Diving Nitrox AND air

[djensen]

Hi,

A buddy and I are having a friendly debate. If we're making two dives in a day, one on Nitrox and one on air, does it matter which gas is used first? For example: Let's do our first dive to, say, 70 feet. I'll use Nitrox on that dive. After an adequate surface interval, we dive again - this time on air to 50 feet. Any advantage to breath the gases in this order (as opposed to diving air first and Nitrox second)?

I took the position that using the Nitrox on the deeper dive is more beneficial as we'll accumulate less nitrogen while under the greater pressure. My bud says using Nitrox on the second dive may be better as it makes your second dive safer (because we're at a higher pressure group on the second dive).

Intuitively it seems that it wouldn't matter much as you either front load the nitrogen or you back load it. Either way, you'll have the same amount of nitrogen in your system at the end of the day, right? Let's assume that we're diving within all limits and our surface interval is adequate to do a nice second dive.

There's a beer riding on this, so it's pretty important... ha ha

 

[lonebrave]

Not positive on this without running tables or graphs on the dives, but I would think the nitrogen loading will not be the same if you dive nitrox then air or air then nitrox (I'm assuming the two dives are different depths/durations). The PP of nitrogen will be different throughout the dives not only due to the fractional nitrogen level of the gas, but also because of difference in ambient pressure.

I just finished Deco for Divers and am working through The Six Skills, so not sure how much of this I've correctly absorbed yet.

 

[Paulmal]

Nitric can do two things; extend dives/ allow more bottom time or make a dive safer but not both. To extend bottom time you have to calculate the NDL for the mix you ate using, to add safety you dive the nitrox on air tables.

So you and your friend are looking at different effects of nitrox. So you both are correct sort of.

As long as you exit the water on dive in a lower pressure group than had you done the dive on air then nitrox will help extend bottom time on dive two, if not then using nitrox on five two will give you a longer dive 2 but shorter dive 1.

Hope you were able to follow me.

 

[live_2_dive]

DISCLAIMER: this is a HIGHLY simplified way of answering this question

Think of the tank of air as another nitrox tank that is simply EAN21. And when diving 2 nitrox tanks you dive the lower EAN first right? So dive the air first, then the nitrox.


However, the explanation given by Paulmal is also totally valid. I don't think there is truly one absolutely correct answer


To be fair, I think this is totally unrealistic why not just dive nitrox on both and boost your bottom times on both dives...but I do understand that when a beer is riding on the answer, you have to at least get AN answer!


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[JohnN]

es...but I do understand that when a beer is riding on the answer, you have to at least get AN answer!

Ha ha! You fool! You fell victim to one of the classic blunders - The most famous of which is "never get involved in a land war in Asia" - but only slightly less well-known is this: "Never go against a Sicilian when death is on the line"!

 

[racerx_]

Heres a quick mockup on divepal. Your answer would probably vary based on diffrent depths/profiles.

Air first, then nitrox



Nitrox first, then air

 

[racerx_]

that's with EAN32 and I did it really fast so I'm not sire if those are accurate ascent rates but should give a decent picture of the nitrogen loading estimates

 

[ajduplessis]

70ft on air tables is going to give you around 35min of BT/NDL diving. Anything more and you start hitting deco obligations. 50ft on air is going to give you around 75min BT/NDL diving. If your SAC will allow you more than 35min @ 70ft I would use Nitrox because you can extend you BT without hitting deco. If not, you will still get out of the water a "cleaner" diver than with air, allowing a longer and "safer" second dive.

 

[Doppler]

Hi,

A buddy and I are having a friendly debate. If we're making two dives in a day, one on Nitrox and one on air, does it matter which gas is used first? For example: Let's do our first dive to, say, 70 feet. I'll use Nitrox on that dive. After an adequate surface interval, we dive again - this time on air to 50 feet. Any advantage to breath the gases in this order (as opposed to diving air first and Nitrox second)?

I took the position that using the Nitrox on the deeper dive is more beneficial as we'll accumulate less nitrogen while under the greater pressure. My bud says using Nitrox on the second dive may be better as it makes your second dive safer (because we're at a higher pressure group on the second dive).

Intuitively it seems that it wouldn't matter much as you either front load the nitrogen or you back load it. Either way, you'll have the same amount of nitrogen in your system at the end of the day, right? Let's assume that we're diving within all limits and our surface interval is adequate to do a nice second dive.

There's a beer riding on this, so it's pretty important... ha ha

For what it's worth, I would use the nitrox first and then the air... the physiology backing this up is tenuously logical at best; however, the nitrogen loading would definitely be best managed if the mix on the first dive were EAN 45 or something similar. This would effectively make the EAD of dive one something like 10 or 11 metres.

Probably, you should buy each other a beer after doing a couple of dives... have fun, stay safe.

 

[djensen]

Excellent discussion. Thanks guys. I'm impressed and distressed that we have well-considered responses that answer with diametrically opposing viewpoints. Very interesting! It's like the "Taste's Great - Less Filling" beer debate... ha ha I'm actually no closer to an answer but I think I'll take Doppler's advice. We'll go for a dive or two and then buy each other a beer...