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

[ScubaBones]

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.

Thanks
don

 

[SuPrBuGmAn]

The only reason I can think your computer says that, is it may not be programmed to allow for an algorithm with two different gas mixtures. I believe most computers do this now.

You can dive Nitrox and air, or different Nitrox mixes in one day. Most Nitrox classes will provide you with equivellant air tables for different mixes of Nitrox anyway. This way you can use a regular air table to plan your dives, just be sure to keep an eye out on your MODs and PPO2s and exposure percentages.

 

[The Kraken]

1 - You're concentration on your O2 accumulation with respect to a 24 hour period.

2 - For the most part

3 - Yes, air is just EAN21.

 

[astrl]

I dove nitrox and regular air just a few weeks ago. I just enter 21% so I don't have to change the mode of the computer.

 

[scubamate]

A lot of the computers will not set back to "air" but you can set the O2 percentage to 21.

 

[jonnythan]

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.

 

[hugo]

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.

Thanks
don

My computer does the same. The reason is that the computer does not monitor the O2 accumulation if it is set to air. Once you dove with nitrox, the O2 level has to be monitored even if you dive air. As you said, just set the computer to EAN21.
Now I wonder what to do if you dive air first and then Nitrox within 24h. I guess you had to set the computer to EAN24 for the air dive too.

 

[simbrooks]

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.

I agree with the others on questions 1 & 2 and the computer clocking stuff, but as for question 3 - you need to run the tables to find out which option is best for you. I did something quick a while back on a previous question like this using PADI tables. air/nitrox combo's.

 

[Cobra852]

I agree with the others on questions 1 & 2 and the computer clocking stuff, but as for question 3 - you need to run the tables to find out which option is best for you. I did something quick a while back on a previous question like this using PADI tables. air/nitrox combo's.

O2 accumulation can continue even using standard air if you push your %limits then switch to air and go deep its possible to push your exposure over the limit

 

[simbrooks]

O2 accumulation can continue even using standard air if you push your %limits then switch to air and go deep its possible to push your exposure over the limit

Not quite sure what you are saying here, if its in reference to the linked post or not. O2 exposure is hard to push compared to NDL, in that linked post i was just matching up different combos of air and nitrox (if you were to only do one tank per pair of dives compared to either just air or just nitrox) as that was part of the original question.

Never said anything about O2 clockwatching as it really is a moot point here, if you did say 32% then air or both 32% you'd still have very little on your clock, but you might max out your NDL. The way the computers work, you have to leave it in nitrox mode to clock up that O2, even though it is next to nothing for a normal dive profile. You'd have to be making really deep dives (>150ft all up to 218ft and beyond if you so wish) to push your O2 clock up on air at which point you'd be deco diving anyway.... I hit at least 3 dives to 100ft on both friday and saturday, all on 32%, all for close to NDL length times with a reasonable SI between each one, plus misc other dives. My O2 clock hardly counted more than a few pixels out of about 10 for the way my computer displays it, looking at the NOAA tables for about PO2 ~1.3 i had 210 mins in 24 hours, i probably was at that kind of depth for only about 50 mins, the rest was getting there or back (i know that isnt how the table works its total BT assumed all at depth, but that wasnt the profile nor how the computer tracked it) which probably totalled about 100 or so mins of travelling time - i havent yet double checked the times this is by memory.