Again, WHY put it into "Air Mode"? If you ever dive nitrox, leave it permanently in "Nitrox mode" and just set it to 21% for your air dives. Silly all this switching, especially if you have to deal with mode lock outs.
If you leave a Vyper2 in nitrox mode, you *must* get in the habit of checking (and if necessary, setting) the O
2 percentage before each dive. While the basic-level Gekko precludes this by resetting to 21% after a few hours (meaning it will be effectively in "basically air" mode by the next outing), the Vyper2 will retain the nitrox mix settings.
If a diver doesn't turn on and check the Vyper2 before the dive, instead just starting the dive and letting the computer self-activate as they cross 4 feet deep, they could find that they have it set to a richer nitrox mix than they are actually diving (eg. perhaps 32% instead of 21%). This would underestimate their nitrogen loading, perhaps significantly. By returning to air mode, they preclude a lack of attention from becoming a potentially dangerous miscalculation.
It's certainly a good idea to *always* activate your computer before you hit the water, even if it can self-activate. Also, once you dive nitrox, you should *always* check your computer's nitrox settings before each and every dive. If you're only an occasional nitrox diver, returning to air mode is just another bit of "just in case" to help keep you from getting in a heap of trouble if you miss a step.
Incidentally, with Suunto computers, if your computer just ticks over into dive mode, and you see you've forgotten to set the mix (perhaps you're diving shallow air after a deep nitrox dive), you must return to the surface and wait. After *five minutes*, the computer will tick back over into surface mode, and *then* it will let you set the correct O
2 settings.
(If you're diving a richer mix, i.e. more O
2 and less N
2, you could just note your MOD and dive it with the reduced NDL your mistake gives you. If you're diving a mix with *more* N
2, the dive computer will underestimate your nitrogen loading if you continue the dive. The prudent thing is to kick yourself for not setting it as you wait for the five minute timer to elapse so you can correct it and get on with the dive. If you only notice well into the dive, well, please be conservative with your bottom time, ascent, and stops...)
