Nitrox/Air - Can you do both in the same day?

[Cobra852]

just trying to come up with a reason why they'd make the computer stay in nitrox mode

 

[doctormike]

I was reading through my dive computers manual, learning how to program the Nitrox modes, and noticed it had a warning that stated once you start your dive in the Nitrox mode, the computer will not let you switch to Air within 24 hours.

After reading that, several questions came to mind.

1 - Why?

2 - Is it considered standard practice to "Start Nitrox / End Nitrox"?

3 - If you only had 1 Nitrox tank, would it be acceptable to dive the 2nd dive with Air, and set the computer to EAN21? Of course this would require planning your dive and standard deco.

Yup, had exactly the same observation/question with my Suunto cobra recently. I guess you would need to ask the engineers what the thinking behind this was, but here is my own guess:

It is probably not because switching to air would reset the O2 clock. I would think that this would be pretty easy to handle in software- just keep the O2 clock running in either mode, using EAN 21 for calculations in air mode. As previously noted, this probably wouldn't add much to the oxygen limit fraction within the non-deco limits. Just because you are not displaying OLF (in air mode), doesn't mean that it can't be kept track of in the background.

I think that this feature is just to make sure that you don't forget to change the % if you are switching between mixes on a given day (sort of a subtle prompt)... of course, you can always just change it to EAN 21, but by making you do that, and not just switching back to "default" air mode, it makes it less likely that you will forget to set the accurate %

Many people who routinely dive nitrox just leave it in nitrox mode and use EAN 21 when they use air. Nothing wrong with this. I assume that the reason that the manufacturers don't do away with the air mode altogether is that it can be intimidating to new divers to have to worry about their O2 % when they may not even be clear on what that implies... Even if you just tell them to leave it at EAN 21, most would probably be happier having an air mode. This also frees up screen real estate, since you don't need to have a part of the screen showing "21" when you are diving air.

 

[Tobagoman]

I believe the computers do this because, when in "Air" mode, they simply don't keep O2 loading information. Therefore, setting the computer into Air mode would erase the O2 clock information.. and if you went back to Nitrox for a third dive, it would think you're at 0% on the O2 clock.

Like other said, just set it to EAN21 for the air dive.

Jonnythan hit it right on the head. This is the exact reason. Just set the computer to 21% for any dives on normal "air".

 

[sharpenu]

Simbrooks-

Not all divers use 32 exclusively. Not too long ago, I did 7 dives in 24 hours. This was the profile, according to my log:

Day..........Time......Max Depth..... time..........02%
Day 1........1621.........67............ 0:51 ..........36
Day 1 .......1758.........62............ 1:01 ..........37
Day 1 ...... 2056 ....... 62............ 0:52 ..........36
Day 1....... 2225 ....... 61 ........... 0:32 ..........35
Day 2 ...... 1207 ....... 64............ 0:41 ......... 36
Day 2 ...... 1327 ....... 65 ........... 1:01 ......... 36
Day 2 ...... 1602 ....... 65 ........... 0:42 ......... 21

Now, when I got back from that last dive, my O2 loading and my nitrogen were both maxed out, but it can be done. I had to dive air for the last dive, because my O2 was maxed. 5 hours and 40 minutes at around 65 feet in about 24 hours. All within NDL. EANx36 is perfect for dives in the 60-65 foot range.

 

[simbrooks]

Simbrooks-

Not all divers use 32 exclusively. Not too long ago, I did 7 dives in 24 hours.

Nor do i use 32% exclusively, i have dove a variety from 21 up to 40% some PP mixed that way, others diluted down from previous mixes. Dont group me with DIR and 32% or trimix all the time kind of stipulation! The example was just that, an example of why you need to run the tables to be sure which option/combo might be best for the dives you intend doing. In the thread which that link was from i had previously stated that i thought it was best to dive nitrox first, air later, in fact to max out your NDL's it turned out with that example that air first, nitrox later worked best for the 100/70' pair of dives if you were cheap and only wanted one tank of nitrox - of course two sets of nitrox would have been best and getting the blends right for those depths gives that extra edge. The reason the combo worked best is all due to the greater amount of time you can spend at depth on the nitrox saturating your body more than a shorter time on air - was a bit of a mind bender for a while until i thought about it, but now i am trying to not skimp on the nitrox!

BTW - most i have done in a day was 5 during daylight hours, probably 6 over 24 hours. Would have been more if buddies had been willing to dive more as well

 

[ScubaBones]

Thanks all! Very good information here.
Glad to know there is a safe alternative method for diving as EAN21.

 

[sharpenu]

HAHAHA! I know, it sucks when your buddies wimp out on ya!!!