2 tanks: Nitrox and Air - Which one do you use first?

[NW_TechDiver]

I took a trip to Canada a few years ago where we were diving single nitrox tanks every dive and getting close to NDLs. On our second dive, I switched to my twins that I had yet to dive that weekend, which were full of air. I didn't do anything long, and I was buddied up with some of my Nitrox diving buddies, but I had racked up 20 minutes of deco.
Lesson to learn? dive air first, and switch to a richer mix as the weekend/day progresses. Not the other way around.

 

[Wilson]

The official version is to move toward the richer mix. That means air first & nitrox 2nd. If you are diving two different nitrox mixes, such as 32% & 36%, dive the 32% first and the 36% second. I know a guy who once tried to do the opposite and dove the air second. He got narked at about 40 ft and swore the fish were ganging up on him (true story). At least he had enough sense left to get out of the water.

 

[Charlie99]

The official version is to move toward the richer mix. That means air first & nitrox 2nd. If you are diving two different nitrox mixes, such as 32% & 36%, dive the 32% first and the 36% second.

If you look at it in a bit more detail, you will really find that "deepest equivalent air depth" first works best.

If the two dives are more or less the same depth, then yes, you dive the lean mix or air first, and then richer mix 2nd.

OTOH, if there is a really big difference in depths, then nitrox on the first works out better. Just like the example I posted two years ago to this thread, when diving a 100' wreck and then a shallow reef, I'll dive nitrox on the first dive and air on the second. Even with nitrox, the first dive still has equivalent air depth deeper than the 2nd dive. The benefit from nitrox on the first dive is signficant, while it has less benefit on the 2nd dive.

 

[J.R.]

I'm not a NITROX diver (yet)... but having learned something from a conversation I heard at the ol' 'Local Dive Shop' last night... whichever you use first... be sure your dive computer gets adjusted to appropriate gas for the second dive!

But that raises an interesting question... if you're doing repetative dives and switching gasses between... how do various computers deal with the adjustments?

I'd *assume* that the residual nitrogen and surface intervals are kept clean to the gas on 'Dive A' and simply picked up for 'Dive B'... but having more experience with computers and programming than with diving... I wouldn't always bet the ranch on it, I've seen some 'interesting' glitches in programs over the years...

J.R.

 

[HowardE]

I'm not a NITROX diver (yet)... but having learned something from a conversation I heard at the ol' 'Local Dive Shop' last night... whichever you use first... be sure your dive computer gets adjusted to appropriate gas for the second dive!

But that raises an interesting question... if you're doing repetative dives and switching gasses between... how do various computers deal with the adjustments?

I'd *assume* that the residual nitrogen and surface intervals are kept clean to the gas on 'Dive A' and simply picked up for 'Dive B'... but having more experience with computers and programming than with diving... I wouldn't always bet the ranch on it, I've seen some 'interesting' glitches in programs over the years...

J.R.

Offgassing on the surface occurs at the same rate regardless of what mix you dived. So this isn't a factor. In other words... If you're in PG "G" on the PADI air table after an air dive, or in PG "G" on the PADI EANx36 table... you're still in PG "G" and offgassing rates are the same for your SI.

Just make sure you set your computer for the correct mix

Once you start diving nitrox, you'll think of air as 21% nitrox, and your other mixes as whatever %o2 you have... Thus it doesn't matter which mix you dive, you just plan your dive accordingly. Switching between nitrox and air (21% nitrox) isn't a probelm.

 

[Stormbringer]

The high partial pressures of O2 and depth tends to reduce one's breathing rate. One way to avoid the buildup and retention of CO2 is to consciously regulate your breathing to a constant rate: fully inhalling for 6 secs holding for 2 secs then fully exhalling for 6 secs and repeating each breath cycle can help to reduce the buildup and retention of CO2 while diving EANx over 21%. I learned this technique from my NITROX Instructor. CO2 retention on EANx is a concern and should be a subject discussed in one's NITROX class and the various methods avail for reducing its buildup and retention. I would be concerned if its not a subject discussed in current NITROX classes as more and more evidence and clinical tests show how EANx effects the breathing rate at depth. Even in the latest Military Channel SEAL TEAM training program shown on TV had a Navy SEAL trainee experience CO2 problems on the Drager 100% O2 rebreather unit at the shallow pool depths they were practicing in.

 

[JeffG]

I realize that I may incur flames and peals of derisive laughter for this question, much as H2Andy did with his question about using water for weight

That was a great thread. I should find it and bump it.

 

[EAN GUY]

We use nitrox to prolong our time down by reducing the amount of nitrogen that we in gas on each breath. Then subsequent dives are effected by that nitrogen from the first dive as residual nitrogen.
Reduce your nitrogen, use the nitrox, on your first dive. That will allow you to dive longer on the air dive.
Remember to reset, if you are using, your computer to the proper O2 mix.
Enjoy
EAN GUY

 

[MASS-Diver]

The high partial pressures of O2 and depth tends to reduce one's breathing rate. One way to avoid the buildup and retention of CO2 is to consciously regulate your breathing to a constant rate: fully inhalling for 6 secs holding for 2 secs then fully exhalling for 6 secs and repeating each breath cycle can help to reduce the buildup and retention of CO2 while diving EANx over 21%. I learned this technique from my NITROX Instructor. CO2 retention on EANx is a concern and should be a subject discussed in one's NITROX class and the various methods avail for reducing its buildup and retention. I would be concerned if its not a subject discussed in current NITROX classes as more and more evidence and clinical tests show how EANx effects the breathing rate at depth. Even in the latest Military Channel SEAL TEAM training program shown on TV had a Navy SEAL trainee experience CO2 problems on the Drager 100% O2 rebreather unit at the shallow pool depths they were practicing in.

You've got to be kidding me. What you have posted in 100% not true. High Po2 only lead to rentention in people with serious medical conditions (COPD), it has no bearing at all for healthy divers.

The mods should really pull your post, to suggest that EANX slows breathing rate is pure misinformation. SCARY

 

[MookieMoose]

The high partial pressures of O2 and depth tends to reduce one's breathing rate.

I don't tknow where you get your information, but there is no research evidence to support this. Increasing gas density (ie increasing depth while breathing air, nitrox, oxygen, anything without helium) can result in decreased ventilation, and therefore CO2 retention, but not increasing FiO2.