Nitrox Tank used first or last?

[adutto]

With nitrox, as you have the constraint of the ppO2, the maximum depth is reduced. So here, what I would do, is go Air in the first one so you can do a deeper dive, and leave nitrox for the second that allows you to stay longer at a descent depth.
If you want to do the other way around, is doable, but you are not going to be as deep as you can in the first dive, nor have a good depth/time relationship in your second dive.
All operators I did 1 air and 1 EANx with, recommended first Nitrox and second Air. In the profile you are proposing, if you do the math you'll see the difference for yourself!

here's my USD 0.02

Alex

 

[knotical]

iztok,
Interesting synthesis to dive the deeper EAD first. And sensible. But I also agree with adutto.

In your particular example of 100 feet and 70 feet, there are four options:
A. 100 foot air followed by 70 foot EANx32
B. 100 foot EANx32 followed by 70 foot air
C. 70 foot air followed by 100 foot EANx32
D. 70 foot EANx32 followed by 100 foot air.

Since:
100 foot EANx32 EAD is 81 feet, and
70 foot EANx32 EAD is 56 feet,
the first two options seem acceptable, being:
a 100 foot actual air depth dive followed by 56 foot EAD, or
an 81 foot EAD dive followed by actual 70 foot air dive.

But the third and fourth options go from lesser EAD to greater and would not meet the test.
As it turns out, in this case you'd do the actual deeper dive first either way.

One might choose option A, doing the deeper air dive first, simply to stay farther away from oxtox limits. (I'm agreeing with adutto)

There are instances in which you might choose to do the physically shallower dive first. For example 70 foot on air followed by 80 foot on EANx32. (70 foot to 64 foot EAD)

The main thing is to stay within the limits of your chosen model. More often than not, the rules of thumb of deeper first and air first make sense.

 

[Rhone Man]

It's impossible to generalise, but if you have to, as a rule of thumb I tend to suggest deferring the nitrox to the second dive.

The second dive is where you have the accumulated RNT, and so it helps having the nitrox to offset that penalty on your bottom time. Otherwise the NDL is more likely to be something that you will bump up against.

 

[iztok]

knotical, as I said I subscribe the "deepest dive first" philosophy and will use EAD to determine that.

I would also generally use best mix for the depth or one of the standard 36 or 32 when available. For that particular dive I would go with 32. A dive place at NC coast only banks 30 so I would use it for both dives.

If I were to mix I would dive deeper first with lesser concentration of oxygen and shallower dive with richer concentration second. Had two are the same depth I would dive lesser first and richer second dive ("shallower dive second").

 

[fdog]

Here's my assumptions:

• Both dives 100' / 25 min
• 1 hour SI
• Diver isn't concerned with light deco

Do the Air dive first. There's more benefit in breathing Nitrox on top of residual nitrogen in the body, than there is in using Nitrox to prevent residual nitrogen buildup.

Using the profiles given, deco is going to be required (especially for the Air dive). However, the deco on that Air dive will be less if it's the first dive. There is a penalty for breathing Air with a residual nitrogen load (as there would be on the second dive, first dive on Nitrox) that makes using Air for the second dive result in significantly more deco.

For example: (V-Planner, +2)

Dive 1 Air
40' 1min
30' 3min
20' 5min
10' 9min

Dive 2 32%
10' 3min

>>>>

Dive 1 32%
10' 2min

Dive 2 Air
50' 1min
40' 2min
30' 4min
20' 6min
10' 12min

>>>>>

Personally I'd take the schedule with less obligation, as it has less headaches with rock bottom, cylinder size, and thermal exposure.


All the best, James

 

[Hatul]

Say you have a 2 tank dive. One filled with EAN32 the other is air. Is it safer to dive the Nitrox first or last? Dive profile 100/25 with 1 hour SI. I'll be swimming at the bottom, +/- 5ft on both dives.
I'm hoping for some technical physiology mumbo-jumbo. Maybe a reference to an article/book that specifically addresses this.

If both dives are identical in profile it's better to dive the second tank nitrox. In terms of nitrogen loading this follows dive-the-deeper-dive-first rule.

The way to quantify this is with air-equivalent depth, which you want deeper on the first dive.

Adam

 

[DaleC]

Using the PADI tables

My assumptions:
Square profile
Wanting to stay out of deco

Dive one: air, 20minutesBT, PG O, 1hour SI, PG E. Dive two: Nitrox32, PG E, 18minutesBT. CombinedBT 38minutes.

Dive one: Nitrox32, 25 minutesBT, PG O, 1hour SI, PG E. Dive two air, PG E, 10minutesBT. CombinedBT 35minutes.

If one wanted to max out the first dive it would be:

Dive one: Nitrox 32, 30minutesBT, PG R, 1hour SI, PG F, Dive two air, PG F, 9minutesBT. Combined BT 39 minutes.

Interesting that, according to the tables, maxing out the EAN only gives 1 minute more combined bottom time than maxing out the air (on the first dives).

 

[MRTdiver]

I've worked the dive both ways using the US Navy tables (Nitrox - Air vs. Air - Nitrox) and the deco requirements were the same. The dives were done in the order - Air tank first then EAN32. And I had reason(s) why, but were they correct? So I was concerned about the physiological aspects of this nitrogen loading order. Which was best for the diver?

For a while I didn't understand what people were talking about (do the deepest dive first) cause I said that both dives were essentially the same depth (100'). Until Adam's post ("If both dives are identical in profile it's better to dive the second tank nitrox... The way to quantify this is with air-equivalent depth, which you want deeper on the first dive.") I finally understood how they were thinking about this... that is many of you were thinking of the EAN32 in terms of EAD conversion. So theoretorically swallower, but only in regard to nitrogen on gasing. Ok got it.
Others said, like James F., "There's more benefit in breathing Nitrox on top of residual nitrogen in the body, than there is in using Nitrox to prevent residual nitrogen buildup."

Yeah good stuff. I just wish I could find this specifically addressed in a DAN article or something. I haven't come across it in any Tech books, but I'm most fimiliar with TDI. Come on Bret Gilliam, Schek?

 

[DaleC]

I've worked the dive both ways using the US Navy tables (Nitrox - Air vs. Air - Nitrox) and the deco requirements were the same. The dives were done in the order - Air tank first then EAN32. And I had reason(s) why, but were they correct? So I was concerned about the physiological aspects of this nitrogen loading order. Which was best for the diver?

For a while I didn't understand what people were talking about (do the deepest dive first) cause I said that both dives were essentially the same depth (100'). Until Adam's post ("If both dives are identical in profile it's better to dive the second tank nitrox... The way to quantify this is with air-equivalent depth, which you want deeper on the first dive.") I finally understood how they were thinking about this... that is many of you were thinking of the EAN32 in terms of EAD conversion. So theoretorically swallower, but only in regard to nitrogen on gasing. Ok got it.
Others said, like James F., "There's more benefit in breathing Nitrox on top of residual nitrogen in the body, than there is in using Nitrox to prevent residual nitrogen buildup."

Yeah good stuff. I just wish I could find this specifically addressed in a DAN article or something. I haven't come across it in any Tech books, but I'm most fimiliar with TDI. Come on Bret Gilliam, Schek?

You may be waiting awhile for Sheck to comment

 

[aquaregia]

Is this an actual dive plan or just an example? The depth, BT and SI are sort of a troublesome combination. Are you just emptying a nitrox tank or something? Why plan different gasses for the same dive? I'm not surprised that this isn't covered in any books just because it seems to be sort of a strange dive plan.