp, conditions, viz, previous dives, physiology of diver, drunk, sober, etc.) it just isn't a big factor.