Nitrox 32 first dive, air second dive?

[MMM]

I have been diving in cozumel with an aeris computer for many years and have never violated NDL on air. I go as deep as I want to (sometimes 110-115 feet and I've done the obligatory devil's throat a few times which is deeper) and I don't understand how you could really have a deco obligation, unless you are staying a lot deeper than I generally would. I personally don't pay too much attention to NDL anyway; I'm much more interested in the N2 loading bar graph towards the end of the dive. With the right profile, you can nudge up close to NDL near the beginning of the dive and easily surface with your N2 load in the 'green' zone. I do extended stops at 10 ft, especially after the 2nd dive, and that has helped quite a bit with post dive fatigue.

I have never paid for nitrox in Cozumel while on 2 dive days. To me it's not worth it, I just do my long stops and I feel fine on surfacing. If you really feel that you are being limited in your profile by air, then it might be worth it for you, but you might be able to enjoy your long deep dives just as much on air as long as you are willing to extend your safety stop.

You don't run into problems unless you are really good on air (and are thus down for longer) or are diving with bigger tanks (that give you more air)...or both. Then nitrox is your friend.

 

[halocline]

Well, I'm decent on air, and I frequently surface from dives in Coz with quite a bit of air in my tank, having done as deep a dive as I care to with a single tank. I would use nitrox if it were free, or if I were doing more than two dives/day. But I'm pretty confident that the long shallow stops provide a similar benefit in my case. I could see running into deco on the 2nd dive if it were as deep as the first, but typically it's a shallower dive, at least with most ops. I suppose if everyone on the boat were using nitrox and there was a plan to do two deep dives, I might feel differently.

 

[eelnoraa]

Air is for car tires.

The proper gas for tires is 100% Nitrogen

 

[BRT]

The proper gas for tires is 100% Nitrogen

But 79% nitrogen works as well.

 

[halocline]

I use trimix in my tires, it gives my car that extra bounce. And if I go over a cliff and plunge into deep water, I can breathe off the tires without getting narced.

 

[Andrew Donn]

DAN studies do not support that myth. Nitrox gives longer NDLs which is great on square bottom wreck diving, but really does not add value or safety on multilevel reef dives. Your ascents, safety stops, floating stops, and minimum activities after the dive are more important.

I don't know what DAN studies you think you are quoting but reduced risk of DCS for the same dive profile with a higher O2 percentage is not a myth it is simple numbers. It is the entire basis for the generation of no-decompression limits. Your exposure made up of depth, time and oxygen percentage (or nitrogen percentage, whichever way you want to look at it) relative to the limit is the primary determining factor for DCS risk. Other factors such as ascent rate, safety stops, hydration, activity are all secondary. They are important and should always be taken into consideration but they are not as important as your exposure to absorbed nitrogen. This applies equally to multi-level or square profile dives.

Nitrox is not in and of itself necessarily safer than air or any other gas. In fact if you dive nitrox to the no-decompression limit of that mix you actually carry the same DCS risk as an air diver diving to the NDL for air. That is actually how the tables and limits are derived. They are all based on a percentage of the population showing signs of DCS after a certain exposure. In practical terms we often find ourselves in the situation like the OP where we are not limited by the NDL for our gas but by other limits such as gas supply or time limits established by the boat. In those cases the higher concentrations of O2 mean that you are further away from for no-decompression limit and therefore have a lower risk of DCS for the same dive.

Whether or not that is worth the additional cost of the nitrox is certainly subjective. You can decrease your risk in other ways like shortening your bottom time or increasing your safety stop like others have pointed out but additional O2 certainly does decrease your DCS risk. One thing that we do know is that current decompression theory does not properly capture is multiple dives over multiple days. Even within established limits people doing multi-day diving have a higher incidence of DCS suggesting that the modeling does not properly capture the loading and off loading of nitrogen in the slowest of the tissues. A little additional safety on multiple days of diving is prudent.

Lastly risk, when talking about nitrox, is not all about the nitrogen. Saying that more is better from a DCS risk has to be balanced against oxygen toxicity risks which also has a cumulative effect over multiple dives.

 

[Andrew Donn]

If someone wants to stay deeper longer Nitrox is certainly the way to go. Everyone needs to decide what they want to make of their dive experiences and if deeper means better to them then that's what they should do and utilize Nitrox.

Nitrox is not for diving deeper. For diving deeper you want a lower concentration of oxygen not higher. Nitrox is most useful for extending bottom time in moderate depth ranges.

 

[ReefHound]

I don't know what DAN studies you think you are quoting but reduced risk of DCS for the same dive profile with a higher O2 percentage is not a myth it is simple numbers. It is the entire basis for the generation of no-decompression limits. Your exposure made up of depth, time and oxygen percentage (or nitrogen percentage, whichever way you want to look at it) relative to the limit is the primary determining factor for DCS risk. Other factors such as ascent rate, safety stops, hydration, activity are all secondary. They are important and should always be taken into consideration but they are not as important as your exposure to absorbed nitrogen. This applies equally to multi-level or square profile dives.

Nitrox is not in and of itself necessarily safer than air or any other gas. In fact if you dive nitrox to the no-decompression limit of that mix you actually carry the same DCS risk as an air diver diving to the NDL for air. That is actually how the tables and limits are derived. They are all based on a percentage of the population showing signs of DCS after a certain exposure. In practical terms we often find ourselves in the situation like the OP where we are not limited by the NDL for our gas but by other limits such as gas supply or time limits established by the boat. In those cases the higher concentrations of O2 mean that you are further away from for no-decompression limit and therefore have a lower risk of DCS for the same dive.

I don't think you two are in disagreement. When he said nitrox "gives longer NDLs" I think he was assuming a diver is actually going to take advantage of that and thus would not be diving "the same dive profile", so his statement that it gives no added safety relates to your second paragraph that nitrox to it's limits carries the same risk as air to it's limits.

 

[Lake Hickory Scuba]

Last year, diving on air in Cozumel for 6 days we came very close to deco obligations on our computers. Something we would like to avoid going forward. We dive AL80's generally.
We are diving in March for 7-8 days and will be doing the usual 2 dives per day, with a night dive mid week (followed by an off day).
It makes sense to me do dive Nitrox 32 on the first dive, as it is usually the deeper one of the two.
My question is regarding the second dive. We could dive Nitrox 36 (as the second dive is generally more shallow), or we could dive air, knowing that we have plenty of recovery time after the second dive.
We tend to do dives around 55-65 minutes following the usual Cozumel dive profiles. We are diving with 3P.
In your opinions, what does diving the second dive on Nitrox 36 buy me vs. diving the second dive on air?
I'm not opposed to shell out the extra $10 per tank, but I'd like to see what other veterans of Cozumel diving AL80's generally do on two tank days. In other words, is it worth it?
Thanks in advance.

Maybe this will help you understand how Erinched Air and 21% Air works when planning dives.

 

[deepsea21]

Nitrox is not for diving deeper. For diving deeper you want a lower concentration of oxygen not higher. Nitrox is most useful for extending bottom time in moderate depth ranges.

Re-read my post... I said "stay deeper longer", not dive deeper. I am guessing all here know "deeper" with Nitrox is a no-no due to O2 toxicity risk.