Nitrox and Atmos ai Question

Hello,

I'm sitting here reading and studying for my nitrox certification and have a question about my Atmos ai computer. According to the Enriched Air Diving book and tables it is possible to combine both air and nitrox dives. But according to the manual that comes with the computer it says:

"Once a dive is made with the unit set as a nitrox computer (F02 set for a numerical value), the unit cannot be programmed to operate as 'Air' computer until 24 hours after the last dive. 'Air' will not be displayed as an option in the F02 Mode. However, you can set F02 for 21% for use with Air."

Is this common when you know that you are or may be diving both Air and nitrox to set it at 21% for Air? Why wouldn't it let you switch back and forth? Also is it better practice to just keep it set for 21% Air instead of putting it back to F02 even if I'm going to dive just Air?

Thanks, sorry if this is a dumb question. It just seems confusing.

Just set it to 21% nitrox. Same as air. It may be this way because in nitrox mode it tracks O2 toxicity and wants to keep tracking it in case you switch back to Nitrox on a subsequent dive. After 24 hrs, the O2 accumulation toxicity clock is reset.

For what it's worth, my suunto does the same thing.

Jason

As indicated above, you just set it to 21%. Until you start diving nitrox, there's really no reason to set the PO2 to 21%, though it might be a good habit so you remember to set it before each dive.

I think the reason for you actually having to set the mix at 21% for air after diving nitrox is so you are forced to actually set your PO2 for each dive. If you forget to set your mix, it defaults to a mix of 50% oxygen and 79% nitrogen (I've been penalized with this more than a few times when forgetting to set my mix). It could be dangerous for it to just default back to air, since you're probably not diving air on your second dive if you were diving nitrox on your first dive.

I wish it would beep at me or something when I begin my descent without setting my mix (or, better yet, allow me to retroactively change it).

bgi:

Just set it to 21% nitrox. Same as air. It may be this way because in nitrox mode it tracks O2 toxicity and wants to keep tracking it in case you switch back to Nitrox on a subsequent dive. After 24 hrs, the O2 accumulation toxicity clock is reset.

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That is how I do it, and it is what the manual says as well!
Great computer! Enjoy!

Thanks for the replies. It's starting to make sense now.

D_O_H:

I wish it would beep at me or something when I begin my descent without setting my mix (or, better yet, allow me to retroactively change it).

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The Atmos 2 has a setting to disable this silly "reset to 50%" feature. Look for it.

bgi:

The Atmos 2 has a setting to disable this silly "reset to 50%" feature. Look for it.

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Yup, I hate when i forget to set it to 21%. And i keep forgetting to disable it too