Nitrox dive one, air dive two

[crispix]

I realize all things being equal (depth, time), in the case where I only have one nitrox tank available, it's beneficial to use air on the first dive and nitrox on the second.

However . . . my local nitrox shop went out of business, and I have been saving my last nitrox fill (EAN32, overfilled to 108CF) for an upcoming dive to the Yukon and then kelp beds.

My second tank only has 90CF of air in it.

Dive 1: max depth 100fsw.
Dive 2: max depth 60fsw.

What would you do? I want the bigger tank on the deeper dive. I'm looking at my dive tables and the difference seems to be that when I use the Nitrox first, I'm getting at least 10 minutes extra bottom time on the first dive at the expense of 6 minutes less NDL time on the second dive.

Of course, I will not be diving a square profile, and I have to factor in my buddy's air situation, so this is a bit academic. Still, I'll use the bigger tank on the deeper dive -- the extra gas overrides any concerns about small differences in nitrogen loading during a second, shallower dive. Comments?

 

[RJP]

I realize all things being equal (depth, time), in the case where I only have one nitrox tank available, it's beneficial to use air on the first dive and nitrox on the second.

???

How'd you come to that "realization"?

 

[boulderjohn]

I believe the only real factor deciding which one to use first in the cases you describe lies in the surface interval. You will be allowed the second dive sooner going one way than the other.

If you have enough SI to do the second dive, there is no reason you can't do the nitrox first.

In this case, I will frequently do the nitrox first if the dive is within PPO2 limits.

 

[crispix]

??? How'd you come to that "realization"?

I love Scuba Board. Excellent passive-aggressive response!

Anyway, here's my cheeky answer: I took a Nitrox class.

Less cheeky example:

Dive 1: 60fsw for 40 minutes.
1 hour surface interval
Dive 2: 60fsw for ??? minutes.

If I do dive 1 on EAN32, I have 20 minutes NDL bottom time available for dive 2 on air.

If I do dive 1 on air, I have 23 minutes NDL bottom time available for dive 2 on EAN32.

So, yes, it's just three minutes difference, and this is using tables and square profiles, which is not realistic. Even so, in this example, it's 15% more bottom time when you save the nitrox for the second dive.

 

[Rhone Man]

This is an old chestnut, but it is fun to kick it around again.

All things being equal I favour Nitrox on the second dive, because (normally) that will be the shallower dive, so it tends to result in a greater stretching of the NDL, and it also offsets the "penalty minutes" that usually accompany RNT.

But in truth all things are never equal, and it all just depends upon profiles and logistics.

 

[willgr1]

based on your dive plan above, if it were me i'd dive the nitrox tank as an air mix. Reasoning being 3 minutes doesn't really make a difference. What is your, and your buddies, air consumption rates? if at 60ft you guys are hitting your turn/end dive pressure before hitting your profile dive times the extra bottom time from the EAN really doesn't matter. I would dive it as an air tank and just use the EAN as an extra safety measure.

 

[Notso_Ken]

I realize all things being equal (depth, time), in the case where I only have one nitrox tank available, it's beneficial to use air on the first dive and nitrox on the second.

However . . . my local nitrox shop went out of business, and I have been saving my last nitrox fill (EAN32, overfilled to 108CF) for an upcoming dive to the Yukon and then kelp beds.

My second tank only has 90CF of air in it.

Dive 1: max depth 100fsw.
Dive 2: max depth 60fsw.

What would you do? I want the bigger tank on the deeper dive. I'm looking at my dive tables and the difference seems to be that when I use the Nitrox first, I'm getting at least 10 minutes extra bottom time on the first dive at the expense of 6 minutes less NDL time on the second dive.

Of course, I will not be diving a square profile, and I have to factor in my buddy's air situation, so this is a bit academic. Still, I'll use the bigger tank on the deeper dive -- the extra gas overrides any concerns about small differences in nitrogen loading during a second, shallower dive. Comments?

Are you diving strictly tables, or are you planning the dive with tables, then backing the dive up with a computer? If the latter, I say do what you want to do. If the former, then you are probably going to have the most bottom time by doing the nitrox on the second dive.

Do the detailed plan and see how much difference it makes for the specific conditions and do what works best for them.

YMMV

Ken

 

[Ann Marie]

Is your tank O2 clean?

Are you diving off the Lois Ann?

If it's not O2 clean and you are diving off the Lois Ann, have them fill it with Nitrox for dive #2 and just enjoy the both dives on nitrox!

Is your buddy diving air or enriched air?

 

[crispix]

But in truth all things are never equal, and it all just depends upon profiles and logistics.

Yes, nicely said.

I find it interesting the way the the dive profiles turn out when looking at these problems. Of course, I'm also the kind of guy that seriously thinks about sucking on my nitrox regulator during the surface interval. Might as well use the leftover 500psi and offgas some more, no?

 

[boulderjohn]

Yes, nicely said.

I find it interesting the way the the dive profiles turn out when looking at these problems. Of course, I'm also the kind of guy that seriously thinks about sucking on my nitrox regulator during the surface interval. Might as well use the leftover 500psi, no?

Another reason to do the nitrox first.