Suunto: Air Model vs. Nitrox Model (21%)

[NudeDiver]

I'm hoping to get an answer from someone who actually knows, fairly definitively, rather than just guessing. So - that said:

Is there any material difference between using the Air Model, and the Nitrox model, with the percent O2 set to 21%? Are there any real reasons to avoid just leaving it on Nitrox 21% whenever diving air? If so - what are these reasons, why, and what are they based on, with respect to how Suunto implements its algorithm, etc.?

The manual doesn't seem to address this issue directly, at least that I can find, for either the Cobra or the Vytec.

Thanks!
ND

 

[Diveralan]

Air mode and 21% are the same as far as the no decompression time. It you dive nitrox you should keep it in nitrox mode all of the time with O2 set to 21% when diving air.
Air mode is intended for divers who never use nitrox. As long as the computer is in air mode divers who have not been trained in nitrox do not have to set O2 % and they can't accidentally change the O2 percentage. I changed my Cobra to nitrox when I first got it and it has never been changed back. The O2% will default to 21% after several hours if you do not set it.

Once you dive in nitrox mode it can't be changed to Air mode for a certain amount of time ( I think 24 hours). It is also not possible to change from Air to Nitrox after making a dive. This is why it is recommended that nitrox divers always keep the computer in nitrox mode.

 

[maged_mmh]

what alan said - in short to: to take the O2 CNS clock effect into consideration

 

[RonFrank]

21% Nitrox = AIR

That is true on EVERY computer.

On some computers if you don't set your specific mix before each dive it may default to some not so desirable value. This is specific to each model, and I have NO CLUE what Suunto does. So for the diver who does air only, keeping it in air only mode is safe as the computer will NOT expect a Nitrox value to be set before each dive.

Suunto does not allow going from Air to Nitrox at ANY point? That is foolish, so is that true? I can understand not going from Nitrox to Air, as 21% IS air. But I've never heard of a computer that won't allow air to Nitrox at any time. IMO that would be a major disadvantage, and a reason NOT to purchase that computer.

 

[Geoff_H]

Suunto does not allow going from Air to Nitrox at ANY point? That is foolish, so is that true? I can understand not going from Nitrox to Air, as 21% IS air. But I've never heard of a computer that won't allow air to Nitrox at any time. IMO that would be a major disadvantage, and a reason NOT to purchase that computer.

I don't know if that is an issue, but even if it is there is a very simple workaround (already stated) - keep it on Nitrox mode and use 21% for Air and 32% (or whatever your mix is) when you switch. So, no reason to NOT purchase the computer at all.

 

[John_B]

Suunto does not allow going from Air to Nitrox at ANY point? That is foolish, so is that true? I can understand not going from Nitrox to Air, as 21% IS air. But I've never heard of a computer that won't allow air to Nitrox at any time. IMO that would be a major disadvantage, and a reason NOT to purchase that computer.

Well you've heard of one now.

IIRC, the manual even specifically recommends that Nitrox divers set it to Nitrox at 21% for air, so you can switch to other Nitrox mixes later. As earlier posters have said, it probably has to do with not tracking the CNS clock with the Air setting.

BTW, its not that you can't change it, you just can't change it until the desat counter/timer has run its course.

There are lots of other hardwired settings on Suunto computers that, IMO, make zero sense. How long until it resets your Nitrox setting back to the default setting of 21% is yet another. Totally arbitrary and not user configurable. Now that I think back, the stuff that is annoying or "lawyerish" is a pretty long list, back when I had my Vytec I think I posted a rant about it. The Uwatec Aladin Tec2g has its issues, but at least the Uwatec designers didn't make a dozen specious decisions for you (in the name of "safety", "we know what's best", its "for the children", whatever) that you can't change or reconfigure.

 

[Hatul]

That's what I do: set Nitrox 21 % to dive with air with my Vyper. I recall the manual advising this as well if you're Nitrox trained.

The difference is the meaning of the bar graph on the left of the display. On Air mode it shows graphically the amount of non dec time left and in Nitrox mode it shows the oxygen toxicity.

Adam

 

[NudeDiver]

All:

Thanks for the responses. This is basically what I figured - but I was unaware if the Suunto computers are "doing something" in Nitrox mode that they do not do in Air mode, that I might find undesirable if I were actually diving air, rather than "nitrox" (despite the fact that yes, air == nitrox 21).

 

[vladimir]

How long until it resets your Nitrox setting back to the default setting of 21% is yet another. Totally arbitrary and not user configurable.

Putting in a value for MIX2 on my Suunto D9 will prevent MIX1 from reverting to its default value of 21%, which it does after a surface interval of a couple of hours. So if I am diving 32% all week, as is often the case, I won't have to reset the FO2 before each dive.

 

[Diveralan]

Why do you think this is a disadvantage? If you are not a nitrox diver why would you need to change it to nitrox mode mid dive trip? If you're a nitrox diver and want to dive air Nitrox = 21%.

I don't know why you can't change from air to nitrox. I can't think of any valid reason for Suunto to prevent this, Just not sure why anyone would want to. It was probably designed this way on the advice on a lawyer.

21% Nitrox = AIR

Suunto does not allow going from Air to Nitrox at ANY point? That is foolish, so is that true? I can understand not going from Nitrox to Air, as 21% IS air. But I've never heard of a computer that won't allow air to Nitrox at any time. IMO that would be a major disadvantage, and a reason NOT to purchase that computer.