Keeping Cobra in Nitrox mode @ 21% 02 when using air?

[azstinger11]

So on a recent trip, a dive instructor recommended we always leave our AI cobra 3's in nitrox mode but to set the O2% @ 21% on repeated air dive days to avoid Suunto's overly conservative air model. I didn't, and while I well know Suunto's conservative nature I didn't like the idea of messing with it without doing some research. Any thoughts on it? At this point i'm just saving up for a perdix, but still want to safely maximize my diving.

 

[tbone1004]

it shouldn't change the conservatism with air vs nitrox mode. that is a new one to me. Me personally? if it's easier to switch it to air mode than it is to change it to EAN21, then I would switch it to air. If it is easier to change the nitrox mix to 21%, then I would change it to 21%

 

[ibj40]

I used to dive a Cobra, and had the same recommendation. Can't say as I recall there being any difference.

 

[boulderjohn]

I heard the same thing with the Cobra I bought in 1999 when I got nitrox certified shortly after that. I don't know if it makes a difference, but it might make a difference for another reason.

I have not owned that Cobra or any Suunto for many years, so I don't know how the current system works, and I don't remember exactly how it worked with that Cobra. The big problem I had with it was that there were 3 modes: gauge, air, and nitrox. If you did a dive in one of those modes, you could not switch to another mode for 24 hours. I learned that if I did a dive with it in air mode and wanted to switch to nitrox for the next dive, I was basically screwed. I therefore kept it in nitrox mode at all times.

Most computers I know today do not even have an air mode, and I don't see the point in having one.

 

[Hoag]

On some computers (I don't know if yours is one of them) if you dive NITROX, then you can not go back to air for 24 hours. If you dive air, the only option is to select 21% O2. The algorithms for air and for 21% O2 should be the same.

 

[Bubblesong]

We have two identical Suunto DC.
Monday, I dove air the two morning dives, rested7 hours, then Two night dives.
One Suunto, set to 21% Nitrox, (air) for morning dives, easily reset to 32% Nitrox.
The other was set to air for morning and I couldn't reset to Nitrox for evening. 24 hours after dives, I was able to reset preferences to 21% Nitrox for this other computer, and that is how I will leave them.

 

[Storker]

on a recent trip, a dive instructor recommended we always leave our AI cobra 3's in nitrox mode but to set the O2% @ 21% on repeated air dive days to avoid Suunto's overly conservative air model.

With all due respect, that's pure and unadulterated bovine manure.

Both the part about air vs 21% nitrox and the blanket statement about being overly conservative.

 

[Bubblesong]

With all due respect, that's pure and unadulterated bovine manure.

the part about air vs 21% nitrox

Why this part is problem?

 

[Storker]

There's no problem. My point was that there's no difference between Suunto's 21% nitrox algorithm and their air algorithm. I get the same NDLs if I plan a dive on my Cobra set to air as if I plan a dive on my Cobra set to 21% nitrox.

 

[RainPilot]

This seems like a classic case of someone teaching the right thing for reasons they haven't understood. Somewhere, someone told this guy to keep the DC in Nitrox mode after a Chinese Telephone sequence and now he has filled in the reasons for himself.

Worries me a bit that an instructor thinks the 21 and Air are going to be different....

The mode issues of that DC makes total sense to leave it on 21, but for convenience not conservatism.